The selected circuit was found in GE's 1977 semiconductor manual. 3N84 is a controlled silicon switch and SCRs support equivalents. The transistor can be a BC558, depending on the supply voltage.
The LED is optically coupled to the LDR1 sensor to provide negative feedback to the circuit and...
The idea of this circuit is a resistance meter or continuity test with a common microammeter or...
This selectable constant current source was found in an English publication in 1983. The...