This circuit was published in a 1979 magazine, but it can still be easily assembled as the components are common. The power source must be isolated (with transformer). The circuit provides for a relay of less than 50 mA that can be connected directly to the 555. For less sensitive relays, it is convenient to use an interface with a BC548 transistor. R1 determines the sensitivity, and can have values between 47k and 10 M.
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