This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can be adapted to use an LED or audible indicator instead of a light bulb.
Texas Instruments SN74HCS32 Q1 2-input OR gate This Texas Instruments integrated circuit has 4...
The output voltage is adjusted by the trimpot. The power transistor mist be mounted on a heatsink. The 3.9...
This alarm circuit was found in a 2004 documentation. The circuit searches for open NC sensors....