$10 Police Flashlight Hack! - Page 2 - XDTalk Forums - Your HS2000/SA-XD Information Source!

**XDAndMe** · 09-12-2007, 02:31 AM

Theoretically you can just put a resistor in series with the batteries and the bulb.

**Houseboat** · 09-12-2007, 03:29 AM

It is a great little project But do not leave the light on for any period of time it will get the bulb hot and start to melt the houseing, It happened to me but it is great for "SHORT BURST"

**XDAndMe** · 09-12-2007, 03:42 AM

I looked up the part number for the krypton bulb that guy was using in the video. It's RadioShack PN# KPR112, which is this: http://www.radioshack.com/product/in...tId=2062383&cp

The bulb conducts 650mA at 6V. That means it needs 0.65 * 6 = 3.9 Watts of power to light up nice & bright. It also means the resistance of the bulb is 6 / .65 = 9.23 Ohms. We're given that our new power supply is 9V.

Using that information, I modeled the circuit on my computer and it looks like this:

After simulating this circuit, it is clear that the poor little krypton bulb is using about 8.8 Watts of power when connected to the 9V battery. That's a bit too much since it is designed for 3.9 Watts (less than half of what it is using in our flashlight).

So I did some quick math and figured out that if you put a 5 Ohm resistor in series with the battery and the bulb (doesn't matter where) then the bulb will only use about 3.7 Watts of power. That's much better and is just under the 3.9 Watt operating power. It should be enough to make the bulb light up bright and last longer, too.

Here's the new circuit:

If you want to be super-accurate, the exact resistance needed is 4.6 Ohms. It's hard to find a 4.6 Ohm resistor, so I just settled for 5 Ohms above. You can pick up a 5 Ohm resistor at RadioShack I would imagine. However you can certainly make a 4.6 Ohm resitor by putting enough smaller ones together (resistance in series adds up just like voltage for batteries in series), or you can mail-order one.

The fine-tuned circuit looks like this:

Those numbers are just about perfect. In this configuration the bulb gets almost exactly 650mA at 6 Volts and uses 3.9 Watts... exactly what it is rated for

It won't shine exactly as bright as it did with all 9 Volts, but it will still be bright and it will last much longer.

IMPORTANT

If you decide to do this mod and add in the resistor, make sure you get a resistor that is rated to handle at least 2 Watts of power. As you can see in the simulations above, R1 uses almost 2W of power. A standard resistor from RadioShack might only be rated for 1/4 Watt and it might burn up.

This one should work: http://www.newark.com/jsp/Passives/R...sp?sku=41K8863
It's a 4.7 Ohm (+/- 1%), 2 Watt resistor. Cost is $0.70 ea.

ichy_trigger · 09-12-2007, 05:47 AM

Show off

**XDAndMe** · 09-12-2007, 02:21 PM

Quote:

Originally Posted by ichy_trigger

Show off

lol, well assuming the resistor idea does indeed work... there's still the problem of how to actually install the damn thing inside the flashlight without breaking it apart

cougar_guy04 · 09-12-2007, 02:26 PM

Quote:

Originally Posted by XDAndMe

lol, well assuming the resistor idea does indeed work... there's still the problem of how to actually install the damn thing inside the flashlight without breaking it apart

Screw that, I'll keep saving my pennies for the Surefire.

87-51 · 09-12-2007, 11:17 PM

That's the correct lamp for that light. 9V flashlights actually use lamps rated around 7.5V since the batteries will have a voltage sag under load. Don't worry about resistors. It will work just fine.

If you're interested in lights like this one, you may take a look over at www.candlepowerforums.com. Lots of good stuff but hold on to your wallet!

**XDAndMe** · 09-12-2007, 11:51 PM

Quote:

Originally Posted by 87-51

That's the correct lamp for that light. 9V flashlights actually use lamps rated around 7.5V since the batteries will have a voltage sag under load. Don't worry about resistors. It will work just fine.

I don't mean to sound like I'm correcting you, but the bulb in the video according to the part number, is not 7.5V. It's a 6V bulb. The difference even between 1.5V is enough that it will cause the bulb to generate excess heat and burn out sooner because it's only a 9 Ohm load.

I guess a simple solution is to not use the bulb from RadioShack. Here's one that is actually 7.5V, instead of 6: http://www.studiodepot.com/store/ind...5-1213&id=3711

Would this bulb still fit the housing? I'm interested in trying this mod.

ichy_trigger · 09-13-2007, 06:29 AM

any buld for use in a 5 cell maglight would work since that would be 7.5 volts. I guess that would include their LED upgrade bulb.

propellerhead · 09-13-2007, 07:50 AM

I just went for the $20-30 Wal-mart ones.

