Two-tone solar buzzer (CB20493E)

This circuit was found in a 1991 publication. The transistor can be a general-purpose type.

Microphone Preamplifier (CB26214E)
We found this circuit in Radio Electronics Special Projects from 1992. The circuit turns on the...
Null Detector (CB11550E)
We found this circuit in the Texas Instruments' Linear Circuits Applications. It uses an...
RF proximity sensor (CB8765E)
This capacitive sensor that operates with a high frequency oscillator was found in Radio...