This circuit is for traditional old-style telephone lines found in an American 1980s documentation. Components are common and the optical coupler can be replaced with an equivalent.
The duty cycle of a Schmitt Oscillator can be programmed by R1 and R2 in this circuit. The...
This circuit is from a Popular Electronics from 1992. It uses two SCRs and can be triggered by...
This simple SCR alarm circuit was found in an old Poptronics, but can be easily assembled as all...