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Light up mouse pad using USB?

C

CrimandEvil

Dick with a heart of gold
Joined
Oct 22, 2003
Messages
19,670
I want to mod my own glowpad powered by USB but unsure if my LEDs are able to.

I know that USB supplies 5v (and a max of 500 mA) and my LEDs are rated at 3.5v Max (4000mcd @ 30ma) so I know that I'll want to run them in parallel so if I'm using four to six LEDs could I safely slide on not using a resistor (I just don't have one handy) and would anyone know if I would be able to put a switch on it to turn the lights on and off?

Sorry, I'm just really green when it comes to some of this stuff. :)
 
simple. yes, you can put a switch on it. yes, you get away without using a resistor, although i don't see why. a 3.5v LED will fry at 5v very quickly, so you need to run 2 in sequence, and as many of these sequences as you want in parallel (well, within reason - you don't want the current regulator to kick in on your usb bus).

if you do decide to use a resistor, use resistor calculator.
 
Okay just so I'm clear on this, one LED will fry but a couple won't..? I tried the calculator and it returns a 56Ohm resistor, I guess I should drop by radioshack too. I just hope they don't charge so damn much for one. :rolleyes:
 
Crim,

Yeah, one LED will fry. The LED rating is the max voltage. Over that, it pops. So, why do multiple work? It depends on A) the LED rating and B) the line voltage used. Take 4 LEDs at 3V each, string them in SERIES, (not parrallel), and drop them on a 12V line. The 12 V is spread across all the LEDs, and equals 12V, so no problem. Drop the same 4 LEDs on the 5V rail, and I doubt that they'd light up, as they're each getting just under half their rated load. Wire all 4 in PARRELLEL (not series) on the 12V line, (splitting the power and ground so they're each getting 12V), and you'll very quickly have melted LEDs.

Here's my suggestion:
Take the USB line, split the 5V line into however many lines you need for one line per LED, calculate the resistor needed to drop the 5V line to the LED rating, put one resistor per LED, and wire them all back to a common line. Take that line, wire it to a switch of your choosing, and wire that back to the USB ground line. Power in, splits and drops to the LEDs, syncs back up at the switch, and back to the PC. Easy.

As for price, I wouldn't go to Rat Shack. I can get 4-8 resistors, depending on size, in a box for a buck or two at my locak Mom and Pop electronics shop. They're cheaper than the LED's, so don't worry about breaking the bank. Oh, and I like Linear's LED calculator. Nothing much different, but his site has some cool stuff too. Check out the "LED center" on his site for more info.

*Edited my terms so not to appear too noobish.... *
 
DOH! I always get the terms confused, but I knew what I meant....

Crim would have figured it our sooner or later, especially the way burning plastic smells. :p
 
Yeah I knew what you meant. :)

So I used this cal to figure out the resistor for running in parallel (five LEDs) and it tells me I'll need a 10 Ohm resistor. I understand the need for a resistor when it comes to needing a 50 Ohm resistor but is a 10 Ohm really needed?

BTW ProphetSix I wish we had some mom and pop shops around here cause they only place I've found to get one is radioshack. :(

Or anyone know a good way cleanly do this with 12v instead? :)

I don't want to do this half assed or anything but it seems like I'm kind of limited since I don't have any resistors laying around and I'm not sure if I can just order a small hand full for cheap (and probably pay more with shipping).
 
ProphetSix said:
The LED rating is the max voltage. Over that, it pops.
Click to expand...
No, it actually is related to current. At 5 volts, the current is so high that the LED frys.

The voltage rating provides an easily measurable operating point at which the current is an acceptable level, which also means the brightness is at the desired level (LED brightness is a function of current). Measuring instruments can more readily measure voltage than current, so that's why there is a max voltage spec more often than a max current spec.
 
Just go to RadioShack. They should have a 5-pack for about a dollar. Per resistor, that's a horrible price, but I think you can spare a buck on this project.
 
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