This circuit was published in an article in Radio Electronics magazine in October 1988. It uses components that are still common today, except for the sensor, which is a special type from that time.
This circuit from Popular Electronics powers a small speaker. The transistor can be of general use...
When the sensor X1 is opened, the relay is energized. When waiting, the current drained is very...
This circuit for measuring bridges was found in the October 1995 issue of Electronics Now...