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The reason behind this question has been asked (and answered) countless times: How to make an automotive dashcam, that plugs into a 12V accessory socket, switch off when the engine is not running. (Or less ideally, switch off when "delayed power-off" accessories like the radio, turn off.)

Most of the answers involve snipping the wire to the 12V accessory socket, and either hackishly tapping it into, say, the radio power - or a little more professionally, running the hot wire to the fusebox, and tapping into an appropriate fuse (possibly using a piggyback device to make two fused circuits out of what was originally one). The reason those options are unpalatable, is:

  1. I've had a few cases of bad luck [or possibly incompetence] running wires of much length through cars, especially near the dashboard where things get dicey [for me].
  2. I don't want to take any part of the car interior apart [again - bad luck with broken hidden fasteners etc.].
  3. I don't want to make lasting physical changes to the vehicle, including routing new wires in and around the dashboard, or through the firewall.

Hard-wiring the device itself to the fusebox is another commonly recommended option. This won't work in my case because I have more than one device that all need to power off with the car, but also must be able to change vehicles. (It's OK if those vehicles have always-on accessory sockets since I don't use them often, e.g. rental cars.)

While researching options, I've formed this idea that seems easy and completely un-original. But after about 10 hours of research, I haven't found an answer to why this idea wouldn't work, nor any mentions of it, much less a how-to guide:

In only the fusebox area, run one or more wires in such a way that the "accessory sockets" circuit is not "always on", but is instead actually powered, or controlled, by a different circuit that is explicitly switched.

Most cars, including my Ford Edge, have both switched and always-on fuse locations. The accessory sockets circuit is unfortunately, inexplicably, "always-on". Not only does this wind up draining the battery (slowly), it also chews through SD memory cards, which have very limited write cycles, even the high-endurance variety. (And it seems I'm always forgetting to unplug the dashcam.)

I can can see a couple of ways of this working. (In my layman's understanding.)

Both involve a first step, of turning one "switched" circuit into two, using one of those $3 doohickies that replaces a fuse, with a stacked rig with two fuse slots: one for the original circuit, another for a new circuit, that has a hot wire just hanging off of it. I'm sure this term is probably incorrect, but I'll call that new circuit, the "cloned" circuit. (Obviously it's not even a "circuit" per-se, with just a hot wire.)

In this scenario, that cloned circuit is "switched". (Ideally, with the ignition. Less ideally, based on a timeout, like the radio.) Options from that point on:

  • Option 1: Bypass the "always on" accessory socket fuse altogether, while still powering the things connected to it - with "switched" power.
    • Method: Permanently remove the accessory socket fuse. (Throw it away, whatever.) Take the wire from the "cloned" circuit, connect it to a fuse-tap (or some conductive pin), that is then inserted into the second half of the now-permanently-interrupted "always on" fuse.
    • Hopeful result (?): Anything connected to that "always on" fuse (in this case, accessory sockets and in turn the devices plugged into them), will now instead be powered by the cloned "switched" fuse.
  • Option 2: Turn the "always-on" circuit on and off, with a relay controlled by the "switched" circuit.
    • Method: Run the hot wire from the cloned "switched" circuit, to the hot control pin of a relay. (Sorry if that's poor terminology.) Then wire the "always on" fuse block, to go through the relay. (With an inline fuse.)
    • Hopeful result (?): The accessory sockets are still powered by the accessory sockets circuit, and still fused by the same fuse, but is interrupted when the controlling circuit (from the "switched" source) powers off.

Option 1 seems much easier. But perhaps that "switched" circuit could be overloaded? And/or, possibly carry a higher risk of RF, electrical, or other crosstalk between the parallel circuits? (My dashcam for example produces RF interference, makes FM radio almost useless.) In many hours of research, I've found zero mentions or descriptions of such an option whatsoever - much less a how-to guide. (And I'd be very reluctant to just "try" this idea, without at least hearing that it isn't completely stupid or dangerous. And ideally, how to specifically do it!)

Option 2 seems more sane, as the circuits remain separate where they are meant to be separate by the factory. But I don't know how to cleanly/quasi-professionally tap into fuse connectors, in order to effectively insert a fuse and relay in between, instead of just a fuse. (The answers on Google seem crude and/or not very physically stable over time.)

I realize I may be betraying that I have no idea what I'm talking about. (In spite of believing I at least have some idea.) I also realize that it seems I have conflicting asks: quick and easy without mucking with the interior, and "cleanly/quasi-professionally". The first one definitely trumps. I also realize the inside of the fusebox will look very messy and ugly - but the point is that I could easily just unplug stuff, put the original fuses back in, and voila - back to stock - in the case of hassle-free warranty service, or selling the car.

So mainly it boils down to this:

  1. Are Options 1 and 2 even viable?
  2. Is there anything about either approach that experts like yourselves would say, "That is a really bad, stupid, potentially harmful, and/or dangerous idea"?
  3. What basic components would you recommend using? (E.g. how to securely tap into an unpopulated fuse post in a way that won't wiggle free over time, and/or risk arcing? What relay would be needed for Option 2 - for say, a 15 amp fuse?

The vehicle is a 2013 Ford Edge.

FYI, I'm not an electrical engineer - not by a long shot. I have a limited understanding of electrical engineering terminology and common phrases. I work in software. But I do have a fair amount of experience designing and wiring up complex circuits for various uses, including coils, multi-switches, potentiometers, etc. (Just by self-taught trial and error without knowing much in the way of terminology.) I know what fuses and relays are used for, and know how to use a multi-meter and soldering iron. I understand the relationship between voltage, current, and resistance (when necessary, with a quick Google refresher).

Many thanks!

migrated from electronics.stackexchange.com Sep 11 '17 at 22:11

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

  • Yes, it's possible, and yes, it's not recommended. You say you're not happy with answers Google gives you, because they are "crude". Arguably, plugging anything at all in your car's fusebox (other than fuses) is crude. – Dmitry Grigoryev Sep 11 '17 at 10:27
  • What I meant by that sentence and context, was specifically that the solutions for tapping into a fuse post were crude. But the third-party product solution for "cloning" a circuit, each with their own fuse, seem pretty elegant - or at least, not crude. It's sturdy, locks in with the same sureness as a fuse, and doesn't seem like it would wiggle loose or corrode. That's separate and apart from the issue of mucking about in the fusebox as being "crude", which I agree with, but am totally OK with. (In part, like I pointed out, because of the ease with which it can be undone, even temporarily.) – Jim Sep 11 '17 at 15:32
  • Both of your questions make it seem as if you didn't actually read the post. Granted, it's long. One of the reasons I didn't post a schematic, is because "I'm not an electrical engineer - not by a long shot. I have a limited understanding of electrical engineering terminology and common phrases..." I would think that would also imply, knowing much or anything about EE pictograms and symbols. – Jim Sep 11 '17 at 16:35
  • You may want to check out this page about what kind of questions could be asked here. If you don't want to understand electrical terminology and schematics, you should ask elsewhere. Like I said, a car-related site could be a good start. – Dmitry Grigoryev Sep 11 '17 at 16:53
  • What you are suggesting would result in the dreaded black hole that it seems so many on stack exchange sites argue over. "Site A said it should go in B. Site B said A." When a more productive solution might be to not demand zero overlap. If the question has a fair amount of relevance, great. If you think it's completely OT, then state that opinion first, rather than after giving up. As I said, I've done much research on this. Including car-related stackexchange sites. Like me, they don't seem to have the understanding of circuits, fuses, and relays necessary to answer this basic EE question. – Jim Sep 11 '17 at 17:02

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