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:
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.

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
- - - - - - -



sfsteamer wrote:
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.

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.


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.

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.