Interesting Information for Students

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Friday, October 28, 2011

Game Show Indicator Lights (Who's First)

The circuit below turns on a light corresponding to the first of several buttons pressed in a "Who's First" game. Three stages are shown but the circuit can be extended to include any number of buttons and lamps.

Three SCRs (silicon controlled rectifiers) are connected with a common cathode resistor (50 ohm) so that when any SCR conducts, the voltage on the cathodes will rise about 7 volts above the voltage at the junction of the 51K and 1K ohm resistors and prevent triggering of a second SCR. When all lamps are off, and a button is pressed, the corresponding SCR is triggered due to the voltage at the divider junction being higher than the cathode. Once triggered, the SCR will remain conducting until current is interrupted by the reset switch. Or, you can just turn the power off and back on.

A 50 ohm, 5 watt resistor was selected to produce a 10 volt drop at 200 mA when a single 25 watt lamp comes on. Higher wattage lamps would require a lower value resistor, and visa versa. For example to use 60 watt lamps and maintain the 10 volt drop, the peak current would be 60/160 = 375 mA and the resistance would be E/I = 10/.375 or about 27 ohms at 3.75 watts. The SCRs are "Sensitive Gate' types which trigger on about 200 uA and the gate current is around 1.5 mA when the first button is pressed. The 1N914 diodes in series with the buttons gates are used to prevent a reverse voltage on the gate when a button is pressed after an SCR is conducting. The two 51 ohm resistors will be fairly large in physical size (compared to a 1/4 watt size) and should be rated for 5 watts of power or more. Use caution and do not touch any components while the circuit is connected to the AC line.

Adding a Buzzer:

The relay shown in parallel with the 50 ohm cathode resistor can be used to momentarily power a buzzer with an external circuit through the contacts. The 1000 uF capacitor causes the relay to energize for about one second, longer times can be obtained with a larger capacitor.

Parts List:
Quantity Description Radio Shack Part Number
1 4 Amp/400 Volt Bridge Rectifier 276-1173
3 Silicon Controlled Rectifier (SCR) NTE5457
3 120 VAC/ 25 Watt incandescent lamp
1 50-100 microfarad/ 200 volt capacitor
1 1000 microfarad / 35 volt capacitor 272-1032
1 50 ohm resistor/ 5 or 10 Watt 271-133
3 Push Button Switch (normally open)
1 Push Button Switch (normally closed)
3 2K resistor, 1/4 watt 271-1325
4 1N914 Diode
1 51K resistor, 1 watt
1 2 Amp Fuse 270-1064
1 Relay (SPDT) 9 Volt DC, 500 ohm coil 275-005

Copyright 2006: Bill Bowden

Monday, October 24, 2011

Light Activated Relay

This is same circuit as above with the addition of a photo resistor to trigger the flip flop instead of a push button. The bias resistor in series with photo resistor was chosen so that sufficient voltage is present at the base of the 2N3904 to supply current to the circuit in ambient lighting conditions. The circuit should toggle when the photo resistor is hit by a flashlight beam or other fast changing light source. Slow changes in light intensity will have no effect unless the light gets too bright to maintain sufficient bias for the 2N3904.

Copyright 2006: Bill Bowden

JAVA Design Pattern

What is the design pattern?

If a problem occurs over and over again, a solution to that problem has been used effectively. That solution is described as a pattern. The design patterns are language-independent strategies for solving common object-oriented design problems. When you make a design, you should know the names of some common solutions. Learning design patterns is good for people to communicate each other effectively. In fact, you may have been familiar with some design patterns, you may not use well-known names to describe them.

Do I have to use the design pattern?

If you want to be a professional Java developer, you should know at least some popular solutions to coding problems. Such solutions have been proved efficient and effective by the experienced developers. These solutions are described as so-called design patterns. Learning design patterns speeds up your experience accumulation in OOA/OOD. Once you grasped them, you would be benefit from them for all your life and jump up yourselves to be a master of designing and developing. Furthermore, you will be able to use these terms to communicate with your fellows or assessors more effectively.

Many programmers with many years experience don't know design patterns, but as an Object-Oriented programmer, you have to know them well, especially for new Java programmers. Actually, when you solved a coding problem, you have used a design pattern. You may not use a popular name to describe it or may not choose an effective way to better intellectually control over what you built. Learning how the experienced developers to solve the coding problems and trying to use them in your project are a best way to earn your experience and certification.

Patterns: According to commonly known practices, there are 23 design patterns in Java. These patterns are grouped under three heads:

1. Creational Patterns
2. Structural Patterns
3. Behavioral Patterns

Thursday, October 20, 2011

Android Developers

Developing applications for Android devices is facilitated by a group of tools that are provided with the SDK. You can access these tools through an Eclipse plugin called ADT (Android Development Tools) or from the command line. Developing with Eclipse is the preferred method because it can directly invoke the tools that you need while developing applications.

However, you may choose to develop with another IDE or a simple text editor and invoke the tools on the command line or with scripts. This is a less streamlined way to develop because you will sometimes have to call command line tools manually, but you will have access to the same number of features that you would have in Eclipse.

Before you begin developing Android applications, make sure you have gone through all of the steps outlined in Installing the SDK.

The basic steps for developing applications with or without Eclipse are the same:

1.  Set up Android Virtual Devices or hardware devices.
  • An Android project contains all source code and resource files for your application. It is built into an .apk package that you can install on Android devices.
  • If you are using Eclipse, builds are generated each time you save changes and you can install your application on a device by clicking Run. If you're using another IDE, you can build your project using Ant and install it on a device using adb.
  • Debugging your application involves using a JDWP-compliant debugger along with the debugging and logging tools that are provided with the Android SDK. Eclipse already comes packaged with a compatible debugger.
  • The Android SDK provides a testing and instrumnetation framework to help you set up and run tests within an emulator or device.

