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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Tricks with
a 4017 chaser

Click for larger image

This circuit uses two SCRs in conjunction with a 4017 decade counter/divider so that its 10-LED output cycles only once after each press of a button. This could be useful for door chimes, an eye-catching LED signal, a countdown timer or a time-out signal for a quiz game.

Initially, both SCRs are off and the circuit does not operate. On pressing switch S1, SCR1 is turned on to apply power to the circuit comprising 555 timer IC1 and a 4017 counter (IC2). The 555 timer sends pulses to the 4017 counter which turns on each of its 10 LEDs in turn. The auto reset components connected to pin 15 of IC2 ensure that its output 0 (pin 3) goes high first.

After a pulse has sent output 9 (pin 11) high, then next pulse sends pin 3 of IC2 high and this sends a trigger pulse to the gate of SCR2 via a 10nF capacitor. SCR2 turns on and commutates SCR1 off, by removing voltage from its anode which stops the entire circuit.

The first press of S1 does not trigger SCR2 because pin 3 of IC2 is already high (by virtue of the auto reset) and therefore no trigger pulse can be delivered (to SCR2).

Pressing switch S1 will then restart the cycle. If you want to alter the speed of operation, just increase or reduce the 1μF at pin 2 of IC1.

To provide repetition of the 10-LED display just connect Circuit 2 to the gate of SCR1. This is another 555 astable timer (IC3) which triggers SCR1 at intervals determined by the timing components connected to pins 7, 6 & 2 of IC3.

A. J. Lowe,
Bardon, Qld. ($40)

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