This Kitchen Timer Used to be a Rotary Phone

Many of us love the look of old rotary phones and enjoy the tactile experience of spinning the dial. But it is very difficult to justify the use of a rotary phone today. Most people have ditched landlines entirely and rotary phones don’t always play nice with modern digital phone systems anyway. The result is a world full of antique stores overflowing with beautiful rotary phones that can’t serve a purpose in the modern age. But with a bit of tinkering, they can take on new roles, as demonstrated by Scott3DPrints’s rotary phone turned kitchen timer.

Scott3DPrints found this telephone in a thrift store and it is quite the specimen. By all appearances, it began its life as a crank phone from around the turn of the century — 1900, not 2000 (note to editor: leave this in to make readers feel old). At some point in the mid-20th century, a previous owner converted it into a rotary phone. That’s intriguing, because the only parts they could have kept would have been the wood enclosure and the ringer. It isn’t clear why that was more practical than buying a new rotary phone at the time, but it makes this project all the more interesting.

Now this phone is a kitchen timer thanks to another conversion. For this conversion, Scott3DPrints added an Arduino Uno Rev3 board. In reads input through the rotary dial by counting pulses and also a momentary push button. That button switches modes (long press) or starts the countdown (short press). The mode selection switches between HH:MM and MM:SS, so this timer works for a variety of situations. Each turn of the rotary dial adds a digit to the trailing end of the entered time, as shown on a four-digit seven-segment display.

When the user presses the “START” button, the Arduino begins counting down one second at a time. When it reaches zero, it activates the ringer. And that was the big challenge with this project, because telephone ringers require alternating current to function and the Arduino can only supply DC.

To provide the AC power the ringer needs, Scott3DPrints connected a LS057020 Black Magic ring generator. That is a sine wave generator similar to what powers an inverter and it can turn 5VDC into 70VAC at 20Hz. The ringer doesn’t need much current, but the LS057020 gets its power through a relay with optocoupler to prevent any back feeding.

Now this old phone has a new purpose to serve for decades to come. And maybe someone in the future will give it new life with another conversion.