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how to wire a 12v light to a led circut


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1redvert 
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Posted: September 14, 2012 at 12:03 AM / IP Logged Link to Post Post Reply Quote 1redvert
Hello, I bought a stand alone adjustable tachometer shift light off of eBay and i want to wire the shift light output to a 12volt led strip(its a led 3rd brake light)so that i can mount the shift light unit under the dash and the led strip in the window in my line of sight. the shift light unit has an led instead of a 12v bulb light that i thought it would have. my question is how do i wire it properly? do i just wire my b+ lead to the pos side of the led and ground the neg light wire or do i have to use a relay from the pos side of the led? i really am unsure because of the led in the circuit and i don't want to burn it up cause it was expensive thanks
oldspark 
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Posted: September 14, 2012 at 11:15 PM / IP Logged Link to Post Post Reply Quote oldspark
If the tacho originally came with a bulb, then powering LEDs instead should be no problem.
If it originally powered a single LED (ie, 20mA), then a single LED string should be no problem.
Parallel LEDs or LED strings (ie, 40mA, 60mA, etc) might be a problem if the added current blows the LED output transistor or circuit.   
A relay coil would need to draw (probably) 20mA or less - ie, LEDs or LED strings often draw less than a relay. A typical automotive relay draws 60mA to 250mA.   
A transistor or MOSFET circuit could or should be used instead of a relay.
1redvert 
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Posted: September 15, 2012 at 3:40 AM / IP Logged Link to Post Post Reply Quote 1redvert
hi there could you recommend a transistor for me to use in this circuit? would a 500 ma 40 v be ok? i looked it up and watched a quick video about it and it looks like i would need a npn transistor. so my voltage would be positive from the led to the center of the transistor and i would power the 3rd brake light led strip by power to b+ and grounding it through the two outer prongs on the transistor right...would i need a resistor in that circuit as well?
KPierson 
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Posted: September 15, 2012 at 2:09 PM / IP Logged Link to Post Post Reply Quote KPierson
I would use a reed relay. Will draw around 5mA (less then an LED) and will provide enough current. The reed relay will be a bit easier to wire and no resistors will be needed.
Kevin Pierson
oldspark 
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Posted: September 15, 2012 at 2:17 PM / IP Logged Link to Post Post Reply Quote oldspark
Assuming the output is 5V or higher, I'd use an N-channel MOSFET. Then you only need a pull-down resistor (~1M between Gate & Source) to ensure is if off when "not on". Otherwise its similar to an NPN transistor except trannys require input-current resistors.
But yeah - assuming at least 0.7V output, thru a resistor to tranny Base; Emitter to GND, Collector to LED- thru LEDs to +12V (IGN +12V etc).   Or probably whatever your web source said...
It'd be nice tho if you found out if you really needed it. Maybe the raw output is fine?   [ Any specs, or did it power a bulb (no), or can you open it up (ie, maybe see what transistor drives the LED)? ]
1redvert 
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Posted: September 17, 2012 at 11:43 AM / IP Logged Link to Post Post Reply Quote 1redvert
Here is the module out of the shift light and the led brake light that i would like to power with it.
https://images.thesamba.com/vw/gallery/pix/971386.jpg
https://images.thesamba.com/vw/gallery/pix/971385.jpg
https://images.thesamba.com/vw/gallery/pix/971384.jpg
1redvert 
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Posted: September 17, 2012 at 11:58 AM / IP Logged Link to Post Post Reply Quote 1redvert
it looks like the two transistors on the board are h945 and a 78L05
1redvert 
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Posted: September 17, 2012 at 12:17 PM / IP Logged Link to Post Post Reply Quote 1redvert
the led brake light draws .25 amps
KPierson 
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Posted: September 17, 2012 at 6:47 PM / IP Logged Link to Post Post Reply Quote KPierson
You need to be extremely careful how you approach this project. Your "expensive" shift light utilizes cheap, poorly designed hardware.
Looking at the picture of the top of the circuit board you can see the "top" lead of the LED is connected directly to 5vdc. The "bottom" lead of the LED is connected directly to the PIC 12f675 through only a resistor!
Generally you NEVER connect a device directly to a pin on a microcontroller for commercial applications. There should be a transistor in there. I've never worked with PIC's but I would assume you have less then 25mA to work with. If you can tell me the colors off of the resistor inline between the 8 pin chip and the LED I might be able to tell you the actual output. V=IR   V=5 R=? I=V/R The colors on the resistor will tell you what R is.
Again, I would desolder the LED, connect a low current reed relay (usually draw 5-6mA and be done with it. Alternatively you can add a transistor but ultimately it's a bit more complicated then a simple relay.
Kevin Pierson
1redvert 
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Posted: September 18, 2012 at 10:37 AM / IP Logged Link to Post Post Reply Quote 1redvert
would some thing like this work to replace the the led with a solid state relay. it is 3v min and 15ma.
the resistors look like ( brown, red, 2black, brown)
http://www./itm/CRYDOM-Solid-State-Relay-SSR-MCX240D5-5A-240Vac-Load-3-15Vdc-Control-New-PCB-/290773757433?pt=LH_DefaultDomain_0&hash=item43b3776df9
thanks for the great help
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