This circuit is based on the light comparison of two LDR sensors. We found it in a 90s Popular Electronics. The transistor must be according to the indicator or load being driven.
This proximity relay circuit was found in the July 1990 Radio Electronics magazine. The supply...
This circuit is from an old operational amplifiers manual. However, the setting applies to modern...
Using an operational amplifier, the figure shows how to convert the waveform to low frequency...