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TIME-BASED DEVICES

Transplant a heartbeat from cheap alarm clocks.

By Larry Cotton

Some projects need a nice, steady, low-voltage pulse at a constant rate. This is relatively easy to do with the ubiquitous 555 IC, but what’s not so easy is keeping a 555-based circuit accurate to within a few seconds a month. For that, you’re better off just gutting a cheap travel clock. This will give you an easy module that generates ultra-accurate pulses at 1Hz — something which, in my opinion, is more valuable than telling time.

Not just any clock will do, however: it should run on one battery (usually AA size) and have hands, not an LED readout. If it has a second hand, it must jerk forward every second (most do), rather than sweep. Clocks like this are easy to disassemble and cost less than $5 at some retailers. They all use the same basic mechanism: a 32,768Hz quartz-crystal oscillator, along with

120 Make: Volume 09

the electronic and mechanical parts that convert its oscillations to clock ticks.

Disassembly demands a bit of finesse. Although you’re not tinkering with a fine Swiss watch, you are filleting some fairly delicate clock guts. What you’re looking for is the electromagnet (solenoid), which turns the first gear in the gear train.

Your surgical procedure will depend on whether your project just needs the electronic pulse generator, or a clock with working hands that continues to tell time. If you just need the clock’s ticker, you can disassemble without worrying about reassembly, and throw away the gears, hands, and even the electromagnet.

Here are the steps I followed with my clock. Yours may differ depending on its age. I’ve also successfully used wall clocks, which also come

Photography by Larry Cotton