reverse function relay

Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=92356
Printed Date: July 22, 2025 at 10:07 PM

Topic: reverse function relay

Posted By: disker
Subject: reverse function relay
Date Posted: March 31, 2007 at 10:17 PM

Greetings,

I have an alarm that has a (-) output when armed, and I am wanting to have it open a relay when armed. I can do this with a normal relay but there would be current draw for the time the vehicle remained with the alarm active. Are there any types of relays or circuits that I can apply the (-) output to and have it open a circuit where there is no current draw? Current draw with the (-) output not applied is of no concern.

I think I know what the answer is going to be, but you never know unless you ask, right?

THANKS!

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Disker

Replies:

Posted By: KarTuneMan
Date Posted: March 31, 2007 at 11:35 PM

What are you using the relay for?

And....what kind of kites do you fly? posted_image

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Posted By: disker
Date Posted: April 01, 2007 at 8:34 AM

I have an accessory that I would like to turn off when the alarm is acticated and turn on when the alarm is deactivated. The easiest way I can think to do this is with the (-) out when armed connected to a relay and use it to break the ground wire in the accessory's power circuit.\ when triggered.

The only problem is the current draw from the relay.

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Disker

Posted By: disker
Date Posted: April 01, 2007 at 8:37 AM

Oh -and I fly the 2 stringed stunt kites.

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Disker

Posted By: KPierson
Date Posted: April 01, 2007 at 1:43 PM

What is the device you are trying to power?

You should be able to do this with a transistor (low current device).

Go with a NPN transistor and wire the base to the (-) when armed wire. Also connect one side of a 10K resistor to the base, connect the other side of the resistor to 12vdc (or ignition if you only want it to work when the ignition is on).

Then, connect the emitter to ground and the collector to the (-) side of a relay. Use a diode across the coil and hook the relay up to the 12vdc source.

Wire up the contacts and you should have it.

When the (-) when armed signal isn't present current will flow through the 10K resistor to the base of the transistor. Since it is a NPN transistor the current will cause the transistor to conduct current from the emitter to the collector, thus powering your relay.

As soon as you arm your alarm the (-) when armed ground will turn the 10K resistor in to a load resistor and apply a (-) to the base of the transistor. This will cause the transistor to turn off, dropping your relay out.

You will still be using a relay when the alarm is disarmed, so you may want to consider using ignition power. Or, if the current is low you can possible eliminate the relay all together.

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Kevin Pierson

Posted By: disker
Date Posted: April 01, 2007 at 8:11 PM

WOW! That sounds VERY close to what I am needing!!!!! AWESOME!

Many thanks!

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Disker