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I saw another thread that was similar, but it didn't a definitive or clear answer on the matter.

I'll be brief. I want to install a Nexus 7 (2012) tablet in the dash of my car. I want it to be able to display video from a backup camera that I will mount in the trunk of the car. The camera will just be a Logitech webcam that I already have, but it uses USB. This is fine, because I'll just use a USB OTG Y cable with the tablet so that I can charge it and send the video data to it as well.

So, I want the camera to automatically display the video on the tablet when I go in reverse, but I need a way to detect this. After some research, I've concluded that I should splice in the power cable from the USB directly to the wire to the reverse lights, connect the ground cable to the car chassis, and then just send the data wires up front to the tablet.

My question is, is it okay if I just splice the power wire from the USB directly into the reverse lights? That way, when the reverse lights go on, the camera goes on, and the video app launches. My concern is that I could either be drawing too much power from the reverse lights, or the reverse lights provide too much power to the camera, which takes 500mA, per the USB 2.0 specification.

Also, if I were to do this, would I have to watch out whether I spliced the wire into a positive or negative wire for the reverse lights? Do those play some part in the switch that determines whether the lights are on or not?

Thanks for any input

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    I think you would need to step down from 12v to 5v? So you'd need a transformer. And you would want to be on the power side of the light. Depending on what car you have the lights are either controlled by a computer or the transmission range switch. or reverse light switch in the case of a manual trans.
    – Ben
    Commented May 4, 2016 at 0:32
  • 1
    Transformers are AC devices. Battery current is DC.
    – Old_Fossil
    Commented May 4, 2016 at 1:23
  • 1
    "Why can't direct current step up or down in a transformer?" quora.com/…
    – Moab
    Commented May 4, 2016 at 1:59
  • 2
    Because a transformer has a primary and secondary coil. It requires the varying voltage and current in the primary coil to induce magnetically a higher or lower voltage in the secondary coil . Because DC is a steady voltage no voltage is induced in the secondary coil except very briefly when the primary is de-enegized. This effect is called inductive kickback.
    – Old_Fossil
    Commented May 4, 2016 at 2:37
  • 1
    @Ben How many relays have 85 pins?
    – Criticizing Israel not allowed
    Commented May 4, 2016 at 9:00

6 Answers 6

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The short answer is that you can splice the camera into the reverse lights, as the current draw is minor. However, since the camera is USB, it is expecting 5V, and your reverse lights operate at 12V, so you will need to regulate the voltage. What you're looking for is a DC-DC converter, and if you look around eBay, you can find one that has USB connector at the output.

To clarify the current draw, the camera draws 500mA at 5V, but since your source is 12V, and accounting for some inefficiency in the conversion, your camera will draw closer to 250mA. Also, as this is often misunderstood, even if you find a DC-DC converter rated for 3A, the camera will only draw what it needs. You just need to make sure the voltage output is 5V.

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  • Well, looks like I'm in luck: [link]ebay.com/itm/…. This converts directly to USB. Just to be certain, this converter has a red wire and a black wire. Obviously, the red wire connects to the reverse light wire, so does the black (presumably ground) wire just connect to a large conductive surface, or does that somehow get attached to the reverse light wire?
    – Chrøme
    Commented May 4, 2016 at 1:04
  • Yes, black is ground and can to to any conductive surface.
    – Paul Dufresne
    Commented May 4, 2016 at 1:05
  • There should be at least one chassis ground wire or terminal by the rear tail lights. Use existing ground wires when possible. The turn-on burst of current will be no higher than the fuse/amp rating of your DC-DC converter. If the video is noisy just add a noise filter (2 amp 100uH to 1mH inductor) to your 5 volt line before the USB connector.
    – user16540
    Commented May 4, 2016 at 1:36
  • Most of those sealed DC-DC converters, like the one in the link have a built-in inductor for noise reduction. I've used many of them, but Sparky is right on both counts. Use an existing ground point if one exists nearby. Since you will probably be wiring to the tail light, look for one near there.
    – Paul Dufresne
    Commented May 4, 2016 at 1:41
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    The relay will not convert voltage to 5V. You still need a DC-DC converter or voltage regulator to get a 5V to pass through the relay. No point in having both a converter and a relay.
    – Paul Dufresne
    Commented May 4, 2016 at 2:12
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No. If I understand your intent correctly. The backup light is activated by a 12 volt source. USB is a 5 volt source. You would cook your camera and Nexus 7 unit.

USB plug pins:(If I remember correctly)

       1: +5 V @ .5A (500mA)

       2:  DATA+

       3:  DATA-

       4:  GROUND(0 V)

If you have a alternative 5 V source like another USB feed that came with the car. Then use a 12V relay to act as the triggering mechanism.

+12V-----from lightbulb------to relay coil----to ground 0V

+5V----from alternate USB plug----relay contact with spring on it. This will power the camera and the Nexus 7

+5V USB from Camera and Nexus USB to relay contact marked N.O.-(Normally Open)- When coil is energized it will close and activate the units.

Connect all the USB grounds together to complete the circuit.

The relay doesn't have to large one. Just a 12V coil with the contact rated about 2A - a solid state one would be OK. Just read up on USB plugs on how to wire up and on relays. Wikipedia would probably have this info.

There are small devices that plug into your 12V power socket that have a USB source. Available near the counter at corner stores, truck stops also sells them as well for about $5 a pop.....

DC to DC converter not required.....

I came up with a another brilliant idea using a 7805 voltage regulator chip. Regulated 5V source straight from the backup light 12V source

12v from backlight or reverse switch connection by shifter.

12v-----7805 regulator----5V out direct to usb

5V feeding usb cabling in a T configuration.

