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Transistor Soft Musical telephone ringer circuit

2016-02-02 21:13  
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The normal telephone bell, at times (specially during night when one does not want to be disturbed), appears to be quite irritating.The incoming ring is detected by transistor T1 and components wired around it. In absence of ringing voltage, transistor T1 is cut off while transistor T2 is forward biased as resistor R2 is returned to the positive supply rails. As a result collector of transistor T2 is at near-ground potential and hence IC1 (UM66) is off. Also capacitor C2 is charged to a slightly positive potential. The circuit shown here converts the loud sounding bell into a soft and pleasing musical tone.The heart of the circuit is IC1, General Instrument's AY-3-1350 melody-synthesizer IC. IC2 is a TCM1512  telephone ring detector IC that is powered by the telephone line. The circuit's operation begins when IC2  senses a ring pulse on the telephone line. The detector (internally) rectifies the ring signal and then outputs a voltage to relay RY1 (an SPST reed-type relay with 5 volt contacts), causing its contacts to close. That pulls pin 12 (the ON/OFF control) of IC1 low (logic `0`), causing it to output a signal the selected tune to transistor amplifier Q2.The amplified signal is then fed to the speaker. The melody continues to play either until the tune is finished (at which time IC1 returns to the standby mode), or until someone takes the phone off the hook. Taking the phone off the hook discontinues the ring pulses to IC2, which opens RY1. When the relay contacts open, pin 12 of IC1 goes high, returning the circuit to the standby mode to wait for the next incoming phone call.The soft musical note into the loudspeaker sounds in synchronism with the ringing signal. When handset is lifted off the cradle, the ringing voltage is no more available and hence the soft musical note switches off.

 

Musical telephone ringer