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Use a relay to turn on lights with a normally-closed circuit


Joined: Feb 12 2003
Posts: 10885

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Post Fri Sep 14, 2007 7:35 pm

This post will explain how to use a relay to turn on your lights or other accessories using a normally-closed circuit.

For example, say you want to turn on lights when your hood is opened. When your hood is closed, a pin switch is sending a constant 12v source to wherever. When your hood is open, the switch "closes" and sends zero volts. This will explain how to use a relay to turn on lights even though the switch is sending 0 volts to it.

Here is a diagram of a standard SPDT (single pull, double throw) relay's connectors:

User posted image

And an actual diagram of the relay function itself:

User posted image

Here's the connections you will use:

30: Fused constant 12v+ power source.
86: Positive current from pin switch (or other normally-closed source).
85: Ground.
87: Not used. (Block off this connector in whatever way, as it will be active.)
87a: Lights or accessories you wish to turn on.

User posted image

How This Works

When the pin switch is depressed and activated, it is actively sending 12v to the relay, thus activating the relay's coil. When this happens, the relay is sending the constant 12v power to pin 87, which is unused. That's why we block this off, so we don't accidentally blow fuses or mess anything up.

When the pin switch is released, it becomes "closed" and sends no voltage. This effectively turns OFF the coil, and switches the constant 12v over to pin 87a, turning on your lights or accessories. =)

This effect can easily be achieved using items from Oznium.com's store:
5 Pin 40 Amp Relay
Relay Harness
Pin Switch
Lights, lights galore!

Note: The pin switch above is a normally open switch, and thus will not work with the above explained diagram. However, you can achieve the same effect using the link at the end of this post.

For a quick demo on using a relay for a simple standard use of controlling one circuit with another, see ImagoX's post here: https://www.oznium.com/forum/topic10892

Joined: Aug 12 2007
Posts: 3095
Location: Fairborn, OH

Are you sure you want to delete this post?
Post Tue Jan 08, 2008 9:29 pm

Will one of you guys make this a sticky... Relays are commonly asked about around here, and even if A user doesn't do a search, it's a good reference for anyone that may be helping them...

Feel free to delete my post, or I will do it once it's a sticky...
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