National Instruments

Engage students and reinforce circuit and electronic concepts with a hands-on dynamic learning environment that is a unique combination of software, hardware, courseware, and textbooks. From teaching circuit basics to facilitating upper-level projects, educators are adopting circuits teaching software such as NI Multisim tightly integrated with the NI Educational Laboratory Virtual Instrumentation Suite (NI ELVIS) design and prototyping hardware, and NI Lab VIEW graphical systems design software to prepare students for tomorrow’s engineering challenges.
Other Topics:
1.  Controls and Mechatronics
2.  Signal and Image Processing
3.  RF and Communications
4.  Embedded System Design

Programmable Digital Code Lock

A programmable code lock can be used for numerous applications in which access to an article/gadget is to be restricted to a limited number of persons. Here is yet another circuit of a code lock employing mainly the CMOS ICs and thumbwheel switches (TWS) besides a few other components. It is rugged and capable of operation on voltages ranging between 6 and 15 volts. The supply current drain of CMOS ICs being quite low, the circuit may be operated even on battery.

The circuit uses two types of thumbwheel switches. switch numbers TWS1 through TWS8 are decimal-to-BCD converter type while switch numbers TWS9 through TWS16 are 10-input multiplexer type in which only one of the ten inputs may be connected to the output (pole). One thumbwheel switch of each of the two types is used in combination with IC CD4028B (BCD to decimal decoder) to provide one digital output.Eight such identical combinations of thumbwheel switches and IC CD4028 are used. The eight digital outputs obtained from these combinations are connected to the input of 8-input NAND gate CD4068.

For getting a logic high output, say at pole-1, it is essential that decimal numbers selected by switch pair TWS1 and TWS9 are identical, as only then the logic high output available at the Specific output pin of IC1 will get transferred to pole-1. Accordingly, when the thumbwheel pair of switches in each combination is individually matched, the outputs at pole-1 to pole-8 will be logic high.

This will cause output of 8-input NAND gate IC CD4068b to change over from logic high to logic low, thereby providing a high-to-low going clock pulse at clock input pin of 7-stage counter CD4024B, which is used here as a flip-flop (only Q0 output is used here).The output (Q0) of the flip-flop is connected to a relay driver circuit consisting of transistors T1 and T2. The relay will operate when Q0 output of flip-flop goes low. As a result transistor T1 cuts off and T2 gets forward biased to operate the relay.Switch S1 is provided to enable switching off (locking) and switching on (unlocking) of the relay as desired, once the correct code has been set.

With the code set correctly, the NAND gate output will stay low and flip-flop can be toggled any number of times, making it possible to switch on or switch off the relay, as desired. Suppose we are using the system for switching-on of a deck for which the power supply is routed via the contacts of the relay. The authorised person would select correct code which would cause the supply to become available to the deck.

After use he will operate switch S1 and then shuffle the thumbwheel switches TWS1 through TWS8 such that none of the switches produces a correct code. Once the code does not match, pressing of switch S1 has no effect on the output of the flip-flop.Switches TWS9 through TWS16 are concealed after setting the desired code. In place of thumbwheel switches TWS1 through TWS8 DIP switches can also be used.

Wednesday, October 19, 2011

Ultrasonic Switch

Circuit of a new type of remote control switch is described here. This circuit functions with inaudible (ultrasonic) sound. Sound of frequency up to 20 kHz is audible to human beings. The sound of frequency above 20 kHz is called ultrasonic sound. The circuit described generates (transmits) ultrasonic sound of frequency between 40 and 50 kHz. As with any other remote control system this cirucit too comprises a mini transmitter and a receiver circuit. Transmitter generates ultrasonic sound and the receiver senses ultrasonic sound from the transmitter and switches on a relay. The ultrasonic transmitter uses a 555 based astable multivibrator. It oscillates at a frequency of 40-50 kHz. An ultrasonic transmitter transducer is used here to transmit ultrasonic sound very effectively.

The transmitter is powered from a 9-volt PP3 single cell. The ultrasonic receiver circuit uses an ultrasonic receiver transducer to sense ultrasonic signals. It also uses a two-stage amplifier, a rectifier stage, and an operational amplifier in inverting mode. Output of op-amp is connected to a relay through a complimentary relay driver stage. A 9-volt battery eliminator can be used for receiver circuit, if required. When switch S1 of transmitter is pressed, it generates ultrasonic sound. The sound is received by ultrasonic receiver transducer. It converts it to electrical variations of the same frequency. These signals are amplified by transistors T3 and T4. The amplified signals are then rectified and filtered. 

The filtered DC voltage is given to inverting pin of op-amp IC2. The non- inverting pin of IC2 is connected to a variable DC voltage via preset VR2 which determines the threshold value of ultrasonic signal received by receiver for operation of relay RL1. The inverted output of IC2 is used to bias transistor T5. When transistor T5 conducts, it supplies base bias to transistor T6. When transistor T6 conducts, it actuates the relay. The relay can be used to control any electrical or electronic equipment.

 Important hints:

1. Frequency of ultrasonic sound generated can be varied from 40 to 50 kHz range by adjusting VR1. Adjust it for maximum performance.

2. Ultrasonic sounds are highly directional. So when you are operating the switch the ultrasonic transmitter transducer of transmitter should be placed towards ultrasonic receiver transducer of receiver circuit for proper functioning.

