Can this be done? Low voltage question... (12 volt DC)

[Tim The Tool Man]

Long story short, I bought a new service truck and I am mounting one of those 48" LED light bars over the back doors for those times when I am working later in the evening or loading the truck and night. It will have a dedicated circuit & switch inside the door but I am now thinking that it might be nice to have it come on when I put the vehicle in reverse (might not be legal, but that is a different story...) Anyhow is there a way to activate it via the reverse lights with a relay as the reverse gear is engaged and with a switch in the back of the truck without crossing the circuits? I would guess it might be bad for the reverse lights to turn on when the light is switched on...

Thanks in advance!
-Tim

 

[gayler]

Sure, just find the wire to the backup lights and use it to active the relay for the LED light. I'm sure it wont be legal tho.

 

[Beemer533]

I'm on my phone, so I can't draw it out, but it would be pretty straight forward I think. .

All you would probably need is a double pole, double throw (on-off-on) switch and a standard 12v Bosch style relay. You might possibly need a couple diodes..

Basically you would cut into the reverse light circuit with the switch; one throw it would operate just the work lights, the other throw both would come on in reverse. The center (off position) would operate normally (just reverse lights).

I'd have to draw it out to make it work...

Sent from my SAMSUNG-SM-G900A using Tapatalk

 

[simpler=better]

I'm on my phone, so I can't draw it out, but it would be pretty straight forward I think. .

All you would probably need is a double pole, double throw (on-off-on) switch and a standard 12v Bosch style relay. You might possibly need a couple diodes..

Basically you would cut into the reverse light circuit with the switch; one throw it would operate just the work lights, the other throw both would come on in reverse. The center (off position) would operate normally (just reverse lights).

I'd have to draw it out to make it work...

Sent from my SAMSUNG-SM-G900A using Tapatalk

This, or just have it on it's own switch. You won't need it every time you back up, and I'm sure you won't ever get a ticket using it when you need it.

 

[gregtwojeeps]

Hmmm ,using a relay that is engaged only when you put the truck in reverse, means the truck would have to be left in the reverse gear while loading it to use the LED's...not safe.

A separate manual switch could accidently be left on while driving down the road, not good. Thinking here...thinking....

In conjunction with a relay used in the back up light circuit... how about a continuously hot circuit wired in to a manual switch then wired thru a door jamb switch located in the back double doors.... so when the doors are closed the light is off /open the light is on. ?

 

[imagineer]

I'm going to do something similar on my Jeep TJ, put high output LED work lights into the rear bumper to serve double duty; as really bright back up lights and independently switched work lights.

Because the LED work lights I will use pull more amps than the OEM incandecents, I'm running them through a relay. The switched (work light) 12V power wire will be attached to the normally closed contact on the relay. (Note, this is a hot lead off the battery, not switched.) The 12V for the lights when being used for reverse lights will be attached to the normally open contact on the relay. The 12v that would normally power the reverse lights will be run to the coil on the relay.

The idea is that under normal conditions, the "work lights" will have hot 12v power via a manual switch off the battery. When I put the Jeep in reverse, the coil on the relay will energize, closing the normally open circuit (and opening the normally closed circuit), thus illuminating the "reverse lights".

It opens the possibility for accidentally leaving the "work lights" on while driving the vehicle, so I'm going to borrow from my "SLOB mod" done on my motorcycle (Signal Left On Beeper), and via another relay, control a small piezo beeper that will sound if I leave the work lights on when the Jeep is running. The coil to this second relay will be energized by a 12v switched supply.
I suppose I could still accidentally leave the lights on while driving, but there would be an annoying beeper letting me know.

I thought also about simply running the 12V hot power for the work lights via the normally closed contact on the 2nd relay, and then have this relay open when the Jeep is running, but I'd rather have the flexibility to be able to turn on the work lights whether the Jeep is running or not.

 

[checkthisout]

Lots of different ways to do it.

A relay will isolate the two circuits from each other. It wouldn't matter in either case if both activated at the same time i.e. it wouldn't be a short circuit or anything bit you don't want the manual switch activating the reverse lights.

 

[nadogail]

With some relays you:
1. Could have the LED's automatically energized when backing up.

2. Trigger an indicator when energized while the Back Up lights are off.

 

[iron block]

You can do it with two ordinary automotive relays.

One coil terminal of the first one goes to the ignition hot lead, the other coil terminal goes to your "Manual override switch", which connects it to ground. So when you turn on the switch, the first relay gets activated.

The wiper arm of that relay goes to the battery +12 terminal. The normally-open contact goes to your LED light bar. So when the relay is activated, your light bar gets +12. So far, so good.

Now the second relay: one of its coil terminals goes to your backup light hot lead. The other terminal goes to ground. So when your backup lights are lit, this second relay gets activated.

The wiper arm and and normally-open contacts of the second relay connect directly to the same contacts on the first relay.

Done. The LED gets +12 from the battery when either the override switch is ON, or the backup lights are ON.

(If your backup lights turn on with a "low-side" switch, the logic is slightly different. Principle is the same, though.)

It is really pretty simple. This write up probably makes it look a lot worse than it really is. Good luck.

