This bidirectional circuit was found in an American documentation from the 80's. The LEDs are common red, and the resistor is calculated according to the formulas next to the diagram.
This proximity sensor circuit is from an 80's documentation. The used integrated circuits are...
This current boosting circuit uses an op-amp and 3 transistors. The current depends on Q1. The...
In the figure we have the circuit of a clamp-type microammeter described in the Radio Electronics...