horn alert when overhead lights left on.

I have a need to be able to sound the vehicle horn on various makes and models when the operator leaves the overhead lights on. I can also install a separate alarm or buzzer rather than activate the horn. Here is what I am looking to do. When the overhead light is on and the key is in the run or aux. position there is no alarm. If the key is off and the overhead light is on, the alarm will sound. If the key is off and the overhead light is off, there is no alarm. Sounds pretty simple. I would like to use standard Bosch style relays. 12v positive ground electrical system. Installing a short timer before setting off the horn would be a plus, but not a necessity. Either a negative or positive output to activate a horn or siren is fine, that is easily changed.

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Blown 1972 Ford F-250 4x4

A lights-on buzzer can be done with the buzzer (or relay) between the light's +12V switched side and IGN +12V.   
A diode may be needed to prevent it triggering with IGN on and lights off.

That is essentially what I am looking for, except by using a relay I have many more options and easier to control.

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Blown 1972 Ford F-250 4x4

Ignition to terminal 85 of the relay. Positive wire from lights to terminal 86 of the relay. Anytime the lights are on without ignition being powered the relay will energize and connect terminal 30 to terminal 87.

Thank you, I knew it was simple, I think for some reason I was overthinking it and trying to install a diode to prevent flash voltage from messing with the ignition

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Blown 1972 Ford F-250 4x4

You will need that diode Oldspark mentioned.

i am an idiot wrote:

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Ignition to terminal 85 of the relay. Positive wire from lights to terminal 86 of the relay. Anytime the lights are on without ignition being powered the relay will energize and connect terminal 30 to terminal 87.

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Am I missing something, but if you supply power from the light to 85 and ignition power to 86, what grounds the coil to make it switch and what is connected to 30, 87 & 87a? I hope I am not missing something obvious here.

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Blown 1972 Ford F-250 4x4

IGN off means the switch IGN side is GND.

This will only work on vehicles that don't feed the light 12vdc constantly. However, I believe the majority of vehicles switch the (-) side of the overhead light.

How many of these are you looking to do? It would be pretty simple to make a small circuit to do everything you want - check for ignition, accessory, + or - light input, and control a small beeper that has a short time delay.


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Kevin Pierson

KPierson wrote:

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This will only work on vehicles that don't feed the light 12vdc constantly. However, I believe the majority of vehicles switch the (-) side of the overhead light.

How many of these are you looking to do? It would be pretty simple to make a small circuit to do everything you want - check for ignition, accessory, + or - light input, and control a small beeper that has a short time delay.

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I can install a power tamer I use on emergency vehicles, they are just a little expensive for the general vehicles. I would hook them up on the ignition of the vehicle, but often times the trucks are parked along or in the road and need the lights on. Trying to conserve fuel, shutting the vehicles off is our goal since we use LED lights, we can get away with that. As for the number of devices, I would like to start installing one in every new truck we put together if the price is right, that is why I would like to use a simple relay setup. I just need to stop long enough to draw it out on paper I guess. The light I use most is a short LED light bar and I switch the 12v power side.

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Blown 1972 Ford F-250 4x4

so the overhead lights are exterior lights on the top of the vehicle? I thought you were referring to dome lights!

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Kevin Pierson