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EMOTE_TELEPHONE_RINGER

2014-11-26 01:10  
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EMOTE_TELEPHONE_RINGER
When there is no ring signal, the phone line's on-hook voltage (about 50 V) keeps the triac (Radio Shack 276-1001 or equivalent) from switching on, so the optocoupler doesn't conduct. When the phone rings, terminal 4 of the optocoupler feeds pulses through R3 and R4 to the gate of SCR1. The activated SCR connects the bell to the 12-V supply. Pulses are about 400 Hz, so the bell might sound a bit rough. The opening of the bell's breaker points cuts the thyristor's holding current to stop it from conducting when the gate signal stops. A 12-V, 2-A power supply operates the unit. Adjust R3 high enough so that the SCR is not triggered between rings, but low enough to trigger it when the phone is ringing.