We found this circuit in the Texas Instruments optoelectronics manual. Field effect transistors can be BF245 and bipolar BC548. The sensor is a photodiode in the photo-generator mode.
This circuit uses a triac to detect the level of a reservoir, and a sensitive SCR connected to the...
This Signetics circuit uses a double 555 (556) to generate tone bursts at frequencies determined by...
This circuit provides an extremely stable, temperature-compensated reference current. The circuit is...