from led to relay?
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=111710
Printed Date: April 29, 2024 at 6:18 AM
Topic: from led to relay?
Posted By: angelars
Subject: from led to relay?
Date Posted: February 17, 2009 at 10:56 PM
What is the best way to wire up a relay that is controlled by a radar detectors LED?
We just want to be able to switch the relay on/off when the LED comes on. The relay will then power something else on/off.
Replies:
Posted By: KPierson
Date Posted: February 17, 2009 at 10:58 PM
It depends what kind of relay you are using. If you are using a solid state relay (or perhaps a reed relay) you should be able to hook it up directly. You could also add a transistor, but you'll need to know how the LED is controlled. ------------- Kevin Pierson
Posted By: angelars
Date Posted: February 17, 2009 at 11:05 PM
KPierson wrote:
It depends what kind of relay you are using.
Standard 30-40 amp relay. Solid state is too costly 
The radar detector is most likely going to be an Escort 9500ci. How do I determine what side of the LED to tap off of?
Posted By: KPierson
Date Posted: February 17, 2009 at 11:38 PM
The easiest way is to use a voltmeter and find a good ground on the circuit board. Once you have a good ground you can determine what pin is ground and power on the LED. Typically, on an LED, one side is constant and the other side is switched. You'll need the LED to turn on and off while testing to verify everything, but that should be simple enough. ------------- Kevin Pierson
Posted By: angelars
Date Posted: February 18, 2009 at 12:00 PM
Are you saying that if one side of the LED shows constant ground, that it has a positive trigger, and/or that if one side of the LED shows constant power that it has a negative trigger?
Posted By: KPierson
Date Posted: February 18, 2009 at 12:01 PM
That is typically the case, but I can obviously make no guarentees.
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Kevin Pierson
Posted By: arhunter
Date Posted: February 18, 2009 at 12:43 PM
What you really need to know to do this is how much current capability the circuitry has that is turning on the led. Most leds only take a few ma current to turn on, but the coil on a 12 volt automotive relay could be much higher than this.
Posted By: KPierson
Date Posted: February 18, 2009 at 12:48 PM
Thats why I said to add a transistor if not using a solid state relay. The LED will most likely operate at ~10mA. More then enough to trigger a BJT, but not near enough to power a standard relay coil.
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Kevin Pierson
Posted By: angelars
Date Posted: February 18, 2009 at 1:33 PM
KPierson wrote:
Thats why I said to add a transistor
What would be the best transistor to use for this, and how would we wire it?
Posted By: KPierson
Date Posted: February 18, 2009 at 5:11 PM
It's hard to spec out a best transistor since we don't know what you are trying to acomplish. I would guess a general PNP or NPN transistor (depending on the switching system). I would throw a resistor on the base and a diode arcoss the coil of the resistor. You will also want to make sure you power the + of the relay with the same power feed as the radar detector (ie don't power the relay with a constant voltage and then power the radar detector with switched voltage). If you use different sources the power on the relay coil can backfeed in to the radar detector and cause it to do unknown things (possibly burning something up).
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Kevin Pierson
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