09-12-2007, 03:29 AM	#12
Houseboat XDTalk 500 Member Join Date: Jan 2007 Location: Tennessee Posts: 747	It is a great little project But do not leave the light on for any period of time it will get the bulb hot and start to melt the houseing, It happened to me but it is great for "SHORT BURST" __________________ www.gun-works.com www.tennesseeholstercompany.com Gun-Works.com is wholly owned subsidiary of Tennessee Holster Company, LLC Quality Leather Holsters for your XDs

09-12-2007, 05:47 AM	#14
ichy_trigger XDTalk 5K Member Join Date: Jan 2004 Location: Dayton, Ohio Posts: 8,013	Show off __________________ "The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." http://militarysignatures.com/signatures/member2645.png

09-12-2007, 11:17 PM	#17
87-51 XDTalk 100 Member Join Date: May 2004 Location: Iowa Posts: 267	That's the correct lamp for that light. 9V flashlights actually use lamps rated around 7.5V since the batteries will have a voltage sag under load. Don't worry about resistors. It will work just fine. If you're interested in lights like this one, you may take a look over at www.candlepowerforums.com. Lots of good stuff but hold on to your wallet! Last edited by 87-51; 09-12-2007 at 11:20 PM.

09-13-2007, 06:29 AM	#19
ichy_trigger XDTalk 5K Member Join Date: Jan 2004 Location: Dayton, Ohio Posts: 8,013	any buld for use in a 5 cell maglight would work since that would be 7.5 volts. I guess that would include their LED upgrade bulb. __________________ "The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." http://militarysignatures.com/signatures/member2645.png

09-13-2007, 07:50 AM	#20
propellerhead XDTalk 3K Member Join Date: Apr 2006 Location: Foat Wurth, TX Posts: 3,559	I just went for the $20-30 Wal-mart ones. __________________ XD-9 Tactical for IDPA, XD-40 Subcompact for concealed carry, XD-45 Service for home defense. Three calibers, three models, three purposes. No compromises.

09-12-2007, 02:31 AM	#11
XDAndMe XDTalk 3K Member Join Date: Sep 2006 Location: PA, USA Posts: 3,402	Theoretically you can just put a resistor in series with the batteries and the bulb.

09-12-2007, 03:42 AM	#13
XDAndMe XDTalk 3K Member Join Date: Sep 2006 Location: PA, USA Posts: 3,402	I looked up the part number for the krypton bulb that guy was using in the video. It's RadioShack PN# KPR112, which is this: http://www.radioshack.com/product/in...tId=2062383&cp The bulb conducts 650mA at 6V. That means it needs 0.65 * 6 = 3.9 Watts of power to light up nice & bright. It also means the resistance of the bulb is 6 / .65 = 9.23 Ohms. We're given that our new power supply is 9V. Using that information, I modeled the circuit on my computer and it looks like this: After simulating this circuit, it is clear that the poor little krypton bulb is using about 8.8 Watts of power when connected to the 9V battery. That's a bit too much since it is designed for 3.9 Watts (less than half of what it is using in our flashlight). So I did some quick math and figured out that if you put a 5 Ohm resistor in series with the battery and the bulb (doesn't matter where) then the bulb will only use about 3.7 Watts of power. That's much better and is just under the 3.9 Watt operating power. It should be enough to make the bulb light up bright and last longer, too. Here's the new circuit: If you want to be super-accurate, the exact resistance needed is 4.6 Ohms. It's hard to find a 4.6 Ohm resistor, so I just settled for 5 Ohms above. You can pick up a 5 Ohm resistor at RadioShack I would imagine. However you can certainly make a 4.6 Ohm resitor by putting enough smaller ones together (resistance in series adds up just like voltage for batteries in series), or you can mail-order one. The fine-tuned circuit looks like this: Those numbers are just about perfect. In this configuration the bulb gets almost exactly 650mA at 6 Volts and uses 3.9 Watts... exactly what it is rated for It won't shine exactly as bright as it did with all 9 Volts, but it will still be bright and it will last much longer. IMPORTANT If you decide to do this mod and add in the resistor, make sure you get a resistor that is rated to handle at least 2 Watts of power. As you can see in the simulations above, R1 uses almost 2W of power. A standard resistor from RadioShack might only be rated for 1/4 Watt and it might burn up. This one should work: http://www.newark.com/jsp/Passives/R...sp?sku=41K8863 It's a 4.7 Ohm (+/- 1%), 2 Watt resistor. Cost is $0.70 ea.

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