Laser operation with a USB 6008 DAQ

[Simba93]

I'm wondering how I go about using the USB 6008 DAQ in conjuction with labView to turn on and off ths laser: 

http://www.maplin.co.uk/p/1mw-3v-red-laser-module-le07h

The laser must be turned on using 3V which isn't possible with this DAQ. I was thinking of usinga BC 109 transistor, then have the laser connected to an external power supply which is supplying 3V to the laser. then have the laser connected to the collector of the transistor.

I set this up and it didn't exactly work out as the laser was drawing to high a current and not working correctly.

Can anyone help me?

[Taki1999]

I suspect that your switching circuit is the part that isn't working right.

Can you post details of how you've wired it up?

Also just to make sure you don't have a bad laser module, does the module work correctly when you connect it directly to 3V?

[Simba93]

The laser module is working correctly when 3V is connected to the power supply. 

Yeah the switching circuit was working correctly, my problem is the laser at 3V isn't at its full intensity but is at its full intensity at 12V which isn't correct. The laser needs to operate fully at 3V.

This is the cirucit I was trying to build as the switching.

** The 5V in the diagram is supposed to be 3V **

[SteveD123]

Have you measured the actual voltage across the laser with your circuit? The foward voltage drop across the transistor will be at least 0.7 volts, so there will be only 2.3 volts maximum across your laser.

Given the low voltage nature of your circuit, I'd try a 2N7000 series "FETlington".

[johnsold]

OK. The problem is that you need to supply about 30 mA to the laser to get it to work. The digital outputs of the USB-9008 are limited to 8.5 mA or less. Furthermore, a laser diode is a current operated device. If you apply a fixed voltage the current will vary depending on the exact doping used when the device was manufactured and on it instantaneous temperature. The curretn rises exponentially with voltage as in any diode. The link you posted does not have any real data about the electrical characteristics of that diode.

Typically a current regulating circuit is used to drive laser diodes.  The simplest circuit is a resistor in series with the diode connected to a power supply with a voltage higher than 3 V. The value of the resistor depends on the power supply voltage, the voltage drop across thes witching transistor, and how much current you want to run through the diode. If you have applied 12 V to that laser from a source which can provide substantial current (>50 mA), you have probably destroyed it.

Lynn