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MCU controlling relay (or a way to simulate a SPDT relay?)

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LiquidKernel

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Hi everyone, I am working on a project where I am going to duplicate the auto-power window feature of the Volvo S60 into my Volvo 850. My car uses a relay to drive the "auto" feature for down on the driver's side window only. The S60 however, does both up and down for all four windows without the use of relays.

I also wanted to avoid using relays as I'd need two per window (based on the schematic in the link below), but as long as I can use a cheap 12V coil voltage relay from Mouser that I can acquire for $0.80/each, I will have no problem with it.

My car uses an interesting way to move the windows up and down. Instead of inverting the ground and +12V line to the motor, it keeps the ground connected to both leads of the motor. Then depending on which way you need to move the window, removes the ground from the required lead and instead puts +12Vs to it.

What I would want to do is have an MCU (Atmel most likely) to detect a 1/2 second pulse (no more, no less) to trigger a SPDT relay that would send +12Vs to the needed lead.

However, how would I drive the coil of let's say a 12V coil voltage relay with an MCU? Optocouplers?

Or, how would I be able to (cheaply) duplicate an SPDT relay using solid-state devices such as power MOSFETs?

Here is the schematic (you have to copy-paste the link):
http://iupload.net/022004/systemwiringdiagrams_45.png

Thank you for any help.
 

LiquidKernel

New Member
Russlk said:
I suspect that grounding both terminals of the motor is how Volvo stops the motor, rather than have the window slam into the stop. Have you figured out how to do that?
Grounding both terminals is how the motor is at an idle state.

To stop the motor after the window has reached it's window will probably be accomplished by the use of a comparator on an Atmel ATtiny12. It will look at the voltage on the line and when it notices a sudden change it will release the relay.
 

Russlk

New Member
When the motor terminals are shorted together, it acts as a generator with a short circuit load, which stops it quickly and safely. This has to be done before the window hits the stop. The way you are doing it, something is apt to break.
 

LiquidKernel

New Member
Russlk said:
When the motor terminals are shorted together, it acts as a generator with a short circuit load, which stops it quickly and safely. This has to be done before the window hits the stop. The way you are doing it, something is apt to break.
I'm quite sure it lets the motor run and jam before it turns it off. Right now I can hear the relay click on when it starts to go down automatically and after it has gone done all the way and stays there for a second the relay turns off. I'm certain that there is no other reason for it to ground both terminals other than to ease the design.
 
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