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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / May 2005JC 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: http://www.maxim-ic.com/appnotes.cfm/appnote_number/555 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. Ya could have gone to wall mart & gotten a watch for $2. No holes in dash, it's right on your arm so even if your not in your car you can still worry about the time. Life's too short to sweat a minuite a week. Slow down man, heart attacks make life even shorter. >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 [quoted text clipped - 43 lines] >1. I could not find another clock that might work. >2. I had cut a hole in my dash which the clock fitted into. Don't you own a watch? -- New service to compete with paypal Get $25 pre-registration bonus by following this link www.greenzap.com/25smackers4u I think he merely wanted to impress all of us with his research on capacitors, in which the values of the reciprocals add up when placed in series. If you only have two, that reciprocal is one half. Big deal. I'm not knocking you Steamer. I love to tinker for the experimental value and to customize everything about as well. > I think he merely wanted to impress all of us with his research on > capacitors, in which the values of the reciprocals add up when placed > in series. If you only have two, that reciprocal is one half. Big > deal. > I'm not knocking you Steamer. I love to tinker for the experimental > value and to customize everything about as well. Many of these little offshore clock gizmos have a little curved wire representing some capacitance that you can simply move around to 'tune' the oscillator. Falling into the believe-it-or-not category, the US import tariffs are higher on adjusted clock movements vs un-adjusted. So its pretty easy for a guy to guess what he's getting straight out of the Dollar Store blister-pack from China. If you have enough spare "time" (how ironic?) you can adjust these cheap movements to pretty good accuracy but they aren't very stable with temperature...particularly not in the constantly changing environment of a car interior. I wouldn't lose much sleep (there it is again!) over a minute per week. Buy a $15 Timex wristwatch and hang it from the dash or put it on your wrist :) -Bill Geeeeesshhh!!!! "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." Yes, lets say that I did wonder why such a minor problem required such a drastic solution. Since the existing clock was fully visible for at least half the day a simpler solution might have been to buy a Timex Ironman or Casio wristwatch. Or tune to one many radio stations that broadcast the time at least every 10 minutes. I agree with the obsessive/compulsive comments - I could have covered up the hole in the dash and stuck on a $2 LCD clock or just looked at my watch but I get a kick out of seeing the clock working like it should where I can see it and without twisting my wrist or pushing a button. |
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