 

[TheEquineFencer]

Run a new fuse protected wire for power to a relay to the Common on a relay. Install it so a relay can energize the lights by way of you B/U lights as planned. The B/U lights will only operate the relay and no power from the B/U lights actually power the LEDs. Then run a wire from the feed from the LED light power wire, through a rotary manual switch to also power the LED lights, but on the switched side, Normally Open contact, that actually powers the LED lights. Then you cannot forget and leave the lights on and there's no way it can back feed to the B/U lights.

If you have it in Reverse, the relay energizes, connects the Com to the NO when it picks up and the LEDs come on. If you have it in Rev and have the Rotary switch on you'll have the Com and also power to the NO at the same time and it still will work and not short anything out as they are both powered from the hot wire landed on the Com.

If you turn the rotary manual switch on, it put power from the Common, bypasses the relay function and puts power to the LEDs.

 

[bczygan]

Don't know why it would be illegal if you just use it when you need light for backing up.

I had a truck with a spotlight bar with one set facing forward and one in reverse. Different switches for different uses. Very handy for backing up in the dark. Saved me from backing into a river once.

Bill

 

[checkthisout]

Like this:

View media item 61458
With the above you can:

1) Turn the light off from inside the cab in case you leave the switch in the work area on or prevent it from turning on whenever you want like backing up or whatever.

2) The light will turn on with reverse

3) The light will turn on with the switch in work area

4) The reverse lights are prevented from turning on with the switch in the work area.

You can get a lot more fancy than this but this keeps it's simple and I think gets you where you want to be.

 

[Ole Slewfoot]

You know those things are blinding in the day right? Auto on in reverse is a fools mission if you ever drive on public roads.

 

[Tim The Tool Man]

Thank you for the replies! I planned to do it just like checkthisout and others have suggested but I was having a brain fart and wasn't sure the relay would back feed to the reverse lights if i had the light on while the truck was parked. I just didn't want to somehow risk blowing my PCU...

I'll post up an image of the setup in a week or so once it is installed. I'm still waiting for it to be delivered at this point.

Thanks!

 

[gregtwojeeps]

Checkthisout done good

I still like my idea of running the manual switch circuit thru a door jamb switch for the back door as a extra precaution . At some point when loading or unloading ...you may get distracted, shut the door with the lights still on and drive off. JMO though. Good luck on your project !

 

[TheEquineFencer]

Thank you for the replies! I planned to do it just like checkthisout and others have suggested but I was having a brain fart and wasn't sure the relay would back feed to the reverse lights if i had the light on while the truck was parked. I just didn't want to somehow risk blowing my PCU...

I'll post up an image of the setup in a week or so once it is installed. I'm still waiting for it to be delivered at this point.

Thanks!

Basically the same thing....

 

[Tim The Tool Man]

Checkthisout done good

I still like my idea of running the manual switch circuit thru a door jamb switch for the back door as a extra precaution . At some point when loading or unloading ...you may get distracted, shut the door with the lights still on and drive off. JMO though. Good luck on your project !

Basically the same thing....

The problem with a back door switch is that it is a box truck and the back door generally does not always close tight. It closes and latches, just not always 'tight' in a way I could reliably count on the switch cutting the circuit. The rotary timer switch is probably my best option because it will eventually shut itself off.

 

[manwithtools]

Couple of comments:

You need a fuse at the source of power supply.

I would also want a master switch that is light indicated. That way you would know to turn it off when driving on the road - just in case you had the remote switch turned on - this would prevent you from blinding people in a parking lot at night unintentionally.

Be sure all the switches are rated for the amperage the light will draw, you may need to use 30 amp relays to turn the light on and off from the lower rated switches. Same goes for wire size.

Be aware that leaving the light on for an extended period could leave you with a dead battery. Seems obvious I realize but first time it happens will be a disappointment for sure.

 

[TheEquineFencer]

The problem with a back door switch is that it is a box truck and the back door generally does not always close tight. It closes and latches, just not always 'tight' in a way I could reliably count on the switch cutting the circuit. The rotary timer switch is probably my best option because it will eventually shut itself off.

Yep, I always try to wire things to make it "idiot proof" , no offence to the OP.

If the OP uses a relay, I'd recommend one with Diode protection, free wheeling diode, to protect the vehicle PCM from a voltage spike when the coil circuit in the relay drops out.

 

[sammer]

5 pin relay...
Fused cab switched power to #30, B/U light circuit through 85-86 to ground, 87 to lights, 87A to switch at rear to lights.

Put an indicator light (led) on the main power wire in the cab after the switch if you're worried about forgetting they are on.

Only one relay!

sam

 

[gtcs]

I just put mine on an illuminated duckbill switch that would annoy me when driving...

I have to ask though: where are you people driving in reverse on public roads so much? Every other work truck here has some kind of worklight/reverse light on it and its not like its that big of a problem, in fact I can't think of a single instance of seeing one of them on on the road

 

[larry_g]

If your worried about driving then add a second relay that is triggered by the neutral safety switch. That way you will have no power to the light if in gear.

lg
no neat sig line