One side of the T goes to camera ,other side goes to Nexus 7 unit

The triggering is done by the swich itself..

Check out this.......

https://youtu.be/GSzVs7_aW-Y

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  • This makes a lot of assumptions as to what the OP has to work with in terms of 'add-on' components. This is a bit more wiring intensive in that multiple wire runs are needed. I think the OP wants this to work with as few parts and wires as possible.
    – user16540
    Commented May 4, 2016 at 1:42
  • @sparky256 Yes, this is what I wanted. -resident_heretic If just wiring in one converter will do the job, then that will be the method. Now, if that were to not work, then I'd be inclined to use a relay, but the evidence seems to indicate that a converter would work. Good idea, but just not as simple. I also would like to point out now (maybe it wasn't clear enough in the original post) that the N7 wouidn't be powered off of the same wire as the camera. The camera will be powered off the light, and the N7 off of a plug in the front of the car. I just need the light so that I can detect reverse
    – Chrøme
    Commented May 4, 2016 at 2:20
  • @Peepnbrick. That was understood when I wrote my comment. You seem to have a good grip on the important stuff. I just commented about the grounding issue, use of an isolated DC-DC converter if possible, and the filter to make sure 5 volt line is noise free. Some converters put out a bit of noise. I was giving you a fix for that if you had noise on the camera image.
    – user16540
    Commented May 4, 2016 at 2:55
  • So what is the benefit of using a regulator over a converter? It seems they provide the same end result.
    – Chrøme
    Commented May 4, 2016 at 21:23
  • Significantly cheaper for one. A 7805 chip costs $0.75 and the 3 capacitors roughly $ 1 each .The most expensive thing would be the USB cable and small board to make the connections . What is the price of a DC-DC convertor just by itself?
    – Old_Fossil
    Commented May 5, 2016 at 3:01
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I would recommend a 12v relay connected to the lights so then when the light comes on the relay will activate the camera running of a steady 5v input provided by the same USB cable you will use for data connected to your tablet's OTG.

This will allow you to not dim the light when powering the camera. And you won't require a DC-DC converter appart from the one you use to power the tablet already.

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  • This makes the assumption that a 5v source is available to be switched by the 12v lights, which 5v rail in the rear of the car do you suggest to use?
    – James T
    Commented May 4, 2016 at 7:57
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    He plans on running a USB all the way from the tablet's OTG to the camera. in any case if by some alternative he wants to get the 5v from somhere else I'm sure he can trace a 12v rail from many components in the rear of the car and use a simple lighter connector adapter. I run mine from the CD switcher in the boot.
    – EChan42
    Commented May 4, 2016 at 8:08
  • a lighter connector adapter is a DC-DC adapter on the inside... same thing.
    – James T
    Commented May 4, 2016 at 8:13
  • But I did mention as a first option the USB cable wich he will have to run down form the camera to the tablet as a first option and the adapter a an alterative, did you chose to ignore the fact I mentioned a perfectly good 5v source or are you simply nitpicking ?
    – EChan42
    Commented May 4, 2016 at 8:19
  • That and the spec for USB 2.0 is 5m, my car from the head unit to the rear of the boot as the crow fly's is ~4m not accounting for extra length required for correct routing & tidying. One way to overcome this limitation is to supply the 5v at the termination point to ensure it can draw the required power easily which is what this would be doing; but that is rather technical so i chose to avoid making a point surrounding it.
    – James T
    Commented May 4, 2016 at 8:26
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I think you'd be better off trying to tap into the reverse switch on the shifter. This is the switch that turns the reverse lights on and off. You'll be running a lot less wire around and will wind up with a neater install.

This may not work with higher-end cars with electronic everything, but for most cars (especially those older than five years or so) the switch should be easy to find and work with. On a higher-end car I'm sure you could tap into the CAN bus and watch for "shift to reverse" events, but that's a lot more work.

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  • Yes, he can use the reverse light but I'd prefer the shifter, if the light goes out the circuit wont close, thus no turn on voltage for the camera.
    – MDMoore313
    Commented May 5, 2016 at 19:23
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Also note that some vehicles use PWM (pulse width modulation) to maintain the same brightness of lights when the engine is running (14.2v) as when it is not (12v) BMW is one I know from the top of my head that does this. Meaning that your circuit needs to be able to handle a PWM signal (it will be at roughly 85% duty cycle, so normally DC DC converters can cope with the signal, but be aware that if you connect for instance a 12v camera to your reverse lights, there is a chance you will get a distorted image or no image at all. Safest, as suggested, is to tap into the reverse switch as a signal to a relay.

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    First off: Wow, cool trivia. Do you have any sources? Second, I think they are planning to wire into the white reverse light (not the brake lights) which would surely not be important enough to get fancy dimming abilities.
    – Zach Mierzejewski
    Commented May 4, 2016 at 18:44
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I did it a simpler way. I bought one of those $40 Chinese radios with a back up display. It was slow to come on, so I took 12v from the reverse bulb hot side into the reverse trigger on the radio, usually a pink wire. The camera gets it's power from the rear cigarette jack, that only goes on when the ignition is on. This way the camera is on all the time, draws little current, but only engages the display when the unit (radio) switches on the display, after receiving a 12v signal from the brake light. Display is instant.

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  • I like the simplicity of this system - using a tablet is complicated and, although it affords more customizability, it's also less integrated into the rest of the vehicle. I enjoyed designing and building this system more than actually using it - in the real world it was a little cumbersome. Maybe I'll revisit it this summer and take inspiration from your project.
    – Chrøme
    Commented Apr 12, 2019 at 17:07

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