crystal timebase for synchronous clocks

The pecularity of this rev . counter is that it responds to differences in luminous intensity. Consequently, if this circuit is to be used as a rev. counter, the motor shaft must be provided with a vane which periodically intercepts the light incident on the light sensor. Little can be said about the choice of light sensitive element, because they come in numerous types. Instead of a photo diode, photo transistors or photo darlingtons can be used. In practically all cases it will be necessary to experiment with the value of R I. A first setting can be obtained by applying half the supply voltage to point A by means of RI .