Low voltage on pin 11 for laser

Hi all, I’m trying to set up a laser using pins 11 and 14 controlling the Jtech 2.8 watt upgrade. I’m issuing the M03 and M05 commands using the macro function on UGS. When I do this, pin 11 only shows about 120 mili volts which goes down to 0 volts on an m05 command. Any ideas?



What S parameter are you sending?

Try sending a “M3 S12500” command. Most likely what you’re seeing is the PWM scaling of the spindle output.

Are you using the Jtech GRBL version or the Inventables GRBL version?

Thanks for getting back Justin. That command works! Can you tell me why it’s different to the M03 command? I guess I’ll have to edit all my Gcode to account for this do I?
Thanks ever so much for your help though.


The S parameter sets the spindle rpm value. For a spindle, you set the rpm value with the level of the pwm out.

The newer version of GRBL has a variable for max/min rpm so you can set it to be 1 so a “M3 S1” to turn on completely.

So would you suggest that I get a newer GRBL version? I want to produce grey scale type work. Isn’t that where the pwm capability comes in? As you’ve probably guessed - I’m a complete novice…
You’ve made my evening - I’ve been on this all day!


I don’t deal with the laser.

@LarryM should be more helpful concerning that.

I have modified 1.0c grbl to support the JTech laser. I would recommend that you download and install it to make working with the spindle and laser more convenient.

Even easier to use if you also get a PicSender license.

.hex link


Cool. I’ll give it a go… Thanks