Changing the timebase on a cheap alarm clock.

Posted by on June 21, 2010

Most alarm clocks run their internal clock oscillator off of the mains frequency by detecting the peak on each cycle and thus

obtaining their timebase. However not every country uses the same grid voltage/frequency! — This poses a problem, if you’re using a 60hz clock on 50hz, it’s going to run at ~83% of it’s intended speed!

I’ve seen three types of clocks so far, the ones that automagically detect the mains frequency, the ones that allow you to choose with a simple switch, and those that just won’t run at your mains frequency!

For the latter, the following applies:

Open it up, find the main IC — it’s usually nearby the 7 segment display and it should be one of the biggest chips as well, write down it’s model number and google the datasheet, now look for a frequency selection pin, it’s usually labeled “50/60Hz select” .

In my case the clock I was modding happened to use the SC8560 IC, which is a rather common one. Here pin 26 works as the selective input — when you tie it to Vss, the chip goes into 50hz mode, otherwise if you leave it open, it assumes the 60hz timebase.

The solution? add a fairly low value resistor in between pins 26 (Select) and 15 (Vss) — This clock happened to use a positive power supply, so I followed the sample schematic from the datasheet to familiarize myself with the design and later on proceeded to solder the resistor once I was confident enough.

After modding the clock, all that’s left to do is to set it on time and put it next to a known good reference clock, leave it for a couple hours and come back later to see the results!

So far after ~3 hours I only noticed a slight delay on the new clock, but we’ll see if it increases/accumulates after a few days, if it does I’ll use a much lower value resistor, if the problem  persists… I’ll simply let it be, it’s not worth the hassle!

As for the input voltage, you can just use an autotransformer (like I did) or go the extra mile and replace the internal transformer for one with equal secondary side, but a 220 primary (extra points for using a tapped primary and adding an input voltage selector!)

Hopefully this quick guide will help someone!

Cheers.

PS: Thanks to the guys from ##electronics @ irc.freenode.net for pointing me toward the right direction on this one! Let’s see if this sucker drifts in time…