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 documentation from the 90s. The transistor can be any general-purpose FET...
Found in an Elektor from the 90's, this double door-bell drive circuit switches a relay by...
Using a CA3130, this circuit generates signals from approximately 550 kHz to 1.6 MHz using a...