Making an old phone ring

It’s that time of the year again. That time were you don’t get anything done. That time were time flies and you can’t stop it. But you know why.

It’s play-time.

And in this year’s play, we need a phone to ring. Now for authenticity, it’s a lot cooler if that ring actually comes from somewhere on scene and not from the speakers. But for that to work, we need a phone on scene which we can control remotely. Preferably an older model, for its characteristic ring.

So that’s what we did.

After doing some research, I found out that the bell ring on an old phone is triggered by an AC-signal of 40 – 100V (Volt) at 20 – 40 Hz (Hertz), depending on the country. Now if that sounds like Chinese, don’t worry.

The easy part to explain is the voltage. If you take an electrical outlet, that’s 230V (Europe) or 110V (US). Common batteries range from 1.5V to 9V (or 12V for a car battery). It is, in general, a measure for the potential of power delivery. Think of it as a height. The higher an object can fall from, the more energy it will have when it reaches the ground (up until a certain point, but that’s perhaps a bit too far for this post), which you can see for instance because it will leave a bigger dent.

The other part is the AC at 20 – 40 Hz. There are two possibilities when it comes to voltage: AC (alternating current) is what you get out of your outlets and changes like a wave, while DC (direct current) is what a battery for instance delivers and remains constant.


And the Hertz part? That’s how many times the wave goes from the top all the way down and back to the top again, per second.

Enough with the lecturing though. For Belgian phones I found out that I needed a signal at 25Hz. You could go down the route to solder something together with I don’t know what components, but it had to be possible to do this an easier way.  I found out that back in the days (all the results I found were at least ten years old, but hey, it’s not like the technology is going to change, is it) people created that waveform to burn it on a CD and then use a stereo amplifier to send it to the phone.

Luckily for us technology did change in those ten years: I didn’t burn a CD, I just put an app that can generate tones on my phone instead, way easier. Then it’s just a matter of connecting the right pins from the phone (A & B in my case) to the amp’s speaker outlets and it should work.

I had it all figured out, but I couldn’t test it because the gear isn’t mine. And then the director decided that we would simply go for a recorded signal playback. It should work, but I can’t confirm it myself. If any of you do, please let me know if it worked for you!

Featured image by Da Sal