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from led to relay?


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angelars 
Copper - Posts: 176
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Posted: February 17, 2009 at 10:56 PM / IP Logged  
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.
KPierson 
Platinum - Posts: 3,527
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Posted: February 17, 2009 at 10:58 PM / IP Logged  

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
angelars 
Copper - Posts: 176
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Posted: February 17, 2009 at 11:05 PM / IP Logged  
KPierson wrote:

It depends what kind of relay you are using.

Standard 30-40 amp relay. Solid state is too costly from led to relay? -- posted image.
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?
KPierson 
Platinum - Posts: 3,527
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Posted: February 17, 2009 at 11:38 PM / IP Logged  

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
angelars 
Copper - Posts: 176
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Posted: February 18, 2009 at 12:00 PM / IP Logged  
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?
KPierson 
Platinum - Posts: 3,527
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Posted: February 18, 2009 at 12:01 PM / IP Logged  
That is typically the case, but I can obviously make no guarentees.
Kevin Pierson
arhunter 
Copper - Posts: 82
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Posted: February 18, 2009 at 12:43 PM / IP Logged  

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.

KPierson 
Platinum - Posts: 3,527
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Posted: February 18, 2009 at 12:48 PM / IP Logged  
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.
Kevin Pierson
angelars 
Copper - Posts: 176
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Posted: February 18, 2009 at 1:33 PM / IP Logged  
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?
KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
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Posted: February 18, 2009 at 5:11 PM / IP Logged  
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).
Kevin Pierson

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