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

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

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

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

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

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

Didn't you mean this? http://www.nxp.com/documents/data_sheet/74HCT9046A.pdf (just discussed to death on s.e.d.)

But seriously, these pulses are likely to be pretty slow, and it ought to be a piece of cake to do this with a little microcontroller. An arduino ought to be able to handle this easily, if you're not prepared to start from scratch. You can get everything you need at Radio Shack, even (glad to see that they're carrying some actually usable stuff again).

If it does turn out to be an analog signal, you can do that with the microcontroller as well.

On 06/27/2013 04:25 AM, Pete Keillor 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.

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

On Thursday, June 27, 2013 10:17:50 AM UTC-4, Jon Danniken wrote:
On 06/27/2013 04:25 AM, Pete Keillor 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.



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

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.