Circuit : Andy Collinson
Description:
A switched timer with equal make and equal space periods
timing adjustable from over 6 minutes to 38 minutes.
Notes:
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
as:-
T = 1.4 R1 C1
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.
