This is the receiver for the previous transmitter. The microphone is of the high impedance ultrasonic type. The circuit activates a relay. The operational amplifier supports equivalent.
We found this simple shortwave receiver in an Electronics Now from the 1990s. Details of the coil...
We found this circuit in a 90's European schematic. The circuit can be implemented with almost any...
In figure 1 we have an application for NLAS5113, a new device from on Semiconductor...