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 circuit allows you to modify the resolution of a measurement bridge. It was found in the...
The four gates of a 40111 IC are used in thes two-out circuit. The output chages their state in a...
This circuit measures the strength of audio signals. We found it in an American publication from...