Friday, March 13, 2009

Car Clocks

Last weekend began Daylight Savings Time. My car has a an analog clock with a simple mechanism for resetting the clock. You put a pen into a small button on the dashboard and the clock runs fast. You release the button at the right time. My wife's digital clock is a bit harder. Either way, we can set the clock and trust it to keep correct time until Daylight Savings Time ends in November. It didn't used to be like that.

When they first started putting clocks in cars they used a simple electric motor. These ran at a consistent rate depending of the voltage applied. The problem is that cars have different voltages. When the motor is turned off the battery puts out 12 volts. When it is running, it is recharging the battery and the voltage is more like 14.5 volts. This can drop if you are running power-hungry things like the rear window defroster. Combine that with headlights, radio, windshield wipers, and the fan and the voltage drops nearly back to 12 volts.

Every time the voltage changes, the old car clocks ran at a different speed. Engineers tried to estimate how much driving the average person would do and adjust the clock so that it would average out. This never seemed to work in real life. Car clocks could gain or lose as much as five or ten minutes in a week. You either constantly adjusted your clock or you gave up and never looked at it.

Sometime in the 1980s they started using quartz movements in car clocks. A quartz crystal vibrates if you apply current to it and it is fairly forgiving about variations in voltage. Problem solved.

1 comment:

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