led lighting on 3 volts need relay

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Forum Name: Vehicle Wiring Information & File Requests
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URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=103100
Printed Date: September 11, 2025 at 12:49 AM

Topic: led lighting on 3 volts need relay

Posted By: dencohe
Subject: led lighting on 3 volts need relay
Date Posted: March 15, 2008 at 10:20 PM

I am making something for my car and need a relay to have the lights go on when the connection is broken or open. The led runs on 3 volts and the coil for the relay can run from 1.5 to 9 volts. I have got relays from radio shack but the SPDT does not work for some reason i run this one https://www.radioshack.com/product/index.jsp?productid=2049716&cp=2032058.2032230.2032277&allCount=24&fbn=Type%2FSPDT&f=PAD%2FProduct+Type%2FSPDT&fbc=1&parentPage=family on a 9 volt battery and it wouldn't work to turn on the light when the connection is broken. Anyone have any ideas on how to do this with a relay that would work on a 1.5-9 volt battery to light a LED?

Thanks

Replies:

Posted By: techman93
Date Posted: March 15, 2008 at 10:54 PM

I believe relays that are designed for 12v application need a car battery 12 volt to operate properly. A regular alkaline battery is not strong enough, let me phrase this differently, the current of the battery not the voltage matters in this application when you look at the current output capability of an auto battery vs. that of a duracell or energizer type battery. if you wire two 1.5 batteries in series and a switch it will work. I would say even a 9 volt battery will be the same, not enough current to operate the coil.
This is just my opinion based on logic.

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The wire I'm test'n isn't doin' what it's supposed to be doin'... I am so glad I printed that tech sheet, with the wrong info.
Do it right the first time... or I might have to fix it for ya

Posted By: KPierson
Date Posted: March 16, 2008 at 8:14 AM

What are you using to power the LED? When you hook the relay from Radioshack up to a 9vdc battery can you hear it click?

That relay should work just fine off of a 9vdc battery. However, if you are using the 9vdc battery to power the LED you need a current limiting resistor in line with the LED, otherwise you'll burn up the LED very quickly.

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Kevin Pierson

Posted By: dencohe
Date Posted: March 16, 2008 at 9:36 AM

I have two different circuits for the led and one for the coil, 9vdc bat for coil and two d cells for the led, is the coil going to overheat if its just a 9 volts directly connected.

Posted By: KPierson
Date Posted: March 16, 2008 at 3:04 PM

The coil will be fine with 9vdc applied directly to it, that is what it is for.

Can you get the LED to light without the relay?

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Kevin Pierson