We found this circuit in an old documentation, but it is still current. General purpose transistors can be used, and the SCR can be the TIC106. The bell is a transducer with an oscillator (buzzer).
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...