Found in a Popular Electronics from the 90s, it applies to the old answering machines that used tapes. It emits a warning beep on the drive.
This circuit was found in a 1987 publication, but it can still be easily assembled today as the...
This circuit uses a common diode as sensor. The output goes from 0 to 10 V with the temperature change in...
This circuit, from an old publication, can also use modern transistors. Note that the unit is...