This circuit uses a 555 (according to the diagram) to generate 50 ms tone bursts at a frequency of 1.5 kHz. The circuit is from an 80's Electronic Design.
We found this circuit in an English documentation from the 60s. The integrated circuits are the...
This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and...
This amplifier circuit applies its signal to an ordinary audio amplifier to pick up signals from a...