Operating at just 1.35 V this circuit was found in a 1969 Electronic Design. The circuit makes use of transistors of the time but can be implemented with current equivalents.
This circuit is from a Popular Electronics from the 90s. The integrated circuit admits equivalents...
This very interesting project was obtained in a 1980 publication. It consists of a device that is...
This circuit beeps when the sensor is triggered by the opening of the freezer door or other...