Photo Event Timer

S2 is used to initiate timing. A light-to-dark or dark-to-light transition stops this timer, depending on the setting of S5. S3 offers a direct operating mode, rather than through the latch. IC3 and IC4 supply 0.1- or 1-second timing pulses. IC7 drives a time display counter, a 12-Vdc unit that draws less than 200mA.

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Photo Event Timer - Circuitos de Electronica