Absolutely totally insane project! Someone decided he needed a new doorbell and chose a vintage outdoor telephone bell from Dutch PTT. Thing is built like a tank. However, one of its coils had gone phut and had to be rewound. This was an effort in itself, see below. Once I got it back together again I needed to know if the supplied doorbell transformer would provide enough juice to actuate the ringer. No, it didn't. Some quick testing using a signal generator showed that the ringer wouldn't even work at 50 Hz. Since ringing voltage in a POTS (Plain Old Telephony Service) system is 200 Vpp at 25 Hz I decided to design a dedicated generator powered by said transformer. PoC (Proof of Concept) testing showed that the bell would work at a lower frequency. I plucked some values out of the air (yes, had to change them later on, see schematic) and made this 40106 based oscillator driving a push-pull totem pole power stage. After fixing the "high" state (not enough base current for the emitter follower), this sucker develops some 80 Vpp across a 1k load. This is enough to make the ringer work: movie
In the end the MPSA42 transistors were a bit overkill. Regular BC547Bs would have done the job just fine. Same for the diodes. But if it's worth doing, it's worth overdoing!
28 April 2015