3. Use a 9-volt PP3 battery for transmitter. The receiver can be powered from a battery eliminator and is always kept in switched on position.

4. For latch facility use a DPDT relay if you want to switch on and switch off the load. A flip-flop can be inserted between IC2 and relay. If you want only an ‘ON-time delay’ use a 555 only at output of IC2. The relay will be energised for the required period determined by the timing components of 555 monostable multivibrator.

5. Ultrasonic waves are emitted by many natural sources. Therefore, sometimes, the circuit might get falsely triggered, espically when a flip-flop is used with the circuit, and there is no remedy for that.

Teach Yourself Graphic Design: A Self-Study Course Outline

Fortunately, it isn’t required to go to design school in order to be a graphic designer. A good foundation in graphic design history, theory, and practical application will help you hit the ground running. There are plenty of resources available in which you can learn graphic design on your own. Don’t set your expectations to high at first, as it will take enthusiastic study for years to become great. You can do it though!

If you would like to learn graphic design from the ground up, through self directed study, then this article lists some great resources that will get you started with your design education. Also, even if you do go to design school, at least three-fifths of your education will be through self directed study anyway. Let’s get to it!

Monday, October 17, 2011

VTC Training CD for C Programming

Author - Mark Virtue
  1. Introduction
  2. A Basic C Program
  3. Basic Elements of a C Program
  4. Conditional Code
  5. Loops
  6. Arrays
  7. Strings and Characters
  8. Advanced Operators
  9. The C Preprocessor
  10. Functions
  11. Structures
  12. The Compilation Process
  13. Basic Pointers
  14. Scope
  15. Dynamic Memory
  16. The Standard C Function Library
  17. Bitwise Operators
  18. Advanced Pointers
  19. Function Pointers

The Gas Chamber Expriment : Written By Ankit Malasi


The biggest worry of a soldier on the battle field is of course the enemy. But when a soldier has been fighting for several days without any time to rest, then his biggest enemy is not a guy with a loaded rifle pointed to his head. But, it is his own body. Sleep, fatigue and pain may not kill you, but it will dull your senses and make you a easy target and a worthless soldier.

The military once tried to address this problem by commissioning a group of scientist to conduct an illegal experiment to develop a stimulant that could make such problems irreverent. These researchers kept 5 people alive for 15 days using an experimental gas based stimulant. They were kept in a sealed environment to carefully monitor their oxygen intake so that the gas didn’t kill them, since the gas was toxic in high concentrations. This was before closed circuit cameras so they had only microphones and 5 inch thick glass porthole sized window into the chamber to monitor them. The chamber contained books, cots to sleep on but no bedding, running water and a toilet, and enough dried food to last them for over a month.

The test subjects were Pakistani prisoners. Each of them had spent at least 5 years in the Indian jails. And, in all this time no one had come to claim them. So it was safe to say that no one would miss them if anything went wrong in the experiment.

Everything was fine for the first 5 days, the subjects hardly complained having been promised (falsely) that they would be freed if they submitted to the test and did not sleep for 30 days. Their conversations and activities were monitored and it was noted that they continued to talk about increasingly traumatic incidents of the past, and the general tone of their conversations took on a darker aspect after the 4 day mark.

After 5 days they started to complain about the circumstances and events that led them to where they were and started to demonstrate severe paranoia. They stopped talking to each other and began alternately whispering to the microphones and one way mirrored port holes. Oddly they all seemed to think that they could win the trust of the experimenters by turning over their comrades, the other subjects in the captivity with them. At first the researchers suspected that this was an effect of the gas itself…

After 9 days the first of them started screaming. He ran the length of the chamber repeatedly screaming at the top of his lungs for 3 hours straight, he continued attempting to scream but was only able to produce occasional squeaks. The researchers postulated that he had physically torn his vocal cords. The most surprising thing about this behavior is how the other captives reacted to it… or rather didn’t react to it. They continued whispering to the microphones until the second of the captives started to scream. The 2 non screaming captives took the books apart, smeared page after page with their own feces and pasted them calmly over the portholes. The screaming promptly stopped.

So did the whispering into the microphones.

After 3 more days passed, the researchers checked the microphones hourly to make sure they were working. Since they thought that it was impossible that no sound could be coming with 5 people inside. The oxygen consumption in the chamber indicated that all 5 must still be alive. In fact it was the amount of oxygen 5 people would consume at a very heavy level of exercise. On the morning of the 14th the researchers did something they said they would not do to get a reaction from the captives, they used the intercom inside the chamber, hoping to provoke any response from the captives they were afraid were either dead of unconscious.

They announced: “We are opening the chamber to test the microphones. Step away from the door and lie flat on the floor or you will be shot. Compliance will earn one of you your immediate freedom.

To their surprise, they heard a single line in a calm voice response: “We no longer want to be freed.”

Debate broke out among the researchers and the military forces funding the research. Unable to provoke any response using the intercom, it was finally decided to open the chamber at midnight on the 15th day.

The chamber was flushed of the stimulant gas and filled with fresh air and immediately voices from the microphone began to object. 3 different voices began begging, as if pleading for the life of loved ones to turn the gas back on. The chamber was opened and soldiers sent in to retrieve the test subjects. They began to scream louder than ever, and so did the soldiers when they saw what was inside. 4 of the 5 subjects’ were still alive, although no one could rightly call the state that they were in as ‘life’.

