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I'm installing a rear camera in my 2004 Jeep TJ with a new radio head unit with a large display. The head unit I'm using automatically switches to the backup camera when the camera video feed is enabled. The camera video is enabled when it receives power.

Typically, the rear camera is wired into the backup light circuit so that it automatically comes on when the car is put into reverse. I would also like to optionally view the camera using a (new) switch installed in the dashboard. Logically, I'm looking for an "or gate" arrangement to power the camera from either the backup light circuit or the switch circuit.

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It's pretty straightforward, but I'm not a car guy and can't find an "automotive OR gate". So I'm looking for some quick guidance on what part will work best for this situation.

2 Answers 2

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I think what you are planning on doing is the way to go (per your diagram) with one small addition. You need to insert a diode between backup light power source and the junction (upside down Star Trek looking symbol) of the power where the goes to the backup camera.

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If you don't have a diode in there, you'll feed power out to the backup lights every time you switch the backup cameral on. Other than that, I think your solution should work just fine.

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  • Thanks for the feedback. I'm looking at this part, do you believe this would be correct?
    – PeterT
    Jul 25, 2023 at 15:14
  • Yes that would work. I was talking about something like this (20A/50V). Put in line where I'm pointing at above will provide the same protection at a cheaper cost (by far). Plus you'd have extras in case this one goes bad (usually wouldn't). Jul 25, 2023 at 15:31
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If the Aux Power is always available, you can simply use a relay that is controlled by the Backup Light Power. Then you wire the relay and Enable Backup Camera switches in parallel, so that either the relay being on, or the Enable Backup Camera switch being on, will let power to the backup camera.

Diode is a poor solution. Diode loses about 0.5-1.0 volts of voltage depending on the type of diode and the current going through the diode. This is 4-7% of the energy going to the backup camera. If the backup camera uses significant energy, the diode can generate significant heat, and in excessive cases it could even burn out unless you use a diode rated for a high current which is very expensive and usually needs a heatsink.

You don't need a diode or an OR gate.

An alternative to a relay could be a MOSFET, but MOSFET on the positive wire would need zero voltage signal to turn on and +12V signal to turn off, which is exactly the inverse of the Backup Light Power logic, so you actually need two MOSFETs, one for converting the logic to an inverse, and another for supplying power.

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  • I understand your point about the voltage drop using a diode. So if I use a 5-pin relay and connect the existing Backup Light Power to pin 87a (NC) and connect the Aux Power to pin 87. The switch would connect to pin 86 to energize the coil (to throw to pin 87). Power to the camera routes through pin 30. In this manner, the normal Backup circuit is the dashboard-switch-is-off and the camera gets power when I put the car into reverse, otherwise I can turn on the camera with the switch to get immediate power?
    – PeterT
    Jul 25, 2023 at 17:25
  • @juhist - To say using a diode is a "poor solution" is quite off base. If sized correctly, the diode will create little if any heat. For that matter, MOSFETs will create more heat then a diode would. As far as it losing voltage, most cars puts out more than enough voltage to make up the difference as well as electronics are capable of using less/more than battery voltage, so it is of no issue. Diodes were created to stymie the reverse flow of electricity. Using one in this case is a very simple and practical solution. While your solution would work, it creates complexity which isn't needed. Jul 25, 2023 at 17:37
  • A properly controlled MOSFET will create far less heat than any diode will. Diode is a constant 0.5 - 1.0 volt drop. MOSFET is a very small resistance, maybe few milliohms. MOSFETs create heat only if improperly controlled, if the current is huge (in which case you probably anyway use many MOSFETs in parallel, reducing the power loss as heat), or if the MOSFET is switched at a rapid rate of 100+ kHz. The gate control of MOSFET doesn't create any heat since MOSFET is an isolated gate transistor. MOSFET doesn't have gate current unlike bipolar transistor which has base current.
    – juhist
    Jul 26, 2023 at 18:57

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