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JC Whitney Digital Auto Clock

I can't read the clock in my radio during the day so I stuck an LCD clock to my dashboard. Problem was I could not read it as night without pressing a button to light the display. Besides, it lost about a minute a week.

I thought I found the solution in the JC Whitney LED DIGITAL QUARTZ CLOCK SKU#ZX812798R $15.95. I cut a hole in my dash and installed it only to find two problems:

  1. I could not read it during the day - The LED display was very dim.
  2. It gained about a minute a week.

This digital was the only one I could find after an extensive search so I resolved to fix these problems.

I ordered another clock to experiment with and took it apart. The first problem resulted from a 0.80 thick plastic lens in front of the actual LED. It was red and dimmed the light from the LED behind it. I cut a slot the actual size of the led in a piece of black plastic the same thickness - a clear piece could be used but the the PC board the LED is mounted on would be visible. I assume this is why they used the red color on the lens.

The second problem was a little harder to figure out - I looked up the quartz crystal (3.93216 MHz ) which the clock depends on for accuracy. There is a pair of capacitors used to load the crystal and the required value was specified at 17 pico-farad.

I found useful information on crystal timing at:

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The first thing was that a clock running fast can be adjusted by increasing the capactor values. The second thing was a formula the total value for capacitors hooked up in series. If they are the same value, the total value is half the value of the individual capacitors.

I found the load capacitors in the clock were 15 pf which calculates out to a load value of 7.5pf versus the 17pf specified. I substituted two 33pf ceramic capacitors for a calculated value of 16.5pf - much closer to the 17pf specified. The result was the clocks (I changed both) now gain or loose 1 second or less in a week of testing.

You may wonder why I went to this much trouble -

  1. I could not find another clock that might work.
  2. I had cut a hole in my dash which the clock fitted into.
Reply to
sfsteamer
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sfsteamer: The moral of your story is: Do NOT cut holes in your automobile dashboard until you are very certain you have a device that meets all of your expectations. electricitym

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sfsteamer wrote:

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Reply to
electricitym

Many devices replace one of those loading caps with a small trimmer capacitor, with careful tweaking you can get it to be quite accurate, as good as a few seconds per month. The next problem you'll have is large temperature swings in the car, but if you're feeling geeky you can overcome that problem by fabricating a small oven for the crystal to sit in. This can be as simple as a power resistor for heat, a thermister for feedback and a simple circuit to control it, with the crystal, resistor and thermister stuck together with heatsink grease and the assembly potted in a small block of polyeurethane foam. With that arrangement you should be able to achieve accuracy of a few seconds a year, but that's probably overkill.

Reply to
James Sweet

Good suggestions - especially about not cutting a hole first. I was going to try a variable capacitor but, as you note, the temperature swings will have a big effect as well. I can live with a few seconds a week - the unmodified clock was gaining a minute a week.

Reply to
sfsteamer

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