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Tuesday, January 17, 2012

Model Railway Level Crossing Lights

A magnet under the train operates reed switches positioned on the track. The trigger reed switch starts the sequence by switching on the amber light, a few seconds later the two red lights start to flash. When the train has passed the level crossing it operates the cancel reed switch which switches off the lights until the next train arrives.

There is a PCB pattern for this project, but if you don't have facilities to make PCBs you can build this project on stripboard instead. Please see the New Railway Modellers website for a stripboard layout and advice on making model lamps and barriers.

This project uses a 555 monostable circuit to switch on the amber LED for a few seconds. When this switches off it triggers a 555 bistable circuit which switches on a 555 astable circuit to flash the red LEDs.

Parts Required
    Level crossing lights
  • resistors: 680 ×3, 1k ×3, 33k, 47k, 82k, 270k
  • capacitors: 0.1µF ×3, 10µF radial ×2
  • red LED (3mm best) ×2
  • amber* (or yellow) LED (3mm best)
    * some amber LEDs are too orange to look correct, yellow may be better.
  • 555 timer IC ×3
  • 8-pin DIL socket for IC ×3
  • on/off switch
  • battery clip
  • reed switch ×2
  • miniature magnet - each locomotive needs one
  • printed circuit board (PCB) - pattern given below
PCB component layout

Track connections

The reed switches can be held in place between the rails with a small piece of blu tac. Each locomotive will need a miniature magnet glued to its underside, test first with blu tac, then use superglue.

Circuit diagram

PCB copper track pattern

                                                       PCB track pattern model railway level crossing lights

Stripboard layout

If you don't have facilities to make your own PCB you can build this project on stripboard. Please see the New Railway Modellers website for a stripboard layout as well as advice on making model lamps and barriers.

Copyright 2011: John Hewes

Dice Project

Press the push switch to 'throw' the dice: this makes the circuit rapidly cycle through the dice numbers so that an effectively random dice number is displayed by the LEDs when the push switch is released.

Drill seven 5mm holes in a dice pattern to mount the LEDs on a panel such as a plastic box lid or sheet of thin plywood. They should be a tight fit but a little glue can be applied from the underside if necessary. 

The 555 astable circuit provides clock pulses at about 5kHz for the 4017 counter which has ten outputs (Q0 to Q9). Each output becomes high in turn as the clock pulses are received. Only six counts (Q0-Q5) are needed so Q6 is connected to reset. Appropriate outputs are combined with diodes to supply the LEDs: 

                              Dice project sequence

For example Q1, Q3 and Q5 are combined for LED A. The dice sequence has been started at 2 so the ÷10 output can be used for LEDs B1 and B2, this saves diodes and simplifies the circuit. Pressing the push switch makes the disable input low so that counting occurs. 

Parts Required
  • resistors: 330 ×3, 470, 10k ×3
  • capacitors: 0.01µF, 0.1µF
  • diodes: 1N4148 ×6
  • LEDs: red 5mm diameter ×7
  • 555 timer IC, such as NE555
  • 4017 counter IC
  • DIL sockets for ICs: 8-pin, 16-pin
  • on/off switch
  • push switch
  • battery clip for 9V PP3
  • stripboard: 20 rows × 22 holes

Stripboard Layout

Circuit diagram

Copyright 2007: John Hewes 

2 km FM transmitter

With a matching antenna, the FM transmitter circuit shown here can transmit signals up to a range of 2 kilo meters. The transistor Q1 and Q2 forms a classic high sensitive preamplifier stage. The audio signal to be transmitted is coupled to the base of Q1 through capacitor C2. R1, R3, R4, R6, R5 and R9 are the biasing resistors for the preamplifier stage comprising of Q1 and Q2. Transistor Q3 performs the collective job of oscillator, mixer and final power amplifier.C9 and L1 forms the tank circuit which is essential for creating oscillations. Inductor L2 couples the FM signal to the antenna.

24V lead acid battery charger circuit

This lead acid battery charger circuit is designed in response to a request from Mr.Devdas .C. His requirement was a circuit to charge two 12V/7AH lead acid batteries in series.Anyway he did not mentioned the no of cells per each 12V battery. The no of cells/battery is also an important parameter and here I designed the circuit assuming each 12V battery containing 6 cells. When two batteries are connected in series, the voltage will add up and the current capacity remains same. So two 12V/7AH batteries connected in series can be considered as a 24V/7AH battery.

The circuit given here is a current limited lead acid battery charger built around the famous variable voltage regulator IC LM 317. The charging current depends on the value of resistor R2 and here it is set to be 700mA. Resistor R3 and POT R4 determines the charging voltage. Transformer T1 steps down the mains voltage and bridge D1 does the job of rectification. C1 is the filter capacitor. Diode D1 prevents the reverse flow of current from the battery when charger is switched OFF or when mains power is not available.

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