This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
The circuit is quite simple, but its choice is due to the suggested application, which shows that...
Found elsewhere in this section, this circuit is from the April 1971 Radio Electronics magazine. Note...
We found this circuit in a Philips documentation from 1965. The circuit uses a common LDR sensor...