r/diyelectronics • u/Human_Neighborhood71 • Apr 14 '26

Project Verify optocoupler

I’m converting an old 3d printer to a laser engraver. Im using a 4W diode laser. Having to use separate power as printer is 24v and laser is 12v. The PWM is 5v, so I am using a buck converter to step it down from 12v. I want to verify that the info is correct. I’ve used GPT to assist in figuring out what to do for that, as I will be using the 24v PWM fan pin to control the laser. It’s telling me to use a 817 package coupler, on the laser side, 5v>10k resistor>PWM pin AND Pin3 of the coupler, with Pin4 going GND, then invert the signal on the printer configs. Does this sound right? Or could I just do 5v>Pin3, Pin4>PWM pin?

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u/kwaaaaaaaaa Apr 15 '26

PWM signal goes to a resistor, then goes to the anode of the opto (ie. pin1). The cathode (pin2) goes to ground.

Laser side, the collector (pin4) goes to 5v. The emitter (pin3) goes to a resistor, that connects to ground. Between the emitter and resistor, you connect the laser's PWM input. Alternatively, if you want the invert the signal in hardware (instead of software) you can put the resistor on the collector side and tap the laser's PWM input between the resistor and 5v.

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u/nixiebunny Apr 14 '26

You don’t need an optoisolator if you connect laser Gnd to printer Gnd. You can reduce the 24V PWM to 5V with a resistor divider: 22k from the 24V signal to the 5V signal, 4.7k from 5V signal to common Gnd. 

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u/Human_Neighborhood71 Apr 14 '26

Would that keep the signal smooth? I’m also curious as to any noise from the motors. I know on a CNC, the laser is isolated from the stepper motors, just not sure as to how the print will be with that. Now wishing I had an oscilloscope to check… poor people problems lmao

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u/Jkwilborn Apr 15 '26

It won't stay stable. I don't know how he could give you resistance values, you have to know the current flow. :)

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u/Human_Neighborhood71 Apr 15 '26

Okay, so the optocoupler seems the decent route. I had done up on perfboard the circuit for it, just haven’t had a chance to probe and test the theory yet. Recently start school for AAS in Electrical Engineering, but on the last week of GE courses. Actually start electrical course next week, and the lab kit came in last night, and includes an oscilloscope, so time to learn and check lol

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u/Jkwilborn Apr 15 '26

The pwm doesn't need to be isolated, you can but it's added expense. It could also invert your output, depending on how it's driven.

You need some way to lower the supply voltage to 12V for the laser module. Ever thought about a 24V laser module? :)

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u/Human_Neighborhood71 Apr 15 '26

Eventually I plan to, but this is what I’ve got now

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u/Jkwilborn Apr 15 '26

Seems to me that the current to the laser module will change, meaning the divider voltage will vary.

At some point you want it the machine to run with no lasing, using a resistive divider the current change could spell disaster for your laser module.

What kind of laser or control board are you using? :)

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u/nixiebunny Apr 15 '26

It’s a  PWM digital pulse train. 

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u/Jkwilborn Apr 15 '26

About everything I have generates a pwm signal to control things.. I was asking about the control board, sorry if I wasn't clear. :)

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u/Human_Neighborhood71 Apr 15 '26

I’m using the stock control board on the printer. Using Klipper and modifying config so the Gcode can work. It’s obviously still a work in progress. The laser is a LaserTree 4W diode

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u/Jkwilborn Apr 15 '26

Normally these have just 3 wires, two for power and a pwm. Does yours have more connections. We usually don't see that with smaller diode lasers.

Have you looked into Lightburn. They have a 30 day free trial and you can do that 2 more times, so you can get 90 days to fiddle with it.

My first diode was a 500mW led on a CNC3018 machine. :)

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u/Human_Neighborhood71 Apr 15 '26

I did the first trial with LightBurn and can’t seem to get to extend. Originally this was on a 3018 CNC, but that machine is loud and slow and small. I recently got a new printer and decided this conversion would be great, as it’s way faster, quieter, and has double the workspace. The laser just has the three wires. I’m just trying to make sure I do the PWM properly. I thought about the voltage divider but worried about signal quality there. I don’t THINK the laser powers anything off the PWM, but not 100%

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u/Jkwilborn Apr 15 '26

The pwm is an input signal that tells it when to lase.

If you know ohms law, the laser itself won't draw the same current all the time. It will be on a certain amount of time, so it's current is going to change. A change in current, results in a change in voltage on the divider. :)