The food rations past day 5 had not been so much as touched. There were chunks of meat from the dead test subject’s thighs and chest stuffed into the drain at the centre of the chamber, blocking the drain and allowing 4 inches of water to accumulate on the floor. Precisely how much of the water on the floor was actually blood was never determined. All 4 ‘surviving’ test subjects had large portions of muscles and skin thrown away from their bodies. The destruction of flesh and the exposed bone on their finger tips indicate that the wounds were inflected by hand, not with teeth as the researchers initially thought. Closer examination of the positions’ and the angles of the wounds indicated that most if not all of them were self-inflicted.

The abdominal organs below the rib cage of all test subjects had been removed. While the heart, lung and diaphragm remained in place, the skin and most of the muscles attached to the ribs had been ripped off exposing the lungs through the rib cage. All blood vessels and organs remained intact, they had just been taken out and laid on the floor, fanning out around the still living bodies of the subjects. The digestive systems of all four could be seen to be working digesting food. It quickly became apparent what they were digesting was their own flesh that they had ripped off and eaten over the course of days.

Most of the soldiers were Indian special operatives at the facility, but still many refused to return to the chamber to remove the test subjects. They continued to scream to be left in the chamber and alternatively begged and demanded that the gas be turned back on least they fall asleep…

Optical Sensor

Electro-optical sensors are those which convert the light rays in to electronic signal very similar to the photo resistor, these are applied in emergency lamps such that when there is light it is made to switch off and when there is no light it automatically made to switch on. In aviation, electro-optical devices have been used as part of the avionics, in order to expand both range and vision at low ambient light levels.

Travel Touch Alarm

The Travel Touch Alarm can be used to provide a audible alarm when someone touches the door knob or handle of your hotel room. The door knob or handle must be made of metal for the circuit to work. The main chip in the circuit is a 555 timer which will be triggered if a hand comes close to or touches the door knob. The circuit attaches to the door knob at the end of the 1 meg ohm resistor. 

Once the timer is triggered the LED will light and the UJT will output a tone to the speaker. The timer will time out in 5 seconds. The sensitivity of the trigger can be changed by changing the 1 meg ohm resistor to another value. The 5 second time out can be adjusted by changing the value of the resistor connected between pin 8 and pin 7. The output tone can be changed by changing the RC values on the base of the UJT.

Loop Sensor

Many security systems use a closed loop of wires and switches arranged so that whenever a door or window is opened, the loop will be broken and the alarm will sound. An obvious problem is that someone can tamper with the system, short out the loop, and later on, come back and burglarize the premises without sounding the alarm. Hiding a known resistance in the loop can prevent this. That way, the alarm can distinguish a short circuit from a correctly functioning closed loop. 

The figure above shows a circuit that does the job. It's a somewhat unusual application of a National Semiconductor LM3915 IC, normally used to drive LED bargraph displays. That chip happens to contain the right combination of comparators and logic circuits to do what you need. 

Step 1 is to translate the loop resistance into a voltage; that's done by putting it into a voltage divider with resistors R1 and R2. Capacitor C2 protects the circuit against electromagnetic noise-important because burglar alarms use long wires, often running near heavy electrical equipment. 

Step 2 is to translate the voltage into a logic signal indicating whether it's in the correct range. That's where the LM3915 comes in. Normally, the LM3 9 15 would drive ten LEDs, one for each of ten small ranges of voltage. The figure below shows the states of outputs A, B, and C under different loop-resistance conditions. obtain logic-level outputs, we have it driving 1K resistors instead of LEDs. Since we only need to distinguish three situations, not ten, we tie some of the outputs together. The LM3915 has open-collector outputs that can be paralleled in that way. 

The truth table in Fig. 2 shows how the outputs work. Note that they use negative logic (OV for "yes", +5V for "no"), the opposite of ordinary logic circuits. You can use inverters such as the 74HC04 to produce positive logic signals if that's what you need. 

Finally, note that the circuit will actually work with any supply voltage from 3 to 25 volts. Of course, if the supply isn't 5 volts, the outputs will not be compatible with j-volt logic circuits.

Digital Electronic Lock

The digital lock shown below uses 4 common logic ICs to allow controlling a relay by entering a 4 digit number on a keypad. The first 4 outputs from the CD4017 decade counter (pins 3,2,4,7) are gated together with 4 digits from a keypad so that as the keys are depressed in the correct order, the counter will advance.

As each correct key is pressed, a low level appears at the output of the dual NAND gate producing a high level at the output of the 8 input NAND at pin 13. The momentary high level from pin 13 activates a one shot circuit which applies an approximate 80 millisecond positive going pulse to the clock line (pin 14) of the decade counter which advances it one count on the rising edge. 

A second monostable, one shot circuit is used to generate an approximate 40 millisecond positive going pulse which is applied to the common point of the keypad so that the appropriate NAND gate will see two logic high levels when the correct key is pressed (one from the counter and the other from the key). The inverted clock pulse (negative going) at pin 12 of the 74C14 and the positive going keypad pulse at pin 6 are gated together using two diodes as an AND gate (shown in lower right corner). 

The output at the junction of the diodes will be positive in the event a wrong key is pressed and will reset the counter. When a correct key is pressed, outputs will be present from both monostable circuits (clock and keypad) causing the reset line to remain low and allowing the counter to advance. However, since the keypad pulse begins slightly before the clock, a 0.1uF capacitor is connected to the reset line to delay the reset until the inverted clock arrives.

The values are not critical and various other timing schemes could be used but the clock signal should be slightly longer than the keypad pulse so that the clock signal can mask out the keypad and avoid resetting the counter in the event the clock pulse ends before the keypad pulse. The fifth output of the counter is on pin 10, so that after four correct key entries have been made, pin 10 will move to a high level and can be used to activate a relay, illuminate an LED, ect. At this point, the lock can be reset simply by pressing any key. The circuit can be extended with additional gates (one more CD4011) to accept up to a 8 digit code.

