Found in an April 1977 Radio Electronics issue, this circuit uses the well-known XR2206. The C1 values for the various frequency bands are given next to the diagram.
This circuit applies to an old dial phone by detecting light pulses in the dial. The circuit is...
This circuit conditions an audio signal, acting as a filter, control, and reaper of signals. The...
Found in old documentation, this timer uses a sensitive relay and a 2N2646. The time is given by...