Found in a 1980s telecommunications documentation, the circuit presented acts as a product detector. The modulator or mixer in the square may be that of the previous circuit.
This circuit is from a 1982 documentation. Here we have an adaptation of the circuit already...
The integrated circuit used in this small transmitter is not very common today. The coil has its...
Like the previous circuits, this one uses a small speaker as a microphone and has two...