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Need help troubleshooting a laser!

  1. Jul 27, 2010 #1
    I bought a red <5mW laser from thinkgeek.com, and it takes 2 AAA batteries. I took it apart though, took off the momentary pushbutton switch and soldered in a wire because i need to solder on a constant on/off switch at some point but I have noticed the laser is not acting how it was before. The laser is extremely dim, and I tested the circuit and it reads about 3 volts. (Should it be 3.3?) But I am using AA batteries also and not AAA, though I thought this wouldn't make a difference as they produce the same voltage.

    One other thing, I don't know how much current it *should* draw, but I measured it anyways, and got some strange readings. On the 10A setting, it reads as 0.02, so 20 mA, and then I switch it to 200mA setting and it reads as 0.2. Now correct me if I'm wrong but shouldn't it go up to 20 instead of .2 since the units changed? Also, as I go down further to 20 mA, then 2mA, and so on, the reading goes to 0, which makes no sense whatsoever. Do you think my multimeter is wrong or am I using it wrong?
  2. jcsd
  3. Jul 27, 2010 #2


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    Staff: Mentor

    When you use a DMM in current measuring mode, it inserts a resistance in series with the circuit, and measures the voltage across that resistance to calculate the current. On the 10A setting, it uses a pretty small resistance, so it won't interfere with your circuit much. As you change to more and more sensitive settings, it's using a pretty big resistor in series, and that will alter the circuit, and decrrease the current that flows through your laser diode.

    If you have a 2nd DMM, you can measure what the current sensing resistor values are in your first meter. You can also measure the voltage across your laser diode as you switch the first meter to different current measuring ranges.

    So that might explain the variation with current meter setting. Is the laser diode still dim with no meter in series at all? If so, it's not the size of the batteries (AA versus AAA). Check all your connections to be sure that they are good (solder joints, battery contact springs, etc. I'm not sure what else might be different. Any chance that the laser diode got hot during your soldering? LEDs don't like to get heated too much during soldering...
  4. Jul 27, 2010 #3
    Yeah I just tested it again it's pretty dim without the multimeter in the circuit, and the batteries read 3.03 volts. I don't know if it's standard laser design but on the tiny pcb end sticking out, there is a really really small chip that says OEE I think on it. The negative terminal runs through this chip, and the positive terminal is the gold plating on the outside of the diode.

    The stock laser simply had the two AAA batteries in the casing with the negative touching a gold contact at the bottom, which ran up the sides of the case and onto the plating. I don't know if any of that makes a difference.

    I'm not sure if the diode was heated during soldering.

    Even more voltage doesn't increase the brightness, I tested it on 4.5 and 6 volts. Maybe it screwed it up even more :/
  5. Jul 30, 2010 #4
  6. Jul 30, 2010 #5
    Are you sure that the push-button switch is a switch only and does not have a resistor or some type of startup circuitry. Lasers are sensitive and it's not a good idea to power them with a simple "noisy" switch because it can cause transients that damage the laser. It's just a guess but if you didn't damage it with static (are you in a dry climate?) then you may have killed it with a voltage transient which was caused when you inserted the batteries directly connected to a laser with no debounce circuitry. (if the debounce circuitry was still in place after your rewiring then this comment is not relevant)

    The use of AA instead of AAA should have been no problem at all, by the way.

    Also, using 4.5 V and 6 V was not a good idea unless you thoroughly understand the circuitry. It might be fine to do this, or it might not, depending on the circuitry.
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