We found this circuit in the 1964 RCA transistors manual. The sensor may be a common LDR and the transistor BC558. The relay has an operating voltage according to the power supply.
This configuration shows the simple way to use a varicap diode to tune an LC circuit. Capacitor C1 must be...
This circuit consists of a touch switch with optical isolation, using CMOS integrated circuits. The...
Within a range of values, this circuit converts input voltages into corresponding currents. The...