Written in an article in Radio Electronics magazine in April 1987, this circuit triggers an alarm via a relay. Details of the sensor are given in the article.
In the figure we have the way to use a stepper motor as an encoder. Operational amplifiers can be of...
Placed inside the refrigerator, this circuit triggers an oscillator when the part is opened...
We found this circuit in an old documentation from the 70s, but it can be assembled with more...