I want to start a circuit in my car when I insert my key in car ignition and let it turn off when the key gets removed. I have Honda Civic 2007. Can someone just help me out as to from where do I take 12v connection which switches ON when a key gets inserted and switches Off when key gets removed. NOTE that I don't want to wait for a key to turn On the main ignition i.e ACC.

Any ideas?


I'm sorry I'm not any expert in electronics or electrical system. I'm a professional software developer and this is my hobby project. Actually my car security system doesn't allow the ignition system to On until I press a provided button switch hidden in console below the steering wheel. So I got an idea to replace that switch with an RFID card system. My RFID card system was a success and I tested that and everything works fine. But my system is connected all the time since I connected it with the main 12v line. Which I don't want. I only want it to run when I insert key so I could touch card which will ultimately switches On the main ignition system. And then only turn my system Off when I remove the key from the ignition system.

  • Is there anything right now that turns on when you just insert your key?
    – Dejvid_no1
    Commented Feb 10, 2016 at 17:43
  • yes, my odometer area lits up a little. And when I remove my key it gets dimmed down and turn off ultimately.
    – Humayun
    Commented Feb 10, 2016 at 17:45
  • There probably is a switch. Does your car warn you if you leave the key in the ignition? But knowing how to connect up to that switch is specialized information. Maybe nobody on this forum will know exactly how to do it without experimenting with it. Maybe the information would be in a wiring diagram for your car.
    – mkeith
    Commented Feb 10, 2016 at 17:46
  • Exactly, car beeps if I leave the key and opens door.
    – Humayun
    Commented Feb 10, 2016 at 17:47
  • 1
    Here's a bunch of a good info for multimeters. You'll need to use one to validate your getting the right circuit and it actually get's power when you turn your key on before you do all of the wiring. mechanics.stackexchange.com/tags/multimeter/info Commented Feb 10, 2016 at 20:40

3 Answers 3


A pretty safe fuse protected accessory circuit in your car is the one that powers your audio system. According to this information, you're looking for the following wires:

Car Radio Switched 12v+ Wire: Violet
Car Radio Ground Wire: Black

By tapping into these wires, you can safely add a low power accessory circuit.

Alternatively, if you need more power than say, 1A, I usually use a piggyback fuse adapter like this:
Piggyback fuse adapter

There are a few styles of this kind of adapter available to match the fuse type in your vehicle. These adapter let you remove an existing fuse, insert the adapter and insert the original fuse and a new fuse for the new circuit that can be powered from the new flying lead. Find a fuse that's only live when the ignition is in ACC mode (audio system radio fuse perhaps?).

If you really want to enable your new accessory circuit only when the key is inserted but not even turned to ACC yet, you will need to sense the "key inserted" signal. This signal is pulled up by the MICU and grounded by the key inserted switch. Here's the schematic: immobilizer 1 of 2 immobilizer 2 of 2

You could add a small +12VDC relay that's connected to "always hot" and protected by a fuse and activated by tapping into the Pink "key detect" wire which goes to ground when a key is inserted. This relay could then provide the power to your new accessory circuit. I would avoid adding a new 1A circuit directly grounded by the "key inserted" switch since this tiny microswitch is not designed to carry significant power but rather only as a small signal switch to ground to be detected by the MICU.

  • I like your idea but I don't want to wait for the ACC mode ON. When you insert key in ignition a little light lits up in odometer. Which indicates that something knows that a key has been just inserted and when I remove the key that light goes Off. So basically I need to find that switch or fuse in the fuse box and then I can use this adapter.
    – Humayun
    Commented Feb 10, 2016 at 18:16
  • And my circuit only needs maybe 500-900 milliamps and won't be needing more than 1A.
    – Humayun
    Commented Feb 10, 2016 at 18:18
  • Exactly what light lights up in the "odometer"? do you mean the instrument panel? Is it the security immobilizer light with a key symbol?
    – BartmanEH
    Commented Feb 10, 2016 at 18:34
  • modified answer to include information on "key detected" switch. now that it's been moved to this new forum, I need upvotes here to get enough reputation to comment everywhere.
    – BartmanEH
    Commented Feb 10, 2016 at 19:22
  • poking around at just the fuses (as suggested by other answer) will not achieve what you need. even if you find the fuse for the circuit that lights up the "odometer" (instrument panel?) light, it will be live all the time and the indicator light controlled by the MICU instead.
    – BartmanEH
    Commented Feb 10, 2016 at 19:25

To find the correct circuit you could probe the fuses. Check the voltage or insert an ampere meter instead of the fuse while inserting the key. Beware that you might blow the fuse of the ampere meter... My bets are on No 10 or 9:

enter image description here

  • That looks something really helpful. I will try probing them and then get back. I'm hopeful now thanks.
    – Humayun
    Commented Feb 10, 2016 at 19:05

It's been a while since I've played in the 12v world but there will be a line (probably provides ground but possibly is powered) that is used to signal the "key reminder". This is the ding that goes off when you forget key in the ignition and open the door. For this to work there is a circuit closure at the ignition switch/assembly just by putting the key in. Find this circuit and you'll have your signalling line.

If that line is just a ground (or a lower voltage i.e. just for signaling), then use that line to trigger a small relay and have the relay switch through the 12v you need.

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