Machine wont operate unless i leave in the old wire where pwm goes

I have K40 with mini GRBL. When i connect it all up it does not work, no movement or laser when i disconnect the wire and replace with pwm. It only works with both connected.

I assume this has never worked?

Can you describe or provide a link to the instructions information on how it’s wired?

To the mini to the lps?


Mini Gerbil hardware installation - K40 laser upgrade - YouTube This is the link to the video. It says remove the existing laser power control wire from the power supply and replace it with the supplied wire to the pwm of the mini gerbil. The laser only moves and fires when the existing wire is left in.

Also the laser is just on, when moving between cuts it stays on and engraves a line. It also doesnt change power when engraving.

The video has a different board in it than this shows…

Which board/lps do you have?

Does the pwm wire you removed go to the pot for power control?


I have the one shown in the video, it is fairly old maybe the first type made.

I don’t know what that is… you can find a link for me to see the pinout.

I’ve spent my time, watching the video, reading the manual… you need to help me help you

You know what it looks like along with the pinouts - I don’t.



I hope these help

The blue wire is the new one the red is the existing one

The blue wire with the black connector

The pwm wire i remove goes to centre connection on pot

Where does the L and P wires lead?

On most lps, the L is for laser enable and the P is for water protect…

You may be using these for test/fire… where do the wires go?

First pin on the left is the ground, then P, then L.

If either of these are not at ground level it will not fire…

So it appear to me that if it’s firing by changing the IN wire, there are other wiring issues… as it shouldn’t fire unless you pull both of them to ground and have some kind of voltage on IN.


Im not sure what your saying. If you are talking about the J2 connector on the mini gerbil there is nothing connected. All connections are shown on the drawing. That is the way it is connected.

Laser on is on right hand side of j3 connector goes to power supply, there is no water protect

I read it here in the photo you posted…


You have a wires in both the P terminal and L terminal… Where do they go?


P goes to laser switch L goes totest switch

The laser should not fire unless, both L and P are low…

When both are low, it should lase at the IN voltage or pwm value.


I don’t think this will work the way it’s wired. You MUST have both fire and test buttons active (pulling P and L to ground) for the lps to fire…

Something is not right with this schematic… P is not held low, so it should never be able to fire from the control board.

Generally these are wired with the pot going to IN and the PWM going to H of the lps, the inverted L input… PWM is active high, L is active low… this lps doesn’t have an H terminal… The video is totally useless.

It’s only firing because you have the pot control still connected… and are manually activating the L and P inputs to the lps.

Which mini grbl board do you have…? I see no way for this wiring scheme to actually work no matter which mini grbl you have.

I’ll have to do some more research, but need to know the actual type of mini grbl controller you have… I need to access the technical documentation for it…

Please send a link to the actual control board you’re using.


This is the only documentation i can find that features my mini gerbil.