This circuit was found in the August 1997 Electronics Now magazine. The circuit uses well-known operational codes and generates signals up to 20 kHz.
Indicated in Maxim's documentation from the 90's, this circuit has a gain of 40 dB and an output...
I found this simple circuit in an English magazine from 1984. The circuit can be assembled easily...
This is the receiver circuit for the transmitter of the previous circuit using an integrated...