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Myke

Sat Feb 24 2007, 04:35AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

How would I test a laser diode from a CD player, DVD reader, CD burner est.? How will I be able to tell if the laser is dead or IR?

LarsE

Sat Feb 24 2007, 04:39AM

Registered Member #153 Joined: Sun Feb 12 2006, 03:44PM
Location: Sweden
Posts: 31

Most digital cameras is capable of seeing infrared light.

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Sat Feb 24 2007, 05:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

OK, to test a laser diode you need a decent power supply (in a pinch the 3.3v line of a computer psu and a variable resistor would work) preferably one that has both voltage and current adjustments.

1. the laser should have 3 pins. One of them is tied to the case, this is usually + (but not always). So start by setting the supply at 10ma, and 0v, connect it to a pair of pins, and slowly increasing the voltage to 2.5v. If it suddenly stars to draw current at about 2v then you have found the laser. If it draws any juice below 1.5v, or doesn't draw any current at 2.5v then is it either backwards, a photodiode, or not connected; turn the voltage back down to 0v and try a different pin. If you don't have any luck with + on the bonded pin, try - on it. Then try all of the other permutations.

Once you found a combination that starts to draw current at about 2v, grab a digital camera (cell phones work pretty good) and point the laser at it. You should see a purple glow. If not, try increasing the current to 50ma, but I wouldn't go any past that (in fact that is probably too much to start with). If you see a red glow, then it is a 650nm diode (note, the beam appears dim, but for comparison 100mw of 650nm is less bright that 5mw of 532nm green).

As to the max current, unless you can find a datasheet for the diode (unlikley) it is hard to say. You can slowly give it more current until the output power starts to drop off (at this point you are doing perminant damage to the diode) and then back it off a little, but this can fry the diode.

A few words of caution about laser diodes:
1. obviosuly, don't look into the beam. The IR diodes have a very weak glow (like 1/1000 as bright as it really is) and the red ones are also quite dim for their power. Sure, when the beam isn't colminated and only a few mw you probably aren't going to blind yourself, but it isn't worth the risk.
2. laser diodes are very sensitive to static discharge. Always keep the pins shorted to each other when not in use.
3. drive laser diodes current mode, not by voltage. As they heat up their 'resistance' changes, so if you just try to run it from a constant voltage it will burn out.
4. laser diodes are very sensitive to over current, and over voltage. If you accidentally have as supply set for current mode and don't turn the voltage down to 0v before connecting the diode the short burst of 5v from the output cap will kill the diode (I have killed way to many diodes because of this, BE VER CAREFUL)
5. make sure that the connections between your diode and the supply are secure. You cannot use alligator clips for playing with laser diodes, as a loose connection can confuse the power supply and blow the diode (killed a few from that too)

Have fun and be safe!

Myke

Sat Feb 24 2007, 04:22PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Could you just use 2 AA batteries for the power supply?

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Sat Feb 24 2007, 04:51PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

you could. but you need a low value resistor (something about 50 ohms would be a start) in series with it. One you have lasing, slowly use a smaller value.

Myke

Mon Feb 26 2007, 02:37AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Ok so I set up this circuit

and did everything you said and it worked!

I didn't expect the beam to be that dim. Even when I was foucoused on it it appeared as a tiny little dot of dim light.

Bjørn

Mon Feb 26 2007, 09:58AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If it does not look like expected it might be because it is infrared and you may damage your eyes. Never assume that the brightness you see is the real brightness. If it is an infrared laser it will look dimly red to the eye but in reality it will be 10 000 times more powerful or more.

Myke

Tue Feb 27 2007, 01:34AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Yup I know. I had the laser facing away from me pointing at the camera and I looked at the screen on the camera pointing up at me.

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Tue Feb 27 2007, 03:49AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Was it red or puprle in the camera?

If it was from a dvd drive it should be about as bright as a cheep red laser pointer...

A tip for messis with drives, just leave the whole optics assembly in tact, and just rip the lense out of the focusing mechanism. Then you have the laser pre mounted/'heatsunk, and a nice colminated beam (although the beam is a little fat)

Myke

Tue Feb 27 2007, 04:00AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Umm, I couldn't tell really because it was so tiny. I think it was ethier red, purple, or red/purple. The brightness depends on the current into it right? The laser isn't visible because I pointed it twards a piece of paper an didn't see anything.

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