Adding LEDs

[Shez]

Hey guys,

I'm in the process of doing a fun, but pointless, mod to an old piece of equipment (an old LCD projection panel). What I'd like to do is add 8-10 LEDs into the case of the LCD. From the reading I've been doing, I know that I can take an old 12V adapter to power the LEDs but that would mean an extra wire (besides the LCD power cable) coming out of the case. Is it possible to power LEDs off the circuit board without screwing with the LCD? And if it is possible, what should I be looking for / how could I test it?

Thanks

 

[gwolfman]

Very interesting, good lucky. Show some pictures once you get it done.

 

[qwertycopter]

Hey guys,

I'm in the process of doing a fun, but pointless, mod to an old piece of equipment (an old LCD projection panel). What I'd like to do is add 8-10 LEDs into the case of the LCD. From the reading I've been doing, I know that I can take an old 12V adapter to power the LEDs but that would mean an extra wire (besides the LCD power cable) coming out of the case. Is it possible to power LEDs off the circuit board without screwing with the LCD? And if it is possible, what should I be looking for / how could I test it?

Thanks

All I can say is don't forget the current-limiting resistors and make sure you solder onto the DC side!

 

[Shez]

I just had a thought about my project. The screen has a fan, which I assume was because it was intended to sit atop an over-head projector. Do you think I could snip the fan out and use the leads powering the fan to light up my LEDs? Do I have to be worried about how much power the LEDs will be pulling?

 

[qwertycopter]

Hey, that could work. Check the voltage listed on the fan. They might run at 5V DC or something similar. LEDs run at different voltages, usually 1-2 V. You'll need those resistors to limit the current so the LEDs don't fry

 

[Shez]

Awesome

Here's a follow up question. If I make two strands of LEDs (in parallel) I could have each strand rated for 5Vs right?

For example:
If I had two strands of 3 LEDs (@ 20mAmps) wired in parallel running off of 5V DC

5V__/ 100 ohm resistor -> 1v LED -> 1v LED -> 1v LED -\____0V
\100 ohm resistor -> 1v LED -> 1v LED -> 1v LED -/

That'd work right?
And is it safe to assume that the fan is running off of DC power? I don't have a multimeter yet but once I get one, how would I check for AC vs DC?

 

[RetiredChief]

What qwertycopter says is correct.

1st verify fan voltage, ie +5 or +12VDC
2nd determine series current limiting resistor value.
Source Voltage - 1.8 Vots (For typical LED, check package for spec.)
divide by 15 ma (20 ma (Max) for brighter
ie 12V - 1.8 V / 15 ma = 700 ohm (68 ohm) series resitor (1/4 Watt)
for 5 V source - 5 - 1.8 /15ma = 213 ohm (220 ohm 1/4 watt)
Editted
When in parallel Source must be able to provid2 # LED x 15 or 20 ma.
in series/parallel 20 ma per sting.
Note: some LED's already have a series resitor . a LED rated at 12 V already has the resitor. Also some LED can handle say 30 ma (super Bright)

Added - your last post - 3 LED would require 5.4 Volts, would be dim and not require Resitor
But yes you can add LED's in series. Ex 6 in series w/12VDC

Plce multimeter on DC and measure, if it is AC The reading should be Zero or very close. If on AC scale and the voltage is DC the reading would be very low to Zero
(12 - 6 x 1.8 ) / 15ma = 80 ohms (80 ohms x .15 ma squared) = .018 W (1/4 watt)

 

[Shez]

So... you can lower the ma calculations to control the brightness of the LEDs? For this project I really do want the lights to be as bright as possible, so I assume I should keep the calculations at 20 milliamps.

Taking all that into account and assuming that the 5V/12V have at least 0.16 amps., would the layouts below be correct?


/-> 70 ohm res. -> 1.8v LED -> 1.8v LED -\
5V --> | -> 70 ohm res. -> 1.8v LED -> 1.8v LED -| ---> 0V
| -> 70 ohm res. -> 1.8v LED -> 1.8v LED -|
\-> 70 ohm res. -> 1.8v LED -> 1.8v LED -/

or

/-> 240 ohm res. -> 1.8v LED -> 1.8v LED -> 1.8v LED -> 1.8v LED -\
12V ->| |-> 0V
\-> 240 ohm res. -> 1.8v LED -> 1.8v LED -> 1.8v LED -> 1.8v LED -/

 

[RetiredChief]

Yes - You have the right Idea. As I mentioned there are "super" bright LED's - Check package for e/I values.
In your 5V example, R = 68 ohms as 70 ohms not a common value unless you get a 1% tolerance resistor. I think 240 is made in 10% tolerance.

 

[Shez]

Thank you all for your feedback. I went to RadioShack today and purchased all the bits and just finished up soldering everything. The fan was indeed a 12V DC source, which made life very easy. Right now I have two parallel strands with 2 LEDs. Each strand ended up being 2x(100 ohm resistor) + 2x(3.3V LED@25mA). I also finished Dremeling the case of the LCD to snake wires through and now I'm just waiting to buy some glue to hold everything down. Once that's finished I'll solder the LEDs onto the old fan cables and be off for the running!

Now lets just hope that the LEDs are bright enough for what they're intended for

 

[Shez]

Just as an update: I glued down all the wires and LEDs into the case this morning and did a cursory check to see if the parts all fit back together, which they did beautifully.

I also did a double check to see if everything was still working properly and again everything was working perfect.

When I get back home all I have to do is solder the fan cables onto the LED leads and then screw everything back into place. If I remember I'll post up some pics of the final product.