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Old 08-22-2012, 08:11 PM #1
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Default Question about power from a USB port

I bought a 1mw laser pointer to use in a computer case I am modding.
I know next to nothing about lasers but just need some help understanding exactly what I need to do in order to power the laser from a USB cable.
I know that inside the cable will be one red wire and one black, and it is obvious to me what these are.

The help I need is in determining whether I need to lower the voltage going to the pointer.
The exact pointer I bought is this one here
it is a 1mw laser and uses 3 LR41 coin type batteries which are 1.5v each so that's a total of 4.5v. USB is 5v so there is a difference of 0.5v
Although I feel this would work is it wise to do something to bring the voltage down and, if so, what should I do?
If it needs a resistor what size and on which wire, positive or negative?

two other questions: is it ok to power the laser this way or could it present a problem to any motherboard components?
Laser light doesnt harm any components does it? (just thinking back to the old days when you could use ultraviolet to wipe an eprom).
The wavelength of the laser I have is 650nm.

My intention is to use it to edge-light a window made of acrylic (either clear or white acrylic - i havent decided)



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Old 08-22-2012, 10:35 PM #2
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Default Re: Question about power from a USB port

Are you in a build war with this guy?

You don't want a pointer - it will not light your window evenly at all. You want LEDs.
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Old 08-23-2012, 01:40 AM #3
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Default Re: Question about power from a USB port

no, actually I want both I have created a really nice effect using a laser pointer and a few led's and managed to get the beam to bounce off one edge and onto another and then across in a beam to hit a 3rd and 4th edge (unusual shaped window it's a HAF X case.
With a few LED's on there at the other side it gives a really excellent effect - cant describe it properly but will post pictures one day
But even if I dont bounce it off the window edges I still might use some kind of fibre-glass strands or similar effect. I am just experimenting at the moment

Oh thanks for showing me that thread.
It still doesnt answer the questions I have about powering a laser from USB though, but I did like his ideas about mirrors at the edge of the glass/acrylic.

Would still like to know the ins-and-outs of USB power though as I had one laser and killed it on a 5v dc wall power supply.
I dont know if I accidentally shorted it or whether the power was too high for it though

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Old 08-23-2012, 03:18 AM #4
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Default Re: Question about power from a USB port

Pointers like that rely on the internal resistance of the button cells. 5V will likely kill it. The simplest modification is simply a resistor in series. The value depends on a few factors, but 220Ω is probably a safe bet.

Since it will be in the case, you should power it from the computer's 5V rail instead.
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Old 08-23-2012, 06:54 PM #5
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Default Re: Question about power from a USB port

Thanks for the information. It's very much appreciated.
I will post some pictures of the case I am working on when it's finished in a week or two
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Old 08-23-2012, 11:42 PM #6
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Default Re: Question about power from a USB port

I've used those Dollar Store red Lasers on a 5Volt regulator before
and they did just fine at that voltage.

The only thing to look out for if using the USB port on a computer is
that it is only 5Volts that is being supplied. I've seen computer USB
ports supply up to 5.6volts.


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Old 08-25-2012, 07:14 PM #7
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Default Re: Question about power from a USB port

Quote:
it is a 1mw laser and uses 3 LR41 coin type batteries which are 1.5v each so that's a total of 4.5v. USB is 5v so there is a difference of 0.5v
Should be fine, would be pretty darn surprised if that would be enough to kill it. That module should have an internal resistor to limit the current and +- 1V shouldn't matter.

Quote:
two other questions: is it ok to power the laser this way or could it present a problem to any motherboard components?
Laser light doesnt harm any components does it? (just thinking back to the old days when you could use ultraviolet to wipe an eprom).
The wavelength of the laser I have is 650nm.
So i'm pretty sure that laser is just a low power laser LED + a resistor in series. As long as you don't bypass that resistor, that resistor should limit the current to the point where it couldn't hurt your computer. Technically your violating the USB 2.0 standards by drawing current from a bus without asking permission....but many appliances do that anyways. USB 2.0 is speced up to 500mA which i'd be pretty darn surprised if your 1mW would exceed provided you don't bypass the internal resistor. In addition, the motherboard should temporarily shutoff that usb port if it draws too much.

and no... there are no modern components used frequently now days that would be harmed by that wavelength/power in motherboards. UV Memory is beyond ancient. Pretty sure everybody from that area is mostly dead by now. =D
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Old 08-30-2012, 12:30 AM #8
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Default Re: Question about power from a USB port

To be on the safe side, you could place a normal rectifier diode in series with the laser pointer, to lower the 5 volts from the supply with about 0.7 volts. A simple diode like 1n4001 or 1n4148 only costs a few pennies and suffices.

650 nm will not damage any componets. UV-erasable eeproms haven't been used in motherboards for a decade or two, but even if there was one on there, it would not be sensitive to the red laser light at all... it takes a decent dose of fairly low wavelength uv (think mercury lamp) to erase those things.
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