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PHONE_MESSAGE_FLASHER

2017-02-05 15:59  
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PHONE_MESSAGE_FLASHER
This circuit flashes an LED to indicate that your phone rang during your absence. A differential amplifier with hysteresis (Q1, Q2, and Q3) detects high line voltage(ringing), which turns on Q4, mulitvibrator Q5/Q6, and flashes the LED via Q7. Q1 and Q2 remain on until the phone-line voltage drops to less than 9 V, which indicates an off-hook condition.