On Wed, 26 Jun 2013 16:57:50 -0700, etpm wrote:

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

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
http://www.wescottdesign.com