Posted on Dec 15, 2012

The two neon bulbs will light when more than 100 V is across the ringing circuit. The bulbs provide line isolation between the unit and the telephone line. Finally, they act as a voltage divider for the bridge rectifier made up of D 1 through D4. That voltage divider creates a positive voltage that is then applied through D5, is filtered by R2, R3, and C2, and causes Q1 and Q2 to conduct.

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When that happens, triac TR1 is fired through the optical coupler IC1; this turns on the triac, which applies 110 Vac to the load.

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