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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

Chrøme
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6 Answers6

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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.

Paul Dufresne
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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

Old_Fossil
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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.

EChan42
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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.

TMN
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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.

LievenV
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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.

Steve
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