Posted on Sep 17, 2012

When the phone rings, the ring signal from the telephone company lights a neon lamp within a CLM3120 optocoupler. That causes a drop in the resistance of the CdS cell output of the device, turning on transistor Ql.

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When Ql turns on, relay Kl is energized. The circuit should be connected in series with the lamp that is to be activated.

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