The diode D1 ensures that the output from pin#11 of IC2 locks the counting of IC1 by providing a feed back latch signal at its pin #11.After the set timing lapses, pin#11 of IC2 goes high activating the transistor/relay stage and the subsequent load connected with the relay contacts.As long as the counting progresses, pin #11 of IC2 remains at logic low, such that the relay driver is held switched OFF.When power is switched ON, capacitor C2 makes sure that the reset pins of both the ICs are appropriately reset, so that the ICs begin counting from zero rather than from some irrelevant intermediate figure.Similarly if the time generated at pin #3 of IC1 is 6 minutes, would mean a high output from pin#11 of IC1 after 60 minutes or 1 hour.It means if the the time interval generated by IC1s pin #3 is 10 hours, the time generated at pin #11 of IC2 would be 10*10 = 100 hours. The next stage consists of decade counter IC 4017 which does nothing but increase the time interval obtained from IC1 to ten folds.The higher the setting of the above components the longer the period at pin #3. ![]() ![]()
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