12v Self reseting switch question

Hello,

I'm an officer with a small volunteer fire department. I'm not an electrical engineer. We have a rescue van that we roll on almost every call. We have a 12v light bar on the rear of the van. The light bar has a yellow rotating light at the rear center. When we are on scene, the yellow light is almost blinding. We would like to be able to turn off the yellow light when we arrive on scene. We would like to put a momentary switch some where near the rear door that will turn off the yellow center light. The catch is that we want the switch to self reset. At 4am after a long call, we don't want to add another item to our crew. When we bring the van back to the station we turn all the lights off. When the next call comes in and the van rolls, we want to be able to turn the rear lights on again from the master switch on the dash.

I've been sitting here trying come up with a diagram. My first thought is to use a switch to control a relay. But, I'm not sure how to get it to self-reset. I'm stumped. I'm open for any suggestion. Myself and the rest of our crew would be very grateful.

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mg
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You better make sure that you are allowed to modify the lights. They are intended to protect you from other approaching vehicles.

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Reply to
Michael A. Terrell

You can link the reset to the turnoff of the vehicle lights. The relay turns on in response to the pushbutton, and has a normally-open holding contact bridging the pushbutton, as well as a normally-closed contact powering the rotating light. But the whole thing is powered by the light bar voltage supply, so it drops out as soon as the light is turned off.

-- John

Reply to
John O'Flaherty

John has it exactly. Copy this to M$ Notepad or any other fixed font, print it out, and give it to your fire department's automotive mechanic. He'll be able to do this easily:

+12V GND | CRY1 | | .---------------. Lightbar | | | Lightbar | ||/ \\.-./ | |-o Control o----. ||-------( X )-----------o | | Circuit | | /||N.C. /'-'\\ | | '---------------' o-----------| | | | COM ||N.O. 1N5402 | | | T ||------o----|
Reply to
Chris

You need a relay with "change-over contacts" (ie both NO and NC)

I'd go with one with spade connectors and a mounting hole, one like that should be available at any auto parts place.

Run the relay off the same power that runs the light, when the power goes off so does the relay.

If this diagram is garbled paste it into notepad etc....

v---------> to lamp from switch __-- --+---------------o~~. | . ^----. | --- . | `--o o------+-----|~| | button | |\\| | | .--|_| | | | | `--|-----------' | -+- //// chassis/ground

another way to do it would be to powe the relay coil off the interior light when you open the door yellow light goes out, when you close it it comes back on.

Bye. Jasen

Reply to
jasen

use a DPDT (double pole, double throw) relay. when the light rack is on you can use the power from that. one pole of the Relay you will use the NC (normally Closed) contacts (this is when the relay is not powered), these contacts will be the switch that allows your power to run the lights. with a momentary push button and using the power that runs the rack (when on), you can energize the relay., the other set of contacts will close which will connect the main power to the coil of the relay at the same time. when you let go of your push button, the relay will stay energized and your light will be off because the first set of contacts are now open. when the rack is turned off, the relay will also turn off because your using the same power as the rack. so the next time you turn on the rack your relay will also be in the off position which will have the NC contacts on the first pole connected to power your light.

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Reply to
Jamie

Please explain if you want to only turn off one light in the light bar and have the others remain in operation. If this is what you want then you will need to alter the connections inside if the light bar unless each light in the bar has a seperate external connection. Either way I would suggest a DPDT relay and a button to activate it. The button should be of the N/O type and connect one side to the chassis(ground). The other should go to one side of the relay coil and the other side of the coil to the power in the light bar. Connect the o/p contact to the chassis(ground) and the n/o contact to the relay coil where the button is attached. If the Light bar has room for the relay then only 1 wire will need to exit the light bar to the button. Using the other set of points of the relay, connect the o/p to the power provided to the bar and the n/c point to the light that you want to be out. Dave

30+ year fire truck driver.
Reply to
CheapscateDave

A safer solution might be not to switch the rotating light completely off , but to dim the light by wiring two bulbs in series.

Just use the suggested relay circuit (see other posts) and wire a second lamp across the normally closed relay contacts. When the normally closed contacts open, the rotating light will be supplied via this extra lamp.

I hope the wiring for the rotating motor can be separated from the wiring of the lamp, else the motor might stall with the reduced voltage available.

By selecting different wattage lamps (or even wiring two lamps in parallel) across the normally closed relay contacts will allow you to set a comfortable brightness (dimness :-) for the rotating light.

Gerhard van den Berg CSIR

Reply to
Gerhard

Thanks to everyone for their ideas. We installed a relay and a momentary switch. This was suggested by Chris and Jason. We had to run a new wire to the light bar to control the amber light. We used a standard 20a automotive relay. It works quite well.

We put the switch just inside the door. This way someone could open the door and turn the light off. We had some of our crew try it out. Everyone is happy.

Thanks again for everyone's help,

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