circuit that supplys voltage when 12v removed

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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=120876
Printed Date: May 01, 2024 at 11:59 AM

Topic: circuit that supplys voltage when 12v removed

Posted By: trekkerjmm
Subject: circuit that supplys voltage when 12v removed
Date Posted: March 22, 2010 at 2:32 PM

Hi . I am a new member to the forum and have been around electronics for many years but need a little help with this one. I did a search and looked through all the relays I found on the website but cant find what I'm looking for. I am looking for a relay that will supply 12v to a buzzer when 12v is removed. What I want to do is when my fog lights are on I want a reminder that they are on when 12v is removed (shut off key. I have a buzzer that I want to energize. Something similar to a headlight or door open buzzer. Thanks...Tj

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As Always Thx for the help...Tj

Replies:

Posted By: Ween
Date Posted: March 22, 2010 at 2:55 PM

hi,

you'll need a relay (spdt bosch type will be fine). connect the positive side of your buzzer to your fog lights (+ term). connect the negative side to NC (term 87a) of the relay. connect ground to C (term 30) and coil-(term 85). connect ignition to coil+ (term 86). the relay will give a ground to the buzzer only when the ignition is off, and the foglights will supply power when the are on.

you could just have your switch that triggers the fog lights or fog light relay connect to ignition, then they'll turn off with the key.

Posted By: trekkerjmm
Date Posted: March 22, 2010 at 3:11 PM

Thank you very much. I will build it tonight...Tj

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As Always Thx for the help...Tj

Posted By: dualsport
Date Posted: March 24, 2010 at 8:02 PM

You can do it without any relays- if your buzzer is low current (I'll assume you're not going to use a car horn or anything extreme like that)
you can just connect it with a diode in series between the fog light power and ignition circuit.
When the ignition is off, it'll provide a virtual ground, and if your fog light is on, it'll provide 12V. The 12V through the diode will buzz your buzzer until you switch the fog lights off.
The diode is to prevent your buzzer from going off when you have the ignition on and the fog lights off.

Posted By: trekkerjmm
Date Posted: March 30, 2011 at 12:53 PM

Hi I had asked a question a long time ago on building a circuit to remind me when I leave my fog lights on with a standard relay for switching and you were very helpful but I lost the diagram and need to make another one.

I have a 12+ wire running off the fog lights and it went to the center of the relay (#87a) and I want the buzzer to go on when I shut the ignition off so I know they were left on and I want the buzzer off when 12+ gets put to it. I had it working great but someone took my console out and disconnected everything. There are 3 or 4 wires running to it if that helps...Thx

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As Always Thx for the help...Tj

Posted By: howie ll
Date Posted: March 30, 2011 at 4:53 PM

Why a relay? 12v+ power to fog lights, join and take to pos side of buzzer, neg side to driver's door switch, assuming the pin is a switched negative.

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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 30, 2011 at 5:05 PM

Buzzer to #87a (other end GND)
+12V to #30.
#85 to GND
#86 to the switched +12V (the one that when off, you want buzzer on).

But you shoud also be able to connect the (low current) buzzer with +ve to +12V and its -ve to that same switched source (#86 above) - hence saving the relay. (Assumes low current buzzer -say 250mA or less....)

Posted By: mtrhd329
Date Posted: March 30, 2011 at 6:06 PM

N-66.

Posted By: howie ll
Date Posted: March 30, 2011 at 6:38 PM

Finally n-66 and I thought the answer was 42.

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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 30, 2011 at 8:30 PM

42 was the original answer. It's a different question this time.

And yes, my answer/reply above is the same as yours. (The old "using load as an earth/ground" or "virtual connection" solution - brilliant, yet so often overlooked!)