The 4017 counting order is 3 2 4 7 10 1 5 6 9 11 so that the first 8 outputs are connected to the NAND gates and pin 9 would be used to drive the relay or light. The 4 additional NAND gate outputs would connect to the 4 remaining inputs of the CD4068 (pins 9,10,11,12). The circuit will operate from 3 to 12 volts on 4000 series CMOS but only 6 volts or less if 74HC parts are used. The circuit draws very little current (about 165 microamps) so it could be powered for several months on 4 AA batteries assuming only intermittent use of the relay.

Copyright 2006: Bill Bowden

Part list:

1x CD4017 decade counter
1x CD4011
1x 74C14 
1x CD4068 
Misc: diodes, resistors, capacitors, etc.

Infrared Remote Control

This circuit will allow you to turn on any piece of equipment that operates on 115 volts ac. The receiver circuit is based on the Radio Shack infrared receiver module (MOD), part number 276-137. It is also available from some of the other sources listed on my Links page. The MOD accepts a 40khz IR signal that is modulated at 4 khz. When a signal is received the MOD will go low. The sensitivity of the MOD is set by different values for R1 and C1. The values for R1 may need to be as high as 10,000 ohms and for C1 40uf. This will prevent the unit from turning on under normal lighting conditions. You will need to experiment with the values that work best for you. The output of the 4013 chip a flip flop toggles on and off with the reception of a IR pulse. The output of the 4013 turns on the MOC optical coupler which in turn switches on the triac and supplies power to the AC load. 

Copyright 1998 Randy Linscott

FM Beacon Transmitter (88-108 MHz)

This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency.

Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery. Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor. The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then stretched to about a 3/8 inch length and tapped near the center. The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be shifted higher or lower by slightly expanding or compressing the inductor.

A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the inductor varies slightly at the audio rate thus causing the oscillator frequency to change at the audio rate (600 Hz). The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pullup resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types (Radio Shack #276-2062 MPF102) or similar. 

Copyright 2006 Bill Bowden

Signal Tracer

The main part of this circuit is the LM386 amplifier chip. It also uses a transistor input to buffer the input signal and provide extra gain for the LM386. The little unit has helped me out on numerous occasions when trouble shooting any amplifier circuit like a stereo receiver, tv / vcr audio section, radios, cd players and car stereos. 

Copyright 1998 Randy Linscott

Triangle Waveform Generator

The Tri-Waveform Generator can be used for a number of different uses. The one that I use it for is a signal generator to test circuits. The frequency range is 20 to 20khz. and can be adjusted by R1. The duty cycle or the time that the waveform is high and the time that the waveform is low can be adjusted by R4. The purpose of R2 and R3 are to clean up any distortion on the sine wave output. To do this you must hook up the sine wave output to and oscilloscope and adjust R2 & R3 to make the sine wave as accurate as possible. 

Copyright 1999 Randy Linscott

Two Component Metal Detector

The circuit shown must represent the limits of simplicity for a metal detector. It uses a single 4093 quad Schmitt NAND IC and a search coil -- and of course a switch and batteries. A lead from IC1d pin 11 needs to be attached to a MW radio aerial, or should be wrapped around the radio. If the radio has a BFO switch, switch this ON. 

Since an inductor resists rapid changes in voltage (called reactance), any change in the logic level at IC1c pin 10 is delayed during transfer back to input pins 1 and 2. This is further delayed through propagation delays within the 4093 IC. This sets up a rapid oscillation (about 2 MHz), which is picked up by a MW radio. Any change to the inductance of L1 (through the presence of metal) brings about a change to the oscillator frequency. Although 2 MHz is out of range of the Medium Waves, a MW radio will clearly pick up harmonics of this frequency. 

The winding of the coil is by no means critical, and a great deal of latitude is permissible. The prototype used 50 turns of 22 awg/30 swg (0.315 mm) enamelled copper wire, wound on a 4.7"/120 mm former. This was then wrapped in insulation tape. The coil then requires a Faraday shield, which is connected to 0V. A Faraday shield is a wrapping of tin foil around the coil, leaving a small gap so that the foil does not complete the entire circumference of the coil. The Faraday shield is again wrapped in insulation tape. A connection may be made to the Faraday shield by wrapping a bare piece of stiff wire around it before adding the tape. Ideally, the seach coil will be wired to the circuit by means of twin-core or figure-8 microphone cable, with the screen being wired to the Faraday shield. 

The metal detector is set up by tuning the MW radio to pick up a whistle (a harmonic of 2 MHz). Note that not every such harmonic works best, and the most suitable one needs to be found. The presence of metal will then clearly change the tone of the whistle. The metal detector has excellent stability, and it should detect a large coin at 80 to 90 mm, which for a BFO detector is relatively good. It will also discriminate between ferrous and non-ferrous metals through a rise or fall in tone. 

Combinational Conjuring Trick

The simple circuit of Fig.1 emulates a similar conjuring trick which sells for hundreds of Pounds. The trick seems to do the almost-impossible from an electronic point of view, let alone from the point of view of common sense.

It consists of a bank of three on-off switches (S19-S21), which have three switch covers, each of a different colour. These switch a bank of three lightbulbs (LP1-LP3), each of a different colour. The colours of the lightbulbs correspond with the colours of the switch covers.

