Off Topic: Cruise control mod?

So I have this aftermarket cruise control in my car that uses the speed signal from the car to set the speed. It does not use magnets on
the driveshaft. The cruise control only starts working above 35 mph. Every cruise control I have ever used, factory or aftermarket, only worked from about 35mph and above. But I do a lot of driving below 35 mph and want to use the cruise control for it. So I'm hoping that if I measure the signal from the car with an oscilloscope it wil show either a chain of pulses or a voltage that changes with the speed. I looked online and found a couple frequency doubling circuits that look like they would work. I'm sure that the extra pulses the circuit would make must be evenly spaced between the pulses from the car for my scheme to work. And at least one of the circuits I found appears to do just that. If it is a rising voltage that I need to modify I'm not sure how I would do that. What if the rise with speed isn't linear? Anyway,any thoughts or advice? Any circuits that would work? Maybe a chip to do all the work? Thanks, Eric
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hi Eric,I too have an after market cruise control. What I have been wanting to do is modify it with some presets for different speeds. Don't know why the manufacturers don't build them that way. Probably some liability issues. Ivan Vegvary
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On Wed, 26 Jun 2013 16:57:50 -0700, etpm wrote:

It's almost certainly pulses. You probably want a circuit based on a phase locked loop (look for a circuit with a 4046 chip in it).
There may be too much delay in the PLL for your cruise control to remain stable -- it kinda depends on how many pulses per second you get at your desired speed, and how the cruise control is setup. Try & see.
--

Tim Wescott
Wescott Design Services
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On Thursday, June 27, 2013 2:23:35 AM UTC-4, Tim Wescott wrote:

d
a
?
Didn't you mean this? http://www.nxp.com/documents/data_sheet/74HCT9046A.pd f (just discussed to death on s.e.d.)
But seriously, these pulses are likely to be pretty slow, and it ought to b e a piece of cake to do this with a little microcontroller. An arduino ough t to be able to handle this easily, if you're not prepared to start from sc ratch. You can get everything you need at Radio Shack, even (glad to see th at they're carrying some actually usable stuff again).
If it does turn out to be an analog signal, you can do that with the microc ontroller as well.
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On Wed, 26 Jun 2013 16:57:50 -0700, snipped-for-privacy@whidbey.com wrote:

The cruise on my 2004 Suburban works above 25 mph. And you're right, it's handy. Drives the speed demons nuts when I go the speed limit through the subdivision.
So they're out there.
Pete Keillor
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On 06/27/2013 04:25 AM, Pete Keillor wrote:

I drive 15 in my neighborhood, which is posted 25; I prefer a certain quality of life, and over time I've found most of my neighbors do, too.
As to the 35MPH lower limit to most cruise controls, I have that on my vehicle as well. My assumption is that it is designed to force people (who are, in large, idiots) to pay attention when driving around town, where there are obstacles which do not exist on the open road.
Jon
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On Thursday, June 27, 2013 10:17:50 AM UTC-4, Jon Danniken wrote:

It's also possible that the 35MPH lower limit is because at lower speeds, we tend to be closer to other cars (or pedestrians), and should the system fail in a "full speed ahead" mode, there'd be little time to react before damage was done.
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