Based on Radio Shack 276-137, this circuit was found in the December 1995 issue of Electronics Now magazine. The circuit drives an LED but could use a relay instead.
This detector with ultra-low deviation was found in an American documentation from the 90s....
This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even...
This is the configuration of this type of oscillator given in an article in Electronics Now...