This circuit that we found in a 1990 documentation uses an LED as a sensor. The operational amplifier is equivalent to the 741 and the trigger adjustment is made in the potentiometer.
This transmitter has a range of a few hundred meters depending on site conditions. The coil...
The circuit shown below was obtained from the National Semiconductor Linear Applications Handbook...
This circuit was found in a Popular Electronics from the 90s. The circuit has 4 frequency ranges...