Mounting a 10 W laser on Geeetech Prusa I3 Pro B?

Question

I recently bought a 10 W laser (12 V) that I'd like to mount on my old 3D printer (Geeetech Prusa I3 Pro B) and control it by adjusting the fan speed. After reading the docs, the laser has 3 wires: PWN/TTL (3 V to 12 V signal), GND, and 12 V (aka - and +).

Here are the laser specifics:

• Model: LD3070HA (by voidmicro)
• Optical Output Power: 4 W
• Operating Voltage: 12 V
• Module Input Power: 10 W
• PWM/TTL Input: DC3.3 V-12 V 100 Hz~50 KHz Preferred: 5 KHz
• Input Interface: XH2.54-3Pin (+, -, PWM/TTL)

My 3D printer has a GT2560 board (docs, scheme) with a PWM FAN socket made up of 3 pins (plus, minus, and C for control I assume to return the fan rpm).

• I tried connecting the laser directly on the PWM FAN; the laser isn't turning on (I assume not enough power since the laser needs approx. 1.2 A).
• Then I tried connecting the laser + and - pins directly on the printer 12 V power supply and the PWN/TTL yellow wire (of the laser) on the + pin (of the PWM FAN). The laser is turning on! But always at 100 % intensity (even when I turn the fan off or change its speed).

I investigated a bit. On the motherboard PWM FAN socket, I got the following voltage value between the plus and minus pins :

Fan speed (%)	Fan speed (out of 255)	Approx. Voltage (V)
0	0	0
0.004	1	5
50	128	9
100	255	12

I don't know if my multimeter shows an average voltage or if it is not PWM used here (even if the socket name is PWM FAN, which for me leaves no doubt...)

Potential issue: However I found out that between the + pin of the PWM FAN (on the board) and the - / ground pin (of the power supply), the voltage is ALWAYS 12 V no matter the speed of the fan! That might explain why the laser is always at 100 % intensity and never shuts down.

I searched online and read that the grounds should always be connected together. So I tried connecting the twos minus pins (from the 12 V power supply and the PWM FAN on the board) and. sadly nothing changed.

A few pictures to illustrate:

To be honest, I'm running low on ideas. Any help would be greatly appreciated!

Answer 1

Fan ports (as well as heater ports) are controlled with MOSFETs that switch the ground connection on the port on and off. The positive connection (+12 / 24 V) is always on - this is not what your laser needs, since that wants a "positive" PWM signal switching between 0V and the quoted 3.3-12V with regards to ground.

To control your laser, you should use a different, easily accessible pin that you can configure as the fan output. Marlin lists PWM-compatible pins for 2560-based boards on their documentation page for laser support.

In the case of your GT2560, there are three viable pin that are both PWM-compatible and reasonably accessible:

• Digital pins 5 (orange) and 6 (green) on the LCD connector

• Digital pin 45 (violet), normally used as the E0 DIR pin

Using the LCD pins means losing the display; using E0-DIR would mean losing the extruder driver or having to move the E0-Extruder to use the E1-Extruders pins in your boards pin configuration file.

To use the pin of your choice as the TTL/PWM pin for your laser, you would define #define SPINDLE_LASER_PWM_PIN 45 and uncomment #define LASER_FEATURE in your configuration_adv.h file.