Now comes the interesting part. The switch covers may be exchanged at will, but still they switch the lightbulbs of corresponding colour. Similarly, the lightbulbs may be exchanged at will, but still they respond to the switches of corresponding colour. On the surface of it, there would seem to be 64 possible connections between switches and lighbulbs, and no possible way that the conjurer can manipulate them all.
However, add some sleight-of-hand, and things become a lot simpler. Each switch cover is symmetrical, in such a way that it looks the same whether facing N, E, or W. Further, each lightbulb is screwed into a circular base, which looks the same whether facing N, E, or W.

Let us consider just one of the switch covers (S19). Three reed switches (S10-S12) are positioned beneath the cover, at positions N, E, and W, and each of these activates a different lightbulb. Any one of the three reed switches may be closed by a single magnet positioned strategically under the switch cover. Depending on the orientation of the switch cover, therefore, the switch will activate any one of the three reed switches, and thus the selected lightbulb.

On discussing this with an accomplished magician, the author was told that this alone would be sufficient for the full effect described - reed switches S1-S9 may be omitted. Nevertheless, the lightbulbs may similarly be surrounded with three reed switches each, which are activated by the orientation of the circular base - a magnet being strategically positioned within it. These reed switches may thus reroute the power to the conjurer's selected lightbulb.

There is just one caveat from an electronic point of view. Carefully consider the voltage and power ratings of the reed switches and on-off switches, to match these with the chosen lightbulbs. Failing this, your trick may demonstrate how none of the switches will activate none of the lightbulbs.

Copyright Rev. Thomas Scarborough
[Contact the author of this article at]

Magnetic Gun

Picured in Figure 1 is a miniature magnetic gun. When optimally tuned, it will propel a small slug about 1.5 metres high, or 2.5 metres horizontally. 

IC1 is a 555 timer in astable mode, sending approx. 10 ms pulses to decade counter IC2. IC2 is continually reset through R3, until pin 15 is taken low through the "Fire" button. IC2 then sequences through outputs Q1 to Q7, to feed power transistors TR1 to TR4, which fire electromagnets L1 to L4 in rapid sequence. 

Transformer T1 secondary is 18 volts 1 amp A.C. When rectified and smoothed, this provides 25.2 V D.C for electromagnets L1 to L4. Resistor R4 drops 12 V to obtain a supply voltage low enough for IC1 and IC2. 

The electromagnets are wound on a 25 cm long, 3 mm dia. copper tube (available at hobby shops). Two "stops" may be cut from tin for each electromagnet, and 500 turns of approx. 30 swg. enamelled copper wire wound between them. The electromagnets should be wound on a base of reversed sellotape, so that one may slide them on the copper tube. The slug (or "bullet") is a 3 cm long piece of 2 mm dia. galvanized wire, which should slide loosely inside the copper tube. 

Most crucial to the effectiveness of the gun are the setting of VR1 and the positions of electromagnets L1 to L4 on the copper tube (the values and measurements shown are merely a guide). Firstly, with L2 to L4 disconnected, VR1 should be tuned and L1 positioned for optimum effectiveness (place a wire inside the tube to feel how far the slug jumps with L1). Then L2 (now connected) should be positioned for optimum effectiveness (the slug will now exit the tube). Repeat with L3 and L4. 

Electromagnets L2 to L4 were each found to substantially increase the range of the gun. In a forthcoming edition of EPE, the author will describe how readers may land a small projectile on Mars. 

Copyright Rev. Thomas Scarborough 
[Contact the author of this article at]

Decimal to BCD Convertor

This circuit will provide an output in Binary Coded Decimal from any of the input switches. The input switches may be expanded to 16 switches, providing a Hexadecimal to BCD conversion. 


When any particular key is pressed, its value will appear in BCD form at the outputs (A, B, C & D). It will remain there until another key is pressed. The 12 keys produce outputs up to "1011". Extended to 16 keys, the circuit will give the full HEX to BCD conversion. 

Memory Module

The above circuit produces an output ONLY while the input switch is depressed. To make a convertor with a latched output, the following modifications are made. Each CMOS 'AND' gate has its free input tied to Vcc, and by the action of R1 through R4 any 'hi input' will therefore cause the output to be latched. The circuit is shown in dec_bcd.png 

When any particular key is pressed, its value will appear in BCD form at the outputs (A, B, C & D). It will remain there until another key is pressed. The 12 keys produce outputs up to "1011". Extended to 16 keys, the circuit will give the full HEX to BCD conversion. 

The LEDs are a visual indication of the value. They are not necessary to the operation of the circuit. If you wish, you may leave them out; together with their associated resistors (R5, R6, R7). 

The circuit works at voltages from 5 to 15 vdc. Please note that A, B, C & D are connected directly to the outputs of the Cmos IC. You will need to regulate the load your application places on these outputs. 


Because the keypad may be used without the memory, the layouts are drawn separately. If you build them both on the same piece of stripboard, isolate them from one another. Cut all of the tracks except for the six that join the keypad terminals to the memory module. Always check carefully that the copper is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. If you don't have the proper track-cutting tool, then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool; there is no need for a drilling machine. 

Board Layout

Board layout for converter without memomry is shown in pad_lay.png. The layout with memory can be found in mem_lay.png 

For clarity, all the components are shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright. Using a socket reduces the chances of damaging the IC; and makes it easier to replace if necessary. The links are bare copper wire on the component side of the board. Two of them need to be fitted before the IC socket. You can make the links from telephone cable:- the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it; and also allow the insulation to slip off.

