The time obtained in this circuit, from a 1978 publication, depends on R3 and C. R3 can have values from 100k to 10 M while C can have values between 1uF and 4700uF. Times of more than 1 hour can be achieved with this circuit. The output of the 741 can be connected to a step with a transistor to activate a relay. Other operational can be used, including comparators.
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