This timer charges the C1 capacitor with constant current, thus determining the SCR triggering time that locks the relay. The relay is 6 V to compensate for the 2 V voltage drop in the SCR. Larger capacitors for C1 can be used. The time with the indicated values is in the order of a few tens of seconds. S2 is used to discharge the capacitor after a delay. The circuit is from the book Fun projects for the Experimenter by Newton C. Braga.
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