Article : Andy Collinson
A switched timer with equal make and equal space periods timing adjustable from over 6 minutes to 38 minutes.
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,
minimum 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
T = 1.4 R1 C1
This gives on and off times of about 379 seconds for postion "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.
Below is my test circuit on my breadboard. The switch assembley is not present for clarity and a single 100k resistor and 33u capacitor (next to the 100nF ceramic)
were used for test purposes.
Please note that electrolytic capacitors have a tolerance of +/- 20%. This means that the nominal value can be higher or lower by 20%. For example, 33uF capacitor may be as high as 39.6uF or as low as 26.4uF. For precise timing periods,
substitute a fixed resistor in series with a preset resistor, or buy high tolerance 1% capacitors and resistors.