I bought an Arduino UNO, a laser, connectors, resistors, transistors. I'm trying to hook up the laser and perhaps a few other components to eventually measure the light intensity frequency of reflected beam.

My first attempt worked for an hour or so. I hooked up the 5v power through a transistor controlled by one of the mainboard's digital pins. Burned out the laser eventually.

Bought a second laser and an assortment of resistors, but I don't know how to design the circuit properly.

Laser: http://www.sparkfun.com/products/9906 UNO reference: http://arduino.cc/en/Main/ArduinoBoardUno

I'm trying http://ledcalc.com/ to determine what resistance I should use according to the laser's specs. But I'm confused about a few things:

  • How ought I power the laser properly? I burned it out (unless I burned out something else instead?) presumably because of too much ... something. Volts?
  • The UNO specs seem to say that the 5V pin, when the board is powered by USB is not available as a power supply (when compared to the 3.3V pin). However, I read about 5V on my multimeter. I am unable to determine how many AMPs are available to draw as the specs don't say it, and I must not know how to read it correctly on my meter. Is it possible to measure available current between the 5V and GND without any load? My meter is an Ideal 61-310 set to DCA 200m with probes in COM and mA.
  • Even if the 5V is available and offers amps, if it's in the ballpark of the 3.3V pin at 50mA, that's not nearly the 300mA that the laser is capable of. How did I burn out the laser?
  • On ledcalc, with 5V input, 4.5V power drop, 300 mA (correct for the laser, I believe), I get a recommendation of 1.8Ohms. I'm also getting 1250mW power dissipation by the laser, which is not the 5mW listed in the spec.
  • If I do need a transistor to control the circuit and a resistor to limit the current, does the order of the components in the circuit matter?

That's a lot of stuff, but I'm confused about a lot, and they're all interdependent.

(I have safety goggles for working with the laser)

  • \$\begingroup\$ What is "light intensity frequency"? \$\endgroup\$ Commented Aug 13, 2011 at 19:24
  • \$\begingroup\$ @Olin - Sorry, I'm trying to differentiate from measuring the frequency of the carrier waveform (fixed at 532nm) to the amplitude modulated intensity of the beam, reflected off of some vibrating surface (500-2000Hz range). \$\endgroup\$ Commented Aug 13, 2011 at 20:11

2 Answers 2


Jason, I'm not trying to discourage you, but perhaps you might want to start with a more basic problem to learn first? My understanding from your post is that you don't have a good grasp of basic electronics - this isn't a problem, because everyone must start somewhere, but laser diodes are notoriously unforgiving for mistakes.

Ok, so here's a few pointers to help you get started. You didn't indicate what your ultimate goal is, so I can't comment on whether what your doing will accomplish it. When working with diode lasers, be very careful as ESD (electro-static discharge) can permanently damage them. Always ground yourself (use a grounding strap, or keep in contact with a metal desk or something like that). Secondly, you might consider placing a LED in place of the laser diode in order to test your circuit first. The LEDs are much more forgiving and that will allow you test things first.

From the Sparkfun laser you linked to, it appears that is a prepackaged unit that you give a "2.5V to 4.5V input voltage" in order to power it. The potentiometer on the laser will control how much current it draws, but any pin on your Arduino will not be able to source that much power. It also appears that that unit does not make it easy for you to cut it on and off via a I/O pin on the Arduino. If you want to be able to control the light via a I/O pin, you'll need something more like this: http://www.sparkfun.com/products/8654

A few comments about your circuit questions:

  • The 5 mW listed on that particular laser diode is the optical power output at 532 nm. This particular diode converts 1064 nm light (infrared) to 532 nm. This conversion process, is very inefficient. That's why the electrical power you came up with (>1W) is very different from the optical power (5 mW).
  • The position of the transistor in the circuit does definitely matter. But, if you were doing a simple circuit with a battery, resistor, and the LED, it wouldn't matter which order they were in. Transistors complicate things.
  • It is not possible to measure the current between 5V and GND without a load. Without a load, there's no current and no way to measure it. If the 5V is coming from the USB port, it usually can draw a few hundred mA. I haven't looked at the UNO schematic, but drawing this much current from the 5V supply might cause the Arduino to be starved for current and "brown out."

If you hooked the laser directly to the 5V pin, when the laser is only rated up to 4.5v, that's a likely cause for why it burned up. Perhaps if you described exactly what you want to do, and drew a picture of what you tried, you'd get better help?

  • \$\begingroup\$ The project is silly - I don't really expect to finish or that it will work. Just taking it in steps. As commented to @Olin, I'm trying to create a laser microphone, basically. Details here \$\endgroup\$ Commented Aug 13, 2011 at 20:14
  • \$\begingroup\$ I appreciate how absurdly unqualified I am for this. I just don't have any interest in SIMON game projects or blinking light patterns. Thanks for your help. \$\endgroup\$ Commented Aug 13, 2011 at 20:17
  • \$\begingroup\$ The original circuit was 3.3v or 5v. Either worked ... for a while. UNO's digital output pin 13 to the transistor's base closed the laser's power circuit (programmatic control of laser is important). That's it. Laser is meant to be used in a laser pointer, includes a battery terminal spring inline with a mechanical switch. A second power lead bypasses mechanical button. \$\endgroup\$ Commented Aug 13, 2011 at 20:27
  • \$\begingroup\$ So the laser itself will be safe at 3.3V, but the current draw would be too high since it will use up to 300mA and only 50mA are available. Then it seems the pins are current-limited, but not fused? But I SHOULD use a resistor to limit the draw to under 50mA if it is to be powered by this board (no minimum power is stated, but the laser was shining, so I guess it was enough). But if I plug in 3.3V supply, 3.3V drop, 25mA draw into the calculator, I get an error. \$\endgroup\$ Commented Aug 13, 2011 at 20:37
  • \$\begingroup\$ Just to clarify, can my multimeter be used to discover the approx. 50mA current availability from UNO's 3.3V pin? I guess in order to determine it, a component would have to attempt to exceed it, which would trip a fuse or damage the circuit. \$\endgroup\$ Commented Aug 13, 2011 at 20:44

Check out this video.

MOSFETS tend to be easier for beginners to understand than BJTs.

Read the documentation on the laser to determine what its electrical characteristics are.

Good luck spying, as I assume that is what this is for ;-)

  • \$\begingroup\$ No, not spying. Mosquitos! I'll check out the video. Thanks. \$\endgroup\$ Commented Aug 14, 2011 at 1:43

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