Mini Rambo FAN1 always on? for Laser TTL/PWM use

Hope someone can shed some light on this. I plugged in a laser (The TTL/PWM pin and a ground) to the FAN1 port on mini rambo. 12V power is going directly into the laser and is not coming from the mini rambo. