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 circuit was found in an old Electronic Design. It makes use of sidacs and operational not very...
This 2009 circuit is a bilateral call warning. Easy to build, using light communication between two...
This circuit for shunt motors was found in an American 1980s documentation. The SCR should be rated...