The existence of field effect transistors at the input of the integrated circuit (operational amplifier) of this application allows its use in timers of long periods. In fact, the time constant obtained with 144 M ohms resistors (7 resistors of 22 M ohms in series) together with the 10 uF polycarbonate capacitor, leads us to a 1000 second time delay. The integrated circuit must have symmetrical power and the clear switch serves to discharge the capacitor at the beginning of a new delay.
