12. The Laser diode

The laser diode I used here is pretty strong. Aimed at 300mW this red laser is a Class 3b laser which means goggles must be worn at all cost.
You will get pinkeye and a cataract. It is not like with smoking were you could possible get cancer. No, looking into the beam will definitly get you a cataract. Even the scattered light the diode produces when bouncing of surfaces is stronger then directly looking into the sun. You don’t want to risk you sight. Period.


The laser goggles should filter 600-670nm (OD4+). Those glasses are not cheap but your eyes are precious !
OD4+ means that it does filter 10^-4 of the incoming (red) light.
300 mW * 10 ^-4 = 0,03 mW.

Laser diode pinout:
The first thing to do when having stripped the diode from an old DVD Burner or got it from the internet is to get the polarity of it.
I just took 2 AA batteries that were in a case with + and – and tried the pins of the diode until it lits up.

Laser diodes of this type are placed into an aixiz housing with heatsink. They often come with a focusable plastic lens. Glass lense are better as they give you about 10-20% more efficiency.

Adjust the power of the laser diode:
Before we want to hook up our laser to the circuit we want to adjust the “power” it will get.
With the blue potentiometer this is easy to do.
The red DVD burner diode can handle just about 300mV (respectively 300mA – with a load) but then I don’t know how long it will last.
If you want to increase the lifetime you may want to reduce the voltage the laser diode gets to around 200mV (respectively 200mA – with a load).
Anyways keep your eyes on cheap or donated DVD burners. Sometime the local recycling plant has some of these pearls in their trash. Kindly asking the service personal may get you fresh meat for your laser grinder.
You don’t want to adjust the power of the laser diode with the actual laser diode. Sounds strange but we will be using a so called dummy load.
A dummy load is placed in the circuit instead of the real diode. It acts as a load and you can steadily increase the power while measuring the voltage without damaging the precious salvage diode.
In the picture above you can see such a dummy load. This one simulates a red laser diode. If you are going to use a blue laser you simply need 6x 1N4001 diodes.


Red laser diode dummy Blue laser diode dummy
1N4001 4x 6x
1 ohm Resistor 1x 1x

Again use your breadboard and put the diodes and the resistor in series. On the resistor I measure the voltage. It doesn’t matter on which side you place the resistor. Set your multimeter to 2000mV and put it to the resistors’ ends. Connect the laser pins from the lasershield to + and – on the breadboard.
Load up gcodesender or your terminal of choice and connect to the Arduino.
Send the command “M3” (Spindle/Laser on) and you should get some value on your multimeter.
Turn the potentiometer on your potentiometer clockwise untill it reaches the desired voltage e.g. 300mV. This corresponds to the mW the laser diode will get.
CW = increase voltage
CCW = decrease voltage
Send “M5” to turn off the laser.

Focusing the laser:
To focus the diode I first turned the lens until I got a very small dot on the wall. Then I tried to light a match.
To get a “rough” focus I taped a ruler to my desk with the laser housing at 0mm.
A black sheet of paper (thicker paper like 450gr photo carton) was placed in front of the laser and moved until it burned.
You may need to play around with the lens and the paper distance.

To do the fine adjustment I proceeded similar again but this time I estimated the time it took the laser to burn a hole through the paper. This way you get very close to the perfect focus of the laser.


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