12v constant on/off to momentary on
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=10101
Printed Date: July 19, 2025 at 9:08 PM
Topic: 12v constant on/off to momentary on
Posted By: Kalus
Subject: 12v constant on/off to momentary on
Date Posted: February 26, 2003 at 5:04 AM
I need help with closing a circuit (A-B) momentarily when the ignition is switched on, and again when it is switched off. I came up with this... not knowing much about any of it, can anyone tell me if it will work??? or make any suggestions??? The A-B circuit is just a low current/voltage (5v). 
Replies:
Posted By: jrilla
Date Posted: February 26, 2003 at 1:36 PM
explain a little further what you are working on: What is your input? It is easier to know if it will work if we know what you are trying to accomplich with a little more detail.
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J Rilla
Owner/Installer
Posted By: Kalus
Date Posted: February 26, 2003 at 3:07 PM
I am trying to startup and shutdown a PC in the car with the ignition. So the A-B circuit is the power switch to the motherboard. The switch on the PC is a momentary switch (ATX power supply). It has to be a contact of less than 3-4 seconds. So, when you turn the key it 'presses' the button and again when you switch the ignition off. I hope I explained this adequately :)
Posted By: TomEllis
Date Posted: February 27, 2003 at 1:31 PM
In the drawing that you show, the Cap. will never drain.
You have constant voltage going to the second relay regardless of the first relay. The coil of the second relay would keep the Cap. charged.
Tom
Posted By: Sonic2002GT
Date Posted: February 27, 2003 at 9:51 PM
This is very similar to what I need, but am VERY new to relays and don't have a clue how to do this... My car is a 2002 Mustang GT... Ford, for some awful reason, put traction control in the car, and you have to push the button every time you start the car to turn it off. What I want to do, is hook up the ignition output to a relay, which will send a short signal to the traction control unit to tell it to go the heck away :) Please list for both negative output and positive output needed. If someone could explain in detail how to do this, I would love you forever... Or at least be very thankful! Also, if theres a good resource on relays (other than here) that would be nice too, that way I can figure this stuff out myself :D
Posted By: jrilla
Date Posted: February 28, 2003 at 8:16 AM
If it is so horrible, you can always just unplug the wires from the switch.
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J Rilla
Owner/Installer
Posted By: Sonic2002GT
Date Posted: March 03, 2003 at 5:56 PM
No, you can't. If it were that easy, I would have already done it. It isn't reliant upon the switch... The switch sends a signal to the ECM telling it to disable.
Posted By: MCubed45
Date Posted: March 05, 2003 at 7:39 PM
what if you just bridge the leads from the switch? isn't that the equivalent of always holding the button down? or does the system require that it be released before you press it when starting?
Posted By: Sonic2002GT
Date Posted: March 05, 2003 at 7:46 PM
Unfortunatly, no. I've bought the book "The Art of Electronics" whch is supposed ot be the best book out there for these kind of things (ought to be for $70). Here's the way the system works: First the car is turned on. About 2 seconds after you start it, a current is sent to the switch. The traction control is automatically on. If you push the switch (and let go,) it completes the circuit, sends the +12V to the computer and says "change status". The computer also sends a +12V signal if it's (traction control) off to the LED light built into the switch. Therefore I need to find a cap that will take about 3 seconds to charge, and then will send a +12V signal only once the entire time the +12V signal is being sent to the switch. I'm sure I'll figure out it after a few chapters, but I figured this would be a good place to ask.
Posted By: MCubed45
Date Posted: March 05, 2003 at 10:45 PM
try using the 'constant to momentary' diagram on the special apps relays page. use the input to the switch as the +12 input signal. that way the relay is only drawig current when you turn on the ignition. momentary +12 output would go to the computer. this setup also allows you to still turn it back on if you wanted i'm pretty sure.
https://www.the12volt.com/relays/page5.asp also just checking but are you sure the computer needs to see only a momentary +12 v pulse? you should double check the output of the switch to make sure. so basically all you want is the car to start with the traction control automatically off right? that constant to momentary setup should do it. 86 & 87 can both be hooked up to the +12 input to the switch and 30 should be hooked up to the output. just tap in, don't cut anything. 85 gets hooked up to that cap an resistor to the ground. idk the system requires any particular length of output from the switch but diff. cap sizes would give diff signal lengths. anyways, hope that helped. oh yea, nice car btw! oh yea apologies to Kalus... can you say thread jacked?
Posted By: Kalus
Date Posted: March 06, 2003 at 4:25 PM
Ok, getting back to the subject at hand..... What do you reckon about this? 
Will it do the job???
Posted By: the12volt
Date Posted: March 06, 2003 at 4:40 PM
That will close A & B momentarily each time you turn the igniton on and off. -------------  the12volt • Support the12volt.com
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