24v DC is a very popular voltage used in industrial settings. This hobby circuit below was designed to accept four different 24v DC alarm input signals, which are then used to drive a single low power beeper. The beeper is a magnetic type with its own oscillator/driver. The four diodes form an “OR” gate so any one of the four inputs will cause the beeper to make noise. A CMOS version of the popular 555 timer is used to strobe the beeper on and off at about 1Hz.
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