High voltage ringer rotary phone to VoIP phone

21 Feb 2018

The next thing I had here on my parade of projects was going to be a classic! My dad had this old rotary ringer telephone in the shop here. It reminded him of the good old days of how telephones were and was using it here for a little while until we switched over to a VoIP telephone system in the office and shop and basically the voltage form the phone lines would surely not make the ringer nor the phone work at all!

So naturally I got my hands on this phone when we took off the wall and I suggested to him, why not turn this into a VoIP phone to work with our current service? The idea was amusing but it was pointed out to me that the ringer bells need at least 80 volts and some high current in order work, and the originality that would be that phone would be strange without. There was a point there, I didn't want to just use a synthesized sound or an MP3 to emulate the ringer at all, there had to be an application where someone must have gotten ringer bells to work with a low voltage and bumped it up to make some high voltage square waves to ring some bells, luckily such an application existed!

https://www.sparkfun.com/tutorials/50

To paraphrase a little, two big DC voltages are going to be created from 3.7 volts, each typically from a LiPo battery or I hope in my case, the supply off the GPIO header pins that have the 3.7 volts there. Once the two big DC voltages are made, they are going to be run through an H-bridge configuration, in this case the SN754410; one Vcc as the supply and the other one, Vcc2 is used for the signals on 1A and 2A which will make a 0 to +V square wave at approximately 22Hz and 180 degrees out of phase with each other. The larger DC voltages are achieved by using a boost converter, in this case a MC34063A which will be doing this for us.

I had previously purchased the parts and had this project on standby for awhile now. Seeing I had a working schematic and knew what to aim for here, I did go ahead and construct the working prototype to see if I could get mine to ring here.

The circuit has been built, as above, but not preliminary testing has been done yet. I will likely make more updates here when I have made sure that all connections are correct.

Raspberry Pi3 VoIP telephony software

Seeing the release of the Pi-zero, thing have certainly now become more compact to fit the brains of the operation in the rotary telephone casing more easier. However, while I would have enough I/O for the dial tone, rotary pulse conversion and so on, I do not know if there will be enough current to source the bells for when they do ring, I do know think there will be enough for a Pi zero to pass through without getting loaded down. I was and probably still going to use a RPi 3 just because that what I can find current on the market right now, still, I wonder if that is going to have current limitation too. I do know there are beefy LiPO batteries out there, if not NiMH batteries too, but of course I have space limitation as well too, so more things to consider here.

Either which way, I am considering a RPi, but importantly enough a suite of software that will emulate (I guess) off the GPIO header the dial tone, convert the rotary pluses, busy signal and so on. When I was actively looking I had found some interesting solutions via Github that some others have done, but over the months I had lost track of what I had intended to use. It looked as if I need three I/O to make the rotary phone work and totally bypass the block transformer.

At the moment, now, if anyone does see any use Githubs for VoIP to rotary phones,please let me know here. As well, I am going to start looking again. I did find one Github that was pretty straightforward as to configuring the software and which pins go where in use the VoIP service one would subscribe or sing up to use. Seeing there is already a VoIP service here, I imagine I would need the VoIP gateway address and specific ports I would need to configure for the phone in order for to be used in our system. I know this is a rather incomplete blog here, however more will be added later one on, and seeing things have calmed down a bit, like I mentioned, I'll be able to research this a bit more and find what software will work here.