using a ground to break connection

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=133925
Printed Date: May 14, 2025 at 4:54 PM

Topic: using a ground to break connection

Posted By: epicminis
Subject: using a ground to break connection
Date Posted: March 25, 2013 at 2:15 PM

I'm probably overthinking this, but I need help to temporarily break a connection with a ground and then have the connection returned when the ground goes away.

Here's what i'm trying to do:

I have emergency lighting on a pickup truck (hide-away LEDS in taillights). What I want to do is have a pin switch mounted so that when the tailgate is closed - lights function normally. When I open the tailgate, pin switch grounds and turns lights off. When I close the tailgate - lights come on again. Basically using the ground as a secondary on-off switch.

I have played with the RAP relay without diodes and it is basically what i'm looking for with the exception being that it does not re-energize when the ground is removed.

Any ideas would be appreciated.

Replies:

Posted By: howie ll
Date Posted: March 25, 2013 at 2:56 PM

This, one 5 pin (87a) relay.
break_a_circuit.bmp

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Posted By: oldspark
Date Posted: March 25, 2013 at 6:00 PM

Noting that in Howard's break_a_circuit.bmp, the #30 & #87a can be in the light's +12V feed assuming the lights are switched with a +12V switch.
The +12V feed to #86 would also be taken from that switched +12V source. (Hence the relay is only activated when the lights are on.)

I can't find a suitable diagram in the12volt's Relay reference pages (surely one exists?!), but there is an equivalent by HotwaterWizard noting that its 86 will go to the +12V supply (eg. #30) and the pin (key switch) is to GND...

posted_image

FYI - the way it works is that 30 to 87a is NC (Normally Closed {connected}) when the relay is de-energised.
When the pin switch connects (tailgate open), the relay energises and switches 30 over to 87, hence breaking the "normal" feed to the lights.

Posted By: epicminis
Date Posted: March 25, 2013 at 8:03 PM

Thanks guys! That will do exactly what I need.

As suspected, I was overlooking the less complicated solution and trying to make it more complicated than necessary.

Posted By: oldspark
Date Posted: March 25, 2013 at 8:48 PM

LOL - fear not. Though somewhat gifted in the simplification of complex solutions, I have many times had amazing confustigations only to have the lot wasted by a later "simpler" realisation. (Pity, some confustigations looked quite expert!)

Relay or switching issues are often solved when realising "inversion" - eg, your "off when on" using 87a.
A similar solution is often "logic inversion" - eg, switching grounds rather than +12V.
Phantom(?) grounds is another example - ie, connecting a relay coil to +12V and the other end to a bulb's +12V so the relay turns off when the bulb is on (eg, fogs off when beams on).

Some are more "system" realisations, one of my favorites being my realisation how flawed electric fuel pump control by oil pressure (switches) is. That has IMO an interesting history, though I find the reaction of others to be somewhat even more interesting - or should I say perplexing? (Though as I found, it can be very tricky to understand...)

Posted By: howie ll
Date Posted: March 26, 2013 at 2:16 AM

Don't worry, job done.
I give you the simple elegant answer and Oldspark explains.
I'm the pro installer, he's the teacher.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.