This simple bridge circuit can be used for impedance or resistance measurement using a high impedance earphone as an indicator. The circuit is from an American 80's documentation.
The sensors in this circuit that triggers when the temperature goes out of a certain range are...
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...
This 10 kHz filter is based on a 741 operational amplifier. The circuit is from Electronics Now...