12 Volt to 120 Volt Inverter

Ever needed a low power 120volt AC power source for your car, van or truck? Well this circuit should do the trick for you. It will supply 15 watts of AC power to a device. It should power lamps, shavers, small stereos and small appliances. If you draw to much power the circuit will shut down all by itself. The output of this circuit is a square wave so there may be some noticeable hum on audio units plugged into it. To reduce some of the hum increase the value of the output capacitor which is at .47uf now. That transistor in the circuit are high power PNP transistors. Radio Shack part number 276-2025 are good ones to use or TIP32. The transformer is a 24 volt 2 amp center tapped secondary Radio Shack part number 273-1512 or equivalent. 

Copyright 2001 Randy Linscott

Colour Sensor

Colour sensor is an interesting project for hobbyists. The circuit can sense eight colours, i.e. blue, green and red (primary colours); magenta, yellow and cyan (secondary colours); and black and white. The circuit is based on the fundamentals of optics and digital electronics. The object whose colour is required to be detected should be placed in front of the system. The light rays reflected from the object will fall on the three convex lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. 

This helps to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2 and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass plates determine which of the LDRs would get triggered. The circuit makes use of only AND gates and gates. 

When a primary coloured light ray falls on the system, the glass plate corresponding to that primary colour will allow that specific light to pass through. But the other two glass plates will not allow any light to pass through. Thus only one LDR will get triggered and the gate output corresponding to that LDR will become logic 1 to indicate which colour it is. 

Similarly, when a secondary coloured light ray falls on the system, the two primary glass plates corresponding to the mixed colour will allow that light to pass through while the remaining one will not allow any light ray to pass through it. As a result two of the LDRs get triggered and the gate output corresponding to these will become logic 1 and indicate which colour it is. 

When all the LDRs get triggered or remain untriggered, you will observe white and black light indications respectively. Following points may be carefully noted :
  1. Potmeters VR1, VR2 and VR3 may be used to adjust the sensitivity of the LDRs.
  2. Common ends of the LDRs should be connected to positive supply.
  3. Use good quality light filters.
The LDR is mounded in a tube, behind a lens, and aimed at the object. The coloured glass filter should be fixed in front of the LDR as shown in the figure. Make three of that kind and fix them in a suitable case. Adjustments are critical and the gadget performance would depend upon its proper fabrication and use of correct filters as well as light conditions.

Beat Balance Metal Detector

A Beat Balance Metal Detector made from discrete components. 

Notes :

Various embodiments of the BB metal detector have been published, and it has been widely described in the press as a new genre. Instead of using a search and a reference oscillator as with BFO, or Tx and Rx coils as with IB, it uses two transmitters or search oscillators with IB-style coil overlap. The frequencies of the two oscillators are then mixed in similar fashion to BFO, to produce an audible heterodyne. On the surface of it, this design would seem to represent little more than a twinned BFO metal detector. However, what makes it different above all else, and significantly increases its range, is that each coil modifies the frequency of the adjacent oscillator through mutual coupling. This introduces the "balance" that is present in an IB metal detector, and boosts sensitivity well beyond that of BFO. Since the concept borrows from both BFO and IB, I have given a nod to each of these by naming it a Beat Balance Metal Detector, or BB for short. Happy hunting! 

Copyright []Rev. Thomas Scarborough

CCO Metal Detector

The metal detector shown here has, in concept, been widely recognised as a new genre. The general concept, of which I have developed three embodiments, is capable in principle of matching the performance of an Induction Balance (IB) metal detector. This is the first embodiment to be released on the Internet (the other two were published in Everyday Practical Electronics and Elektor magazines). When this circuit is correctly set up, an old Victorian penny (30mm diameter) should induce a shift in frequency of at least one tone on the Medium Wave band at 140mm (5½"). 

Apart from using two overlapping coils, the concept is fundamentally different to IB. Unlike IB, its Rx section is an integral part of the oscillator. Further, unlike IB, the design does not require the critical placement of the coils, which should have significant advantages for manufacture. A special characteristic is that sensitivity covers a wide area of the coils, thus making the design well suited to sweeping. It also provides discrimination. Further, while the design uses a beat frequency oscillator (in this case a MW radio), it differs fundamentally from a BFO metal detector. Its performance far outstrips that of BFO -- and further, unlike BFO, it is dependent on the mutual inductance of two coils (BFO, of course, uses only one). 

The circuit should be instantly recognisable as a transformer coupled oscillator (TCO) -- a well known oscillator type. This essentially consists of an amplifier which, by means of a transformer, feeds the output back to the input, thus sustaining oscillation. In the circuit, the TCO transformer is replaced with two search coils, L1 and L2. These have the same action as the transformer in a TCO, L2 being the "transmitter", and L1 the "receiver". On the basis of its similarity with a TCO, I named this metal detector a Coil Coupled Operation (CCO) Metal Detector. The presence of metal induces changes both in the inductance and the coupling of the two coils, thereby inducing a shift in the oscillator frequency. A single stage common emitter amplifier provides 180 degrees phase shift, and the "transformer" provides a further 180 degrees. Base bias is provided by R1, and C1 provides decoupling. Depending on the placement of the coils, the oscillator frequency is around 200kHz. In the absence of a 2N3904 for TR1, a BF494 or BC109C may be pressed into service. 

The two coils are each made of 50 turns 30swg (0.315mm, or 22awg) enamelled copper wire, wound on a 120mm (4¾") diameter former. Each coil has a Faraday shield, which is connected to 0V as shown. This is essentially a tin or aluminium foil screen, which does not quite make the full circuit of the coil -- a gap of 10mm or so is left open. The coils are positioned side by side on the search head, with their beginning (B) wires to the left, and end (E) wires to the right. They are wired to the circuit as shown. The circuit will sustain oscillation with wide variations of coil overlap, and the best degree of overlap may be found through trial and error. The circuit is connected to a Medium Wave radio aerial by means of a screened cable as shown, and a suitable heterodyne is tuned in. 

