Office Phone

Step 7 Make it Ring

While the solution for this step ended up being straightforward, it was hard to find. It turns out that the ring voltage on a standard telephone is essentially only found on telephone lines, and used only there, so there are not many components that convert the voltage specifically. The only transformer I could find of the correct range was a variac, by far the most expensive component in the project: ~$90 with a custom made housing. That said, a cleverer or less deadline-fearing craftsman might come up with a more elegant solution.

Once the voltage conversion is decided upon, it is straightforward to hook up. The ringer circuit will have some power in, two wires. Run the output of the variac through another simple NO relay (make sure this one is rated for 90+ VAC if you do it my way!) and then connect that to the ringer circuit. When the output pin you connect to is high, the ringer will sound.

After hooking the ringer up, however, I discovered that it did not actually ring the way it did with an incoming phone call. It really just buzzed. The problem is that the distinctive "ring" of a phone (and they vary by country...) is a function of its on and off pattern. By trial and error, I discovered that mine sounded most convincing with a pattern of one second on, one and a half seconds off. I am more comfortable with software, so, under pressure, that's where I chose to manage the ring pattern. I imagine that some combination of 5-5-5 chips and capacitors could take care of this in hardware, which might be advisable.

Now all the hardware is in place... let's get things under control.