This circuit was found in the 1992 Radio Electronics Handbook. It is used to test VCR recording heads. It can also be used for other purposes.
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...