Adapting cruise control using relays, want to go solid state

Thread Starter

8.Lug

Joined Mar 7, 2018
6
The cruise control turns on when it sees +12v but my steering wheel only has momentary switches so I need to do this. Since these switches are just rubber like in a remote control, I don’t think they can handle the current a normal relay draws - or at least not for very long - so I want to go solid state. Then I was thinking - is there already an IC that does what I’m trying to do here?


Basically one switch turns on a NO relay which then powers itself to stay closed. The second switch turns off a NC relay which disconnects power to the first relay. This allows me to have two momentary switches perform the duty of a toggle switch. You can ignore the COAST and ACCEL wiring.
 

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shortbus

Joined Sep 30, 2009
7,851
The cruise control turns on when it sees +12v but my steering wheel only has momentary switches so I need to do this. Since these switches are just rubber like in a remote control, I don’t think they can handle the current a normal relay draws - or at least not for very long - so I want to go solid state. Then I was thinking - is there already an IC that does what I’m trying to do here?
So your adapting a different steering wheel into your car? Does the car have cruise now? Pretty sure that they all use relays even the ones that have buttons in the wheel instead of a stalk or lever, since those type are momentary contact too.
 

Thread Starter

8.Lug

Joined Mar 7, 2018
6
No, I put a different engine into the car from a different manufacturer. My wheel has a “set” button, an on/off button, and the accel and decel buttons - all of which are momentary switches. I don’t want a useless button on the wheel so I’m making the Set button the On button and the I/O button will be the Off.
 

Thread Starter

8.Lug

Joined Mar 7, 2018
6
The engine has all of its original factory equipment like ecu, electronic throttle body, even the drive-by-wire gas pedal. It would have been much easier to revert to a regular throttle cable when I did this, but then I’d lose the cruise control option.

Cruise is very important. I do lots of long-distance driving. I drive across the country and up and down the coasts a bit more often than a normal person.
 

crutschow

Joined Mar 14, 2008
25,267
Basically one switch turns on a NO relay which then powers itself to stay closed. The second switch turns off a NC relay which disconnects power to the first relay. This allows me to have two momentary switches perform the duty of a toggle switch.
You could do that with a latch circuit.

How much current does the relay switch?
Are the push-buttons connected to ground or 12V?
 
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shortbus

Joined Sep 30, 2009
7,851
No, I put a different engine into the car from a different manufacturer. My wheel has a “set” button, an on/off button, and the accel and decel buttons - all of which are momentary switches. I don’t want a useless button on the wheel so I’m making the Set button the On button and the I/O button will be the Off.
First place I would start when doing something like this is with a wiring schematic from both vehicles, most Haynes manuals have the schematics. This would show you how both controls are meant to work, and if they could be made to interface.

Are you using "drive by wire" for the throttle control? Did the donor vehicle have cruise? These questions are because many of the cruise controls now are done with the ECM or BCM. If that is how the donor car did it you may not need relays at all, just the push buttons connected to the correct pin of the control module.

Back in the day I put a few Mopar cruise controls in cars I built, because they were the easiest to use. They used the speedo cable and switches with a cable to the throttle.
 

Thread Starter

8.Lug

Joined Mar 7, 2018
6
I honestly have no idea how much current the system draws through them. The original control wiring is very tiny so I can’t imagine more than a few hundred mA. Unfortunately I can’t test it because I’ve already ripped everything apart and pulled the engine from the car to do some other work.

As far as my steering wheel buttons - they aren’t connected to anything right now. When I figure out how I want everything wired up, I will connect them however I need.
 

Thread Starter

8.Lug

Joined Mar 7, 2018
6
Cruise is part of the ECU. 3 pins on the ECU control the cruise function and they all require +12v signal. For the cruise to be “on” one pin requires a constant +12v. The other two pins are for accel and decel.
 

crutschow

Joined Mar 14, 2008
25,267
Here's a simple latch circuit that requires only one N-MOSFET, one P-MOSFET, and three resistors, which can be switched on and off with your two pushbuttons.
The LTspice simulation is below.
S_PBon and S_PBoff are the pushbutton signals to ground (controlled by V1 and V2 for the simulation).
M1 can be any ≥60V P-MOSFET that can handle the current of the load (which apparently is small in your case).
The circuit inherently powers up in the Off condition and draws no current in that state.

If the buttons need to be connected to 12V to be compatible with the other buttons, that change is easily made.

upload_2018-3-7_22-22-33.png
 
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