How to connect a cheap laser kit to your reprap
If you want to engrave things with your reprap you can add a laser diode. I have come across some guides that explain how to salvage one from an old DVD burner. There is also the quite expensive JTech Photonics addon which I'm certain would provide some advantages over this solution, but it was out of budget.
However I saw that these things are available quite cheaply from the usual places on the net and promise 3x the power of the DVD burner diode.
You will need a heatsink for that and a driver, that looks like this
There are quite some variations so I will not show a pinout, but it should be fairly easy to distinguish the correct pins. They are clearly marked on the backside of the PCB on mine.
Before connecting the board to the printer we have to tune the output power. I have found this procedure to work okay (after burning through my first diode).
- Disconnect the laser from the driver.
- Connect the driver to a 12v power supply as indicated on the board.
- With a multimeter test the voltage between the Laser + and - terminals. Adjust the voltage knob until you are just about the rating of your diode (mine is 5v)
- Turn the current knob a few whole circles to the left, to make sure we won't fry your diode.
- Set your multimeter to measure current and make sure it can go up to 1A DC.
- Wear your protective goggles and connect your diode, and your multimeter in series. This might require cutting the cables or using jumpers.
- Turn the current up slowly until you reach the rating of your diode (1A in my case).
I saw a sudden drop in my measurements at around 0.5A (current went down to 0.25A) without an obvious drop in optical power (I have no way to measure that, I just observed the laser power through the goggles). I was not willing to sacrifice another diode so I dialled back and up to 0.49 again, just before the drop and didn't push it towards its rating of 1A. I would love an explanation from somebody who understands lasers better than me.
While searching for info on how to connect this thing to my reprap, I read the recommendation to connect the TTL input of the driver to the fan output. This didn't work for me (probably because the TTL pins want 5v and the fan is 12v). Even if it did work however, I would have to disconnect the fan and connect the laser every time I wanted to switch from 3D printing to laser engraving.
The solution I found to be adequately working is to connect the 12v to the power supply (obviously) the laser to the laser, and the TTL+ to one of the spare pins of my Melzi (D10). I suppose every board should have one or two spare digital pins. The TTL- goes to the 5v ground.
You can then control laser ON/OFF by issuing
m42 P10 S0 for OFF and
m42 P10 S255
There are two drawbacks with this setup that I haven't found a way to fix. One is
that the laser turns on when I turn the printer on and I have to turn it off
by issuing the
m42 P10 S0 command from pronterface. The other one is that
PWM does not work, just on and off. This isn't much of a problem since you can
control the thickness of your lines by varying the speed, or by making multiple
Here's a video of it in action: