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Author Topic: Dim laser – where's the resistance?  (Read 838 times)
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Good evening, all.  I'm working on a high-speed flash trigger (what all the cool kids are doing these days).  I've got everything working as I want, except for my laser trigger.  I've got a laser level I bought off Woot, but it was anything but level, so I ripped it open and have modified it for my purposes.

The laser originally ran off two AAA batteries, so I wired it up to a voltage divider to drop the voltage down to 3.4V (the best I could do with 100 and 220 ohm resistors).  When connected directly to the power rails of my breadboard (which is providing a full 5.02V, currently), the laser lights up brilliantly and draws about 25mA.  However, when I hook it up to one of the digital pins on my Arduino and flip the output HIGH, the laser is very dim and only draws 21mA.  Putting a potentiometer in line with my multi-meter and the laser, I can confirm that the 4mA difference is enough to dim the laser.

I've also tried putting (separately, of course) TIP120 and BC547 transistors in to act as a switch for the laser and I'm having the same problem (base to digital pin on AVR, emitter to GND, collector to negative side of laser).  It seems that as soon as I put any kind of switch in place to turn the laser on and off, I lose those precious 4mA which cause my laser to go dim.

Can anyone help out this pathetic newbie?  

Thanks,
Brian
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you really shouldnt be using a voltage divider for anything that draws a significant amount of current. perhaps you could use an lm317 to get the required laser voltage. also, perhaps you should try using a mosfet transistor instead, they dont have much resistance losses as compared to a bipolar (is that what you call them?) transistor.
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Thanks.  I'd pretty much come to the same conclusion last night, after a bit more Googling around.  It figures that that was the neatest part of the project, so far.  :smiley
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Well, logical output don't delivers 5V. If you look at the datasheet of the ATmega you see that Voh is 4.2V at 20mA load. As you laser need more current this will decrease the output voltage a little more (say around 4V).

The same way, when you use a transistor to switch the power supply of your laser, there is a voltage drop in the transistor and the output is less than the 5V input. You should take into account this voltage loss when you compute the voltage divider.

As weirdo557 said a voltage regulator is a more secure solution.
« Last Edit: December 12, 2008, 07:12:20 am by fdufnews » Logged

New England
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Thanks for the help, folks.  Dropping the voltage divider for an LM317 regulator did the trick.
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glad to see it worked, good luck with your project.
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what can you do with a high-speed flash trigger? Is it for special light effects?
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It's the starting point for pictures like these:


More here: High Speed Photography 1/10000th Second Flash Exposure Or Faster
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You can just use a 2N2222 to in emitter follwer config to provide up to 1000mA to your laser without drawing too much current from the Arduino.  Also, if you can find the laser part number/datasheet, you might be able to just use a 5v output to drive the laser (and make it real bright).

Emitter followers are "good" to put in between whimpy current sources (like MCUs) and things that need lots of power (motors, relays, LEDs/Lasers/death rays).  
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