This circuit is from an old American documentation from the 70s. Transistor 2N3904 can be replaced by BC548. The relay must have 100 mA of maximum current.
This circuit is from an old manual of operational amplifiers. The setting can be adapted to work...
This 2009 circuit is a bilateral call warning. Easy to build, using light communication between two...
Found in Popular Electronics from the 90s, this circuit generates a 1 kHz signal. The coil is from an...