Convert That Old Rotary Phone Into an Arduino Cellphone

Rotary phones have a certain nostalgic charm and tactile appeal that our modern smartphones lack. Our smartphones are perfect for their intended purpose — which is definitely not making calls — but they offer very little fanfare when it’s time to call up your mom or doctor’s office. The sensation of dialing a number on a rotary phone is quite satisfying; in the same way that putting the turntable needle down on a vinyl record triggers a pleasing little tingle in your brain. If you want to mesh the old and the new, Proton Gamer has a guide on how to convert a rotary phone into a cellphone.

To tackle this project, you will, of course, need a rotary phone. Pretty much any model will work, so long as it is actually vintage and not a modern lookalike. Virtually all vintage rotary phones have a dial that works in the same way. After you rotate that dial to a specific number, a spring will pull the dial back to the “home” position. As it passes each number on the way, it will cause a switch to momentarily make contact or break contact. That causes a brief electrical pulse that can be detected by something like an Arduino board. Just count the pulses and you know which number was dialed.

For this project, Proton Gamer is using an Arduino Uno to monitor the dial. Pretty much any development board will work, though. That was been paired with a SIM900 GSM shield, which provides the actual cellphone functionality. The original telephone handset’s microphone and speaker are connected to speak and hear through. The ringer is a little tricky, as it is probably based on a solenoid that requires quite a bit of voltage in reversing polarity. Proton Gamer used a DRV8833 motor driver to control the solenoid, which has an H-bridge to reverse the polarity on command. The voltage is still lower than the ideal amount, so it rings quieter than it should. Even so, the conversion works well and lets you make modern GSM calls through a vintage rotary phone!