# UV flashlight hack question?



## mroct31 (Nov 10, 2007)

A project I did awhile ago with CalHaunts was to hack those small 21 LED black light flashlights so they could be run off a wall wart. What we did was use a resistor on the positive side of the battery holder and the + side of the wall wart went to that and the - side went directly to the - side of the battery holder. I made 2 at the the time and they work great as small spots. I'm trying to make more and no one seems to remember the resistor size other than maybe it being 1 ohm. I got the 1k ohm 1/2 watt ones at RS and while they work the light output is very low not like the others I made before which are nice and bright. I"m guessing the resistor is wrong but I'm not sure which to go with, more ohms, more watts. Any ideas resistor masters? FYI the battery pack is a 3 AAA style if that helps.


----------



## Otaku (Dec 3, 2004)

A 1K ohm resistor is pretty high for a 3AAA (4.5VDC) source. I'm guessing the LEDs are wired in parallalel with a single resistor. There's a big difference between a 1 ohm and a 1K, but go the safe route and wire in a 150 to 220 ohm resistor. If you can see the OEM resistor try to get a value with a DMM, or get the color code and look it up online.


----------



## TroyO (Sep 7, 2011)

Whats the voltage of the wall wart?


----------



## mroct31 (Nov 10, 2007)

I looked up the color code and it's a 15 Ohm, 1/2W which I guess would be a difference compared to a 1K-ohm 1/2 watt resistor? Should of done this to begin with!

The original wall wart was a 5 VDC 2000mah but I have them working off a 
6 VDC 800 mah just as well. I couldn't find another of the original warts so I went to my box of warts and found one that powered the 2 I had working just as well so I'm pretty sure it's OK.


----------



## TroyO (Sep 7, 2011)

Yeah, 15 Ohm is in the ballpark. I ran some calculations using my TLAR enginering handbook (That Looks About Right) and came up with a 3.3 Ohm to 8.2 Ohm resistor depending on whether you wanted to run the light "hot" for max bright or cool for max life. 15 Ohm is close enough.

All that being said, it would probably be just fine straight off that 5V transformer. I usually use a 20% rule with cheap electronics and 10% with expensive stuff. (4.5v x1.2 = 5.4V so the 6V would probably be pushing it.)

More of a concern is the wattage rating.... 

Most LEDs are around 20mA current draw and that thing has 21...

21 x 20 = 420 mA (Which is .420 Amps) and at 4.5V

4.5V x .420A = 1.89 Watts

On the plus side.... your 15 Ohm resistor is cutting that to about 150 mA so the 1/2 watt resistor will be fine.

On the down side you are missing some available power if you got something in the 5 to 7.5 Ohm/2 Watt ballpark instead.

Of course, it's late and I am tired so any math I do has a plus or minus 1000% margin of error... ;-P


----------

