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May 25, 2012 AT 8:03 am

ASK AN EDUCATOR! “How can I keep power to an AUX device in my car?”

Aaron asks:

How can I keep power to an AUX device in my car?

I’ve got a Bluetooth stereo adapter I’m powering with the Aux port in my car. When I go from Aux standby to the engine running, the power cuts for a moment and I have to reconnect. Can I add a capacitor, or some sort of battery backup that will keep the device on for the 2-3 seconds the port switches off?

Part 2: The same bluetooth adapter makes audible noise on the stereo not connected to it’s audio output. For instance, a blinking light shows the device isn’t connected, and while it’s blinking, you can hear ground hum coming on and off (matching the light). As well, when the adapter is connecting to another device, you can hear muffled, modulated sounds (reminds me a bit of an old modem) – these are separate from the audible beep that happens when the bluetooth connection completes.

What are these noises? Can they be isolated from the audio output?

This actually isn’t too hard of a task. Right now the power going to your 12V AUX port in your car is connected to the ignition circuit. Power is cut to this circuit when you start the engine because of the large power demands your starter puts on your car’s 12V battery. There are 12V circuits in your car that never cut off during this period, like power to your ECU. If you look at the wiring diagram for your car’s ignition, you can isolate which lines are controlled by the different positions of your ignition switch. I would recommend that you connect to a circuit that stays on during all key positions except for OFF. This way you don’t run your battery down while you are not in your car.

In order to make this modification, you are typically going to need to remove the your center console bezel to expose the lines running to your outlet. Before you try and tackle any electrical modification to your car DISCONNECT THE BATTERY! BOTH + and – just to be one the safe side. At the outlet there will most likely be three lines: one for 12V, one for GND, and one for the light. You will want to cut the 12V and heat shrink the end on the hot side to prevent it from shorting. You will then need to run a simillary sized gauge wire from the outlet to the determined electrical line and solder splice the connection. You can use mechanical splices like wire nuts, wire taps or crimps, but they will eventually fail from vibration. Make sure you use heat shrink or electrical tape to prevent shortage. Finally, reconnect your battery and check the power to the outlet with a multimeter prior to plugging in your bluetooth unit.

Regarding your audio noise problem, it happens to be very common. You have an AC generator (or alternator) connected to your engine that is used to recharge and assist your battery. If the alternator is not properly electrically isolated or your radio is not well grounded, the radio will pick up these electrical emissions resulting in an annoying hum or buzz. Crutchfield has put together a pretty good guide for determining the source of your noise. You will most likely need a ground loop isolator, which you can pick up at most automotive stores for cheap.

Good luck with your project and I hope you can kill that annoying noise!

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“Ask an Educator” questions are answered by Adam Kemp, a high school teacher who has been teaching courses in Energy Systems, Systems Engineering, Robotics and Prototyping since 2005.


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