Switched Radio power (without key)

Discussion in 'Audio, Video & Gadget Tech' started by pmf608, Jul 27, 2009.

  1. pmf608

    pmf608 Epic Member 5+ Years 1000 Posts

    I was at a party a while back and using my truck for the music (i had the best sound system of the vehicles there). Problem was, to power the radio, the key must be in. A few drinks later, a couple of friends decided they wanted to be idiots and go for a ride. Luckily i was nearby and could stop them.

    My solution to this is a switch and relay setup, using a SPDT relay in reverse (two sources to one output). I don't know if this will work, so i wanted to run it by some people who are good with electrical. Here's my plan. 85 and 86 will be wired as usual to energize when the switch is flipped. This will change the power from ignition switched on 87a to 12V input on 87 and will run through the relay and ouput on 30.

    Does anyone know if a relay will work in reverse like this?

    Attached Files:

  2. bill190

    bill190 Rockstar 100 Posts

    Yes that looks like it would work fine.

    Be sure to use a "continuous duty" rated relay or sometimes called a "power relay". These relays are designed to be on all the time for long periods.

    And these relays would be used for headlights, radiator fans, A/C compressors, etc. (As opposed to a horn or door lock relay which would only be used momentarily.)

    Then relays which are always on can get to be quite hot. So best to place it in the engine compartment where it will get good air circulation. In this case the vehicle would not be moving, but best in the engine compartment where it has a lot of breathing room!

    Also you might want to learn about diodes. Diodes are a one way street so to say. You could do something similar with a switch and a diode (no relay), where you would place a diode on the line to the radio power, then provide the switched power after that. The diode would prevent the power you are feeding to the radio from "backfeeding" the rest of the vehicle's electrical system.

    The band (-) on the diode would go the direction of the radio.

  3. pmf608

    pmf608 Epic Member 5+ Years 1000 Posts

    My first thought on this was to use diodes, but I am yet to see a diode heavy-duty enough (and with thick enough wire) that I would feel comfortable putting it in line on the power for my stereo system. Although the head unit itself does not draw a tremendous amount of power, I do not want to take that chance. It was out of that concern that I started thinking of other ways, and came up with the relay, which i have a bit more faith in. To limit the number of wire I need to run between the engine compartment and interior (power for the set of 4 switches I will be installing) I would likely mount the relay somewhere on the very bottom of the dash where it has all the open space underneath. In the settings it would be used in, the doors will be open and there should be sufficient airflow there...

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