Posted on Sep 7, 2012

This accessory connects to phone and will activate an attention-getting 120-V bell whenever a ringing voltage appears on the ph

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one line. The four-terminal transducer has a neon bulb close to a photocell, which are both enclosed in a light-tight tube. When the lamp is off, the cell is dark and its resistance is very high. When a ringing voltage appears on the phone line, the neon bulb glows brightly and illuminates the photocell whose resistance then drops to about 1000 . The photocell is in the gate circuit of a triac that will turn on whenever the cell resistance drops. The triac is connected across the switching terminals of the isolation relay. Thus, the relay closes and applies 120 V to the output socket whenever the triac is on.

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