High isolation telephone ringer

  
The diode rectifies the ringing signal to supply the operating power to the audio relaxation oscillator made up of LI, L2, Rl, R2, and C. Moreover, L2 together with Ql acts as an opto-isolator, totally isolating the telephone line from the rest of the circuit. The oscillator audio frequency is optically coupled to the photo-Darlington which drives Q2 and thus the speaker.
High isolation telephone ringer  - schematic

The 10 µf capacitor is not large enough to smooth the ringing ripple completely. This results in frequency modulation of the audio oscillator giving it an attention-getting warble.




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