UK Clock Project Inspires Custom 60Hz to 50Hz Phase-Locked Inverter

Hacker “Mitxela” was recently contacted by someone trying to build a replica of the alarm clock from Groundhog Day. In this movie, the same day is repeated over and over, starting with the alarm clock blaring Sonny and Cher’s “I Got You Babe,” on a 1970s-era Panasonic flip-clock. While that may or may not sound like a pleasant way to wake up, this hack isn’t so much about the clock itself, as it’s about the power source.

The problem here is that the flip clock used is for the US market, operating at 115V, 60Hz, but if you want to use it in the UK at 240V, 50Hz, this input has to be properly transformed. What makes things complicated is that the clock actually uses the mains power frequency, not a crystal oscillator or other means to keep track of the progressing time. So a 50Hz frequency would make things 16% slow. An inverter would work, but relying on its frequency generation would be unreliable.

What follows is Mitxela first reverse-engineering the motor used to advance the clock flaps. The hack continues with how the phase conversion was accomplished, using an ATtiny85 chip as well as an SN754410 motor driver IC. While there are a wide variety of approaches here, even just using a quartz crystal in the inverter to generate timing pulses, the method used of actually measuring and converting the input sine wave leads to a very interesting hardware discussion.