Last updated: November 29, 2007

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  LED Traffic Signals

A request that keeps popping up in modelling circles is that of a circuit for traffic signals. While several circuits exist, most are for only one direction, while others are unprototypical in that they abruptly change from red to green without the brief red-red overlap used in modern systems.

When I set out to design a system myself, I wanted it to handle two directions and also include the red-red overlap. I love playing with digital chips and came up with the following TTL-based circuit. I am sure there may be simpler, more elegant solutions but this one really works very well and uses chips and discrete components that are very easy to work with.

For a larger (800K) version of the above drawing, suitable for full-page landscape printing, click here.

Parts List:

1 Linear IC, timer, 555
1 Digital IC, binary counter, 74LS93
1 Digital IC, 4-to-16 line decoder/demultiplexer, 74154
1 Potentiometer, 1/4 watt, multi-turn, 1M ohm
2 Resistor, 1/4 watt, 10K ohm
5 Resistor, 1/4 watt, 2.2K ohm
5 Resistor, 1/4 watt, 1K ohm
2 Resistor, 1/4 watt, 180 ohm
1 Resistor, 1/4 watt, 120 ohm
1 Capacitor, electrolytic, 10uF, 16V
1 Capacitor, 0.1 uF
18 Diode, small signal, 1N4148 (or 1N914)
5 Transistor, PNP, 2N3906
- LEDs, red, yellow (amber), and green - quantities as needed

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