The signal from this transmitter is injected into a cable that you want to follow along a pipeline. Monitoring is done using an ordinary AM receiver out of season.
This circuit is from a Popular Electronics from the 90s. The circuit has a tone frequency...
This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose...
This circuit is based on a common cathode 7-segment display. The circuit is based on CMOS...