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I’ve got an old car that I am building a circuit to add automatic window functionality to. The circuit operates like this - when the window switch is pressed for less than .3 seconds it continues to run the motor until the window is open or closed. Window switch held longer than .3 seconds and the motor will stop once the switch is released. This works well on the driver’s side where it has a constant 12v wire available to feed my circuit. The passenger side is another story. On the passenger side door, the switch has 2 wires that are fed from the passenger switch on the driver’s door. If the passenger switch on the driver’s door is not pressed up or down, both wires are 12v. If the passenger switch on the driver’s door is pressed up or down, one of the wires leading to the passenger door will be 12v and the other pulled to ground.

My circuit has inputs for 12v, ground, window switch up, window switch down, and 2 outputs from an h-bridge to power the motor. The motor pulls around 2 amps while running, and roughly 7 amps when stalled or starting.

In order to preserve functionality on the passenger window with both the passenger and driver’s side passenger switch, I think I need to modify my circuit to handle being powered by these 2 wires leading from the driver’s side to passenger switch knowing that both wires can be at 12v or either but not both (at the same time) can be pulled to ground.

Does anyone have a suggestion on how I might power my circuit with these 2 wires? There is a dedicated ground wire available. However these wires are OR’d together they will be powering an h-bridge (to drive window motor) in my circuit so there will be up to a peak of 7 amps of current running through them. These window switches form what I believe to be a mechanical h-bridge, so generically what I am attempting to do is power my circuit with the output of this mechanical h-bridge.

As a side note, I'm attempting to design the circuit so it is plug and play and there are no modifications needed to the existing car circuitry.

If anyone has a suggestion, it is much appreciated. Thank you!

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    \$\begingroup\$ Post schematic! \$\endgroup\$ – winny Aug 4 '17 at 20:27
  • \$\begingroup\$ You can use two relays - one for up and one for down. Wire them in such way so the NC contact goes to the wire coming from driver's door, center goes to motor terminal and NO contact pulls down to GND. When relay coil is powered this contact of the motor will be negative and the motor will operate. When the other relay is powered the other terminal will be connected to GND and the motor will go in the other direction. When no relay is powered NC contacts will pass trough the current from driver's door. You can get power for relays' coils from two wires ORing them with diodes. \$\endgroup\$ – Todor Simeonov Aug 4 '17 at 20:43
  • \$\begingroup\$ Sorry for not answering with a schematic, but I'm writing from a phone. \$\endgroup\$ – Todor Simeonov Aug 4 '17 at 20:45
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I ended up using a schottky barrier rectifier to OR the 2 wires together to have a constant 12v. I went with MBRF20H150CTG which can handle 20 amps, 10 per side, which is comfortably higher than I needed. Thanks Todor Simeonov for your assistance.

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