by pebe » Thu Aug 22, 2013 8:55 pm
Hi beholt,
Here's the explanation.
IC1 is triggered by taking the TR pin 2 below 4V. After triggering, TR has to be taken high again or else it will override the timed o/p pulse and keep O/P high. Hence the arrangement R1,C1, R2.
Normally , both sides of C1 are at 12V. Closing S1 takes the LH side of C1 down to V. Because C1 cannot change its charge instantly, its RH side will also go down to 0V and triggering the TR pin. R2 will start to charge up the RH side of C1 to 12V so the timer will function normally, even if S1 is held closed. When S1 is opened both sides of C1 will again be at 12V ready for the next trigger.
When IC1 o/p is high the motor is turned on, and both sides of C3 are at 12V. At the end of the timed period, IC1 o/p goes low to turn off the motor. The RH side of C3 also goes low and triggers IC2. Its o/p pin goes high for 3minutes. During that period, if S1 was closed again then D1 would conduct and stop TR pin of IC1 from being triggered. That was to satisfy your requirement that once the timed 'on' period for the motor had ended and the motor had switched off it must not be possible to turn it on again for 3 minutes.
I hope that all makes sense.