I present the circuit here merely as a bare bones or experimental idea, and look forward to seeing its further development in the future. To give an indication of what the concept is capable of, the Elektor design obtained nearly one-third better performance. I would welcome comments at my e-mail address However, while I would like to reply to all mail, I cannot guarantee that I shall be able. Happy hunting! 

Voice Scrambler

With this circuit you can modify how your voice sounds by changing the pitch of your voice. This circuit can be connected to a phone and with a duplicate circuit on the end of the phone line, you can have a scrambled voice communication. The way the circuit works is as follows: If we cut the circuit in half at the T2 transformer and include the LM324 on the left side, you will see that the LM324 portion of the circuit is a tone oscillator which shifts the frequency of all input signals to a new higher frequency. When the voice and the tone oscillator mix frequencies the voice is not recognized. The voice signal is then inputted to the second stage which again shifts the voice signal again. I recommend that the first stage be tuned to a frequency that is 100hz lower then the second stage. 

Copyright 1999 Randy Linscott


This circuit was requested by an school teacher. It is a simple intercom that anyone can put together and get to work. It is based on the LM380 IC chip. This chip is able to put out 2 watts of power if it is heat sink properly. The following pins should be grounded and attached to a foil to dissipate the heat. Pins 3,4,5,10,11,12 should all be grounded. The circuit works as follows. Switch 1 is a double pole double throw switch. In one position is the talk position and in the other is the listen position. In the diagram shown the switch is in the talk position for the speaker on the left. Talking into the speaker inputs a signal to the IC chip through the matching transformer T1. The output from the IC chip goes to the speaker on the right. If you put the switch in the other position the speaker on the right is the talking unit and the speaker on the left listens. Volume is controlled by the 1meg ohm pot R1. This circuit is very basic but is a good start for a child or anyone starting new in electronics. 

Copyright 1998 Randy Linscott

Voice Record / Playback Circuit

The ISD1000A is a Direct Analog Storage device which allows you to store 20 seconds worth of voice data on an IC chip which can be play backed anytime. The data stored will stay in memory even if the power is removed. To use the circuit below simple apply power to the circuit, press the record button and hold. Speak clearly into the microphone. You have up to 20 seconds of voice message that you can store. If you talk beyond that time the chip will only store the first 20 seconds. After recording, release the record button. To playback the message, press the playback message and the message you recorded will play back. The microphone is an electret mic and the speaker is a 8 ohm speaker. If you use a 16 ohm speaker then the 10 ohm resistor marked optional, can be eliminated. This circuit can be the basis of many other larger projects. For example it could be part of an alarm circuit which plays back a voice warning when the alarm circuit is triggered. 

Copyright 2000 Randy Linscott

PC / Laptop to MP3

I rustled up this simple electronic circuit “of necessity”. It takes audio straight off the Internet, and puts it on a digital recorder. There are various advantages to this, not least that one can pull the audio off any computer without having to have this or that software in place first. Note that this is a digital-to-analog-to-digital conversion, so the sound will only be as good as one's computer's / recorder's analog circuits.

After wiring up the circuit, set your digital recorder's sensitivity to Lo or Dictation mode. Turn potentiometers VR1 and VR2 back about three-quarters, to avoid overload of the recorder (you might prefer a dual potentiometer here). Turn your computer volume up to about three-quarters, both to minimise hiss and avoid amplifier distortion.

Stereo Jack Plug A goes into your computer's headphone output, and stereo Jack Plug B goes into your digital recorder's microphone input. You will probably require 3 mm jack plugs at both ends. Further adjust VR1/VR2 as required for optimal recording. Happy listening!

You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog (

Decibel Meter

The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section. You can also use a dynamic microphone but I found the speaker was more sensitive. The remaining 3 sections of the LM324 quad op-amp are used as voltage comparators and drive 3 indicator LEDs or incandescents which are spaced about 3dB apart. An additional transistor is needed for incandescent lights as shown with the lower lamp. I used 12 volt, 50mA lamps. Each light represents about a 3dB change in sound level so that when all 3 lights are on, the sound level is about 4 times greater than the level needed to light one lamp. The sensitivity can be adjusted with the 500K pot so that one lamp comes on with a reference sound level. The other two lamps will then indicate about a 2X and 4X increase in volume.

In operation, with no input, the DC voltage at pins 1,2 and 3 of the op-amp will be about 4 volts, and the voltage on the (+) inputs to the 3 comparators (pins 5,10,12) will be about a half volt less due to the 1N914 diode drop. The voltage on the (-) comparator inputs will be around 5.1 and 6.5 which is set by the 560 and 750 ohm resistors.

When an audio signal is present, the 10uF capacitor connected to the diode will charge toward the peak audio level at the op-amp output at pin 1. As the volume increases, the DC voltage on the capacitor and also (+) comparator inputs will increase and the lamp will turn on when the (+) input goes above the (-) input. As the volume decreases, the capacitor discharges through the parallel 100K resistor and the lamps go out. You can change the response time with a larger or smaller capacitor.

This circuit requires a well filtered power source, it will respond to very small changes in supply voltage, so you probably will need a large filter capacitor connected directly to the 330 ohm resistor. I managed to get it to work with an unregulated wall transformer power source, but I had to use 4700uF. It worked well on a regulated supply with only 1000uF.

Copyright 2006: Bill Bowden

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