Seemingly simple problem - make a phone ring on stage for a local theatre play. We used to have a magic black box which ran off 12v and would make the phone ring when you pressed a button - and would stop when the phone was picked up. But that was lent out to someone and never came back.
Some googling later and I find what the requirements are - a 20Hz 90v AC waveform, overlaid on a -48v DC signal. Sounds simple enough, but then there is also controlling the ring pulses to do: ring, ... ring, ......... ring, ... ring, ..........
Some other people have tackled it, from using a stereo with 180 degree out of phase sine waves to drive the phone, to IC controlled sine generators with transistor switching to control the on/off phases. But I'm not great with building circuits, so fewer components is better for me. Hence why I got an Arduino, as it can be hooked up to a few components to do a lot of different things, and if I want to change certain characteristics, it's often possible in code (eg. style of the phone ringing - frequency, length, gap in between - so much easier than changing resistors and capacitors).
The current state: After more research, I believe if I get the Arduino to output a PWM signal representing a 20Hz sine wave, amplify it with a MOSFET, filter the amped output with a first order low pass filter, it will produce a DC biased (50%) AC waveform. The bit I'm not clear about is whether I need to AC couple that before running it through the step-up transformer to avoid saturation? Or will the output of the transformer simply pass the AC component without distortion? A little distortion should still allow the phone to ring OK, too much and who knows.
My current design is this:
I may be missing a capacitor after the transformer? I really don't understand the AC coupling very well, or certainly the implementation of it.
The right side of the circuit should run at about 7V, and close to 1A (about 5W total), so R4 would therefore need to be 5 - 8 ohm, making C1 rather large at 1600uF (so that 20Hz is the 'cutoff' frequency). Which would mean using an electrolytic I think also.
The other capacitor(s) I'm not sure about size - as I'm not sure if there is a frequency discrimination, or is that only if a subsequent resistor then connects to ground as in the typical high pass filter? My understanding is that C2 could be an electrolytic cap, as the DC side will always be more positive due to there being a constant DC offset.
As for the transformer, it would be stepping up from 7v peak to peak, to 90v AC (approx. 250V peak to peak) - a ratio of 36x. I'm still trying to figure out what size transformer I can sacrifice for the task - 9V output or 18-20V output from mains power (250V RMS here)? I'm confused about the rectification step in calculating the ratio. Or is it possible to wind my own transformer?
Any advice and guidance would be greatly appreciated. I have contacted a friend who has a lot of experience in electronics for advice, but he's a little snowed under with paperwork / legal issues subsequent to his partner passing away.
I get computers and software more than electronics. I already have the Arduino programmed for what it needs to do - verified with outputting to the line-in on my sound card (after voltage dividing it), but it's now hooked up to a LED through the mosfet (no filtering) and it flashes at 20Hz with the right on/off phases. I'm hopeful I'm at least most of the way there with the design, but it's been a lot to learn (and I've probably overlooked things) - hopefully worth it though for future projects.
An earlier design had a H-bridge to make it AC. I could have generate a 'full rectified' sine wave (all troughs inverted to be peaks as well), and flip every second one, but that's 4 more MOSFETS, and probably 2 drivers - over complicated for my liking. Or 2 PWM outputs (equivalent to half-rectified waves) alternating in direction through a double tap transformer, a bit easier, but so far this 'solution' above seems easier and simpler - if it works... I'll be happy to share the outcome, designs and code for anyone else interested in tinkering with old phones.