9 Ringing and Answering Phones
Andrews circuit design to detect when a ringing phone has been answered actually gave me a fair amount of trouble to understand. If I'm honest I would confess I still don't really understand on a technical level exactly how it works, more that it does "work". But what really didn't help was not realizing that there are two types of transistors, PNP and NPN, and for a long time was using the wrong one - no wonder it wouldn't work! I guess we learn by our mistakes and once I had it figured out I was up and running. Andrew was very helpful during this time patiently answering all my questions. As Andrew explained, and I hopefully I have this right, the way I see it is that the 47 ohm resistor generates a small current before the destination phone is answered, enough to stop the base on the transistor passing current. When the phone is answered the base current flows to earth activating the transistor which in turn activates the photo cell in the optocoupler which...