Pulse Generator - LED Odometer Display - Defender Source
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Old November 25th, 2008, 06:10 PM
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Pulse Generator - LED Odometer Display

I would like to advance the miles on a new replacement speedo/LED odometer unit to match the mileage of an original LED odo display that is flashing and soon to die.

I searched and read that this can be done with a pulse generator. Has anyone ever done this? Is it worth trying to get one, or should I just suck it up and get it done at the local speedo shop?

(They want $125 to advance the mileage on the new unit on the bench.)

Thanks.
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Old November 25th, 2008, 06:37 PM
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they want HOW much? Damn! You can build the parts for just $10 or so for a pulse generator.

-Hans
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Old November 25th, 2008, 08:38 PM
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I to want to Know I have set my new speedo up on the bench. I got it powered up but was unsure of where to hook up the frequency generator. any help would work.
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Old November 26th, 2008, 09:08 AM
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I'll see what I can dig up for schematics for this.

Shouldn't be too hard, a pulse generator circuit is basically just a 555 timer chip, a couple of resistors and capacitors. Maybe throw in some variable items to speed it up and down.

-Hans
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Old November 26th, 2008, 09:34 AM
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Here is a very simple one: http://electronicdesign.com/Articles...ArticleID=6235
Anyone knows what the output of the stock sender is voltage wise?
Also, how fast can you make the pulse generator go?
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Old November 26th, 2008, 09:56 AM
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Most resistors/capacitor pairs I've seen have the pulse generators running at 68khz, which would equate to about 4 miles per second on your average speedometer. I doubt the speedo can go that fast, so throw in a couple variable resistors to make that slower if needed. You will need to do that near the end anyways, or you will rapidly blow past the desired mileage.

Your average 555 chip can run at a pretty wide variety of voltage ranges. Power the speedo with a 12v power supply, and the wiring for the Vehicle Speed Sensor should theoretically power the pulse generator.

-Hans
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Old November 26th, 2008, 10:26 AM
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Quote:
Originally Posted by Hans
they want HOW much? Damn! You can build the parts for just $10 or so for a pulse generator.

-Hans
Thanks; I couldn't believe it either.
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Old November 26th, 2008, 11:31 AM
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I experimented with this and it does work. My problem was that it was going to take something like 45 days of constant running to take the speedo to around 100k miles. If I turned up the pulse rate to fast, it stopped working. This was on a VDO Speedo. I opted to install it instead of waiting, but may go back this winter and let it run.
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