This circuit was published in a 1979 magazine, but it can still be easily assembled, as the components are common. The circuit generates an audio tone when one of the system's sensors (reed) is activated by a magnet. Other types of sensors can be used. The signal is applied to a loudspeaker or transducer and the power can be supplied with voltages from 5 to 12 V. The diodes can be 1N4148.
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