Interactive Toy Traffic Lights

Posted on Feb 4, 2014

This toy traffic signal uses a single low-cost hex Schmitt-trigger inverter IC (IC1a-IC1f) to directly drive three coloured LEDs (red, green and amber). At switch-on, the circuit lights the red signal for 30s, then shows green for 6s, then amber for 3s. It then repeats the sequence. Interaction is provided by pushbutton S1 which abbreviates the re

Interactive Toy Traffic Lights
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d period to a further 3s only, if it is pressed while the red signal is showing. Sequencing of the three LEDs is controlled by inverters IC1c, IC1d & IC1e, while the electrolytic capacitors at the inverter outputs and their associated 2. 7MO resistors determine how long each LED stays on. Diodes D3, D4 & D5 discharge the timing capacitors for the next two LEDs in the sequence while the current LED is on. Note also the 10kO resistor at the input of each inverter. These protect the inverter inputs from being damaged by the negative voltage produced when the previous output goes low while its timing capacitor is fully charged. The circuit is forced into the red state at switch-on by IC1f and its associated circuitry. What happens is that IC1f briefly pulls the negative end of the amber timing capacitor (C4) low via D6 at switch-on. As a result, IC1e`s output goes high and turns the amber LED (LED3) off. The red timing capacitor (C5) is in a discharged state at power-up because D5 and the 10kO resistor at the output of IC1e discharge it when the power is off. As a result, when IC1e`s output goes high, IC1c`s output goes low and turns LED1 (red) on. This also pulls the input of IC1d low, so IC1d`s output goes high, turning the green LED off. The amber timing capacitor (C4) at the output of IC1d charges rapidly when it receives the negative pulse from IC1f. That`s because its positive end is high when the green LED is off and the...

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