Saturday, September 06, 2008

fixed laptop USB LED

Some time ago I bought a cheap-o laptop keyboard light. It used a white LED for illumination. It stole power from a USB port.

It was my intention on buying it to hack it, figure out how it worked (although I did not expect there to be much to it), and possibly convert it. I wanted to make it astronomer-friendly, changing it to emit red light.

I started to use the keyboard light (in bed) with some laptops, primarily the Dell Latitude D800. Curiously, in very short order, I noticed the metal shield at the end getting very hot! Then the colour and intensity of the white LED changed. Huh. I guess you get what you pay for... To be honest, I simply assumed that they didn't bother to put a resistor on the white LED and it overloaded.

Today, when I finally opened it up, I discovered, in fact, the maker had included a resistor. ¼ watt. But 15 ohms?! That's a weird number. I backwards computed the values. Assuming the white LED had a forward voltage of 4.5, then a 15 ohm resistor would dump 35 to 40 milliamps into the LED. That's a little high. But. If the white LED was a low-voltage type, then this resistor would be way too low.

I used a 100 ohm resistor with a super-bright red LED. I assumed the forward-voltage was 2.1. The 100 ohms would put about 30 mA into the LED. A tad high but I want maximum brightness.

We'll see how long it lasts...


Even though the old IBM ThinkPad has a super-cool integrated keyboard LED, I will make use of this hacked keyboard light, with it's good colour, high brightness, and long flexible neck.

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