Wednesday, January 26, 2011

Aixiz module finally arrived!

After waiting for a month, I finally got the module from Aixiz. I had been very busy anyway, so the delay didn't really matter.

Here it is: the 650nm 5mW Aixiz module!

I also got an extra lens for it, made of glass. I'm sure it's better suited for a laser CNC than the plastic one the module came with.

This module is needed for collimation and heat sinking. I will put the laser diode inside the module and focus the beam with the glass lens.

Next up: how to insert the 250mW laser diode into the Aixiz module.

Sketching the CNC

So, at this point, I have a scanner that I'm going to use as a base, with a stepper controlling the movement along one axis.
This is the scanner I'm going to use as the base.
I also have a stepper motor, a metal rail and a pulley system that will be used to move along the other axis.

A stepper motor from an old printer.
A metal rail & old printhead.

I was thinking about the possibilities how to create the XY-controlled CNC from these parts, and came up with three sketches.

I admit, I'm very bad at drawing, but anyway, I decided to share these sketches with you.

My first idea on how to construct the CNC was not a very brilliant one:

First idea of the CNC, a very shitty one. The laser points up.

The surface to be engraved would be held upside down over the CNC byt something, and the beam is pointed upwards from the scanner. Well, you can imagine how this would make the whole set-up and engraving process a pain in the ass. Especially with heavier items.

Next idea, already a bit better one:

2nd sketch of the CNC.

Now the scanner is held upside down, and the engraved surface is lying beneath it. The rail is sticking a bit out from the side (it's wider than the scanner). The laser points down. The problem with this one is that it limits the engravable things a bit. It would be hard to put bigger items under it, like a guitar, a synth, or things like that.

With the third idea I think I nailed it.

Third and probably the final sketch of the CNC.

Now you can put whatever items under it. The metal rod & pulley system is held high by a big piece of something. I just wonder how the scanner motor & tray will pull it off with all that weight. Obviously I'll need to make it as light & balanced as possible. Or then I need something to hold the rail from the other end...
I have no idea yet how I will do all of that. These are just the first ideas I came up with.

Laser diode driver circuit

Here's the schematic of an adjustable laser diode driver circuit. It's based on this post by nickname Daedalus on the Laser Pointer Forums. This post has gotten so popular that people are referring to the circuit as a 'DDL circuit' (DDL is Daedalus' signature in the forums). It's just a constant current driver, but Daedalus was the one who popularized the use of this in DIY laser scene.


From various sources in the internet, the voltage of the red 16x DVD burner led is found to be 3.2V and it's said (in various sources in the 'net) to withstand 250mA current. The laser output power is said to be 250mW. That is the power of the light coming out of the diode. It's not needed for these calculations.

The circuit, like this one, is using two 10 ohm resistors in parallel for the limiting resistor. So the total resistance is 5 ohms, and the resulting wattage is 2x the wattage of the resistors. 5 ohms results in a maximum output current of 250mA.

The two capacitors in the input and in the output can be for example 10 microfarads both. They are there to flatten any voltage spikes coming from the input. The bigger the caps, the flatter the voltages. 10 microfarads should be more than enough.

According to this Davshomepage article, the optimum input voltage is: LD voltage 3.2V + 3V needed by LM317 + 1V reserve = 7.2 V.

However, I'm planning to use this one with a 9V power supply. That means the LM317 will have to dissipate some extra power. Thus, I calculated some power dissipation figures. These are not necessary - if you use the same parts, the circuit will work just the same.

The maximum power dissipation in the whole circuit (including LD) is 9V*0.25A = 2.25 W.

Maximum power dissipation in the driver circuitry is 9V-3.2V*0.25A = 1.45 W.

Maximum power dissipation in the limiting resistor 1.25V * 0.25A = 0.3 W
(the LM317 makes sure there's always 1.25V over the limiting resistor. 0.25A is the maximum current)

Maximum power dissipation in the laser diode is going to be 3.2V * 0.25A = 0.8 W.
(3.2V is the laser diode voltage and 0.25A is the maximum current)

The main thing to look at here is the maximum power dissipation in the limiting resistor. So if I use two 1/4 W resistors, I get about 1/2 W rating for the parallel connection and that's more than 0.3W, so the resistors shouldn't burn out even with maximum output.

I will probably use some salvaged heat sink, at least for the LM317. For the LD (laser diode), the Aixiz module works as a heat sink and should be enough.

Parts list:
Laser diode from a 16x DVD burner drive
LM317 adjustable regulator (preferably in TO-220 package)
2 x 10 ohm 1/4 W resistors
100 ohm potentiometer
2 x capacitors, for example 10uF (microfarads)
A piece of metal as a heat sink for the LM317.
650nm 5.0mW 12x30mm Aixiz module (to house the laser diode)

Stay tuned for part II, where I'll build this circuit.

Related links: 
Davshomepage - Constant current source - A good page that helped me with the power calculations.
Laser Pointer Forums - Daedalus' DIY Homemade laser diode driver - The classic DIY Laser driver.
It can be done - Laser driver - Another page on the laser driver circuit.
LM317 on Wikipedia
National Semiconductor LM317 regulator datasheet
Resistors: color codes, wattages... - A great tutorial page for dummies.