Help: speeding up an analog clock with higher frequency AC?

[rea5245]

Hi,

I rescued an old flip-style digital clock from someone's trash. I'd like to hack it with a uC and GPS receiver so I can set the time automatically. But I need some advice.

My first thought was to put a stepper motor on the knob that sets the time, but that would be ugly.

The alternative would be to produce 120V at the highest frequency the clock's motor can handle. I can either shut the power to the motor off if it's running fast or run it faster if the clock is slow (I realize that setting the time could take several hours this way). So I need to build an inverter with, say, 120 Hz output. (The clock draws 7 watts.)

I've been trying to learn about inverters on the web, and it seems that the output frequency is dependent upon the transformer. I've seen references to "60Hz transformers". If I go to Digi-Key and look at power transformers, they don't offer a choice of frequency.

What sort of transformer should I look for? Have I misunderstood inverters, and maybe there's another way to adjust the frequency?

(BTW, I hope to detect the clock's current time by dabbing some reflective paint on the 12 midnight flap. It means I can only detect the transition from 11:59 PM to 12 midnight, but it seems the simplest way to go.)

Thank you for any help you can give,
   Bob

[Paul Price]

If it is a "flip-type" digital clock, then how come it you are talking about working with an analog clock?

[Richard Crowley]

Huh?  Why not simply run a local "disciplined" 60 Hz oscillator and step that up to feed the motor directly?
You can "discipline" the oscillator by phase-locking it to the 1PPS (or 10MHz) out of a GPS receiver, etc.

[NiHaoMike]

Easiest way to get the inverter you're looking for it to just hack one of the little cheap inverters. On most of them, it's very easy to change the frequency of the oscillator that drives the output stage. You'll also need to use V/Hz control to get expected results, though that's essentially negligible over a narrow adjustment range.

Or replace the existing motor with a stepper or even a DC motor plus a speed sensor.

[SeanB]

Just use the GPS to generate 120Hz and divide by 2 to get a 60Hz output. then use this to drive a small transformer in reverse, to give a 120V 60Hz clock signal to feed to the clock motor. If your reference is stable and accurate you will never need to set the time. Speeding up is simple, just drive the clock with 120Hz and most will work and count at 2 minutes per minute.

[rea5245]

If it is a "flip-type" digital clock, then how come it you are talking about working with an analog clock?

The electronics are analog even if the output is digits.

- Bob