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Alarm clock hacking, and other rainy-day pursuits

Clock readout at 23:37, with conspicuously-missing segments in digit 1

I have never liked the 12-hour time display which comes standard on most alarm clocks sold in the U.S., so I cracked open my RadioShack alarm clock was able to reconfigure it to display the time in 24-hour format with very little effort. The operation was quite simple and straightforward; however, because the extra four display segments needed to display the number 2 in the tens-of-hours place were not occupied with LEDs on this US model, the result is less than fantastic (see photo).
Unfortunately, the LED display module in my clock only contained the necessary LEDs to display "18:88", even though through the red plastic on the front you can see cavities for the other four segments. I wonder how much money the manufacturer saved! Still, the clock works fine in 24-hour mode; hours 20 through 23 look a little weird, of course, but the clock is still perfectly useful.

The LM8560 and you

The LM8560 is by far the most common alarm clock chip I have encountered in the AC-powered LED alarm clocks that I have taken apart, and the datasheet is pretty easy to find on the Web.

24-hour time display

Jumper pulling pin 28 to ground to set 24-hour display

To activate the 24-hour time display mode, simply connect a jumper between pin 15 (VSS) and pin 28 on the LM8560 (if you've got a different chip, go find a datasheet for it). If you're really lucky, your clock won't be missing the necessary LEDs to display a "2" in the first digit...

Controlling stuff

Modification to the alarm clock to provide a control output

To make things interesting, I added a control output to the clock as well. The 8560's ALARM output is filtered and used to energize a small reed relay which closes a pair of contacts on a 2.5mm phono jack mounted in the battery compartment. This allows the clock to be easily plugged into Mr. Switchy and used to control sirens, a strobe light, explosions, etc...

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