![]() ![]() Since the wires between the phones can be quite long and possibly subject to interference, the part to detect on/off hook considers a reading valid only if it has been stable for 50mS or more. The software uses a simple state machine model and sleeps when the phone is not in use. However, it also runs off a standard (two amp, not travel) cell phone charger. It consumes around 100mA in use and 300uA but now only about 1uA when sleeping so there is still some scope for optimisation. ![]() I was able compromise a bit by reducing the bell voltage to 24 volts and increasing the frequency to 50Hz to still provide an adequate ring, but you may have to experiment here. This all means that the boost converter is running when phones are being used, not just when these are ringing. It is also so that 5 volts is not sufficient (actually depending on the phone) to keep the voice path open. The boost converters are not particularly efficient when stepping up to the multiples of the supply voltage necessary to operate the bell. One challenge is getting a reasonable power consumption for battery operation. I have already published a bell ringer circuit and the development here is an extension of that design. Some discrete transistors are used to detect on/off hook. The MCU is an ATtiny 84 to control it all. ![]() It is based on an boost converter and and a L293D H-bridge chip to power the bell. There is an interlock to prevent unwanted ringing if, after a conversation, both phones are not replaced on hook simultaneously. When both phones are back on hook, the system sleeps. When the other phone is picked up, or the first phone hangs up, the ringing stops and the voice channel is open. Lifting one receiver will cause the other phone to ring. I wanted to do better and use the original bell and also make no internal modifications to the telephones, which are anyway now becoming collectors' items. ![]()
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