This tone recognizer circuit that relies on R5 and C1 was found in the December 1994 issue of Electronics Now. The circuit operates on voltages from 6 to 12 V typically.
When the light that falls on the LDR is cut off, this 1992 circuit activates a relay for a set...
We found this circuit in a 1979 French publication, but it can still be assembled today with...
We found this circuit in a Radio Electronics from May 1966. Its purpose is to generate pulses...