Periodic Timer Timing

Posted on Feb 7, 2014

This timer circuit is similar to the 5 to 30 minute timer except that when switch S1 is closed, the on/off action of the circuit will continue indefinitely until S1 is opened again. A 7555 time and low leakage type capacitor for C1 must be used. The 6 way rotary switch S3 adds extra resistance in series to the timing chain with each rotation, mini

Periodic Timer Timing
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mum resistance point "a" maximum point "f". The 7555 is wired as an equal mark/space ratio oscillator, the timing resistor chain R1 to R6, being connected back to the output of the timer at pin 3. The output pulse duration is defined as:- This gives on and off times of about 379 seconds for position "a" of S3 (just over 6 minutes), to about 38 minutes at point "f". The times may of course be varied by altering R1 to R6 or C1.

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