This circuit was found in Radio Electronics from the 90s. The sensor is an oscillator whose frequency depends on humidity, an uncommon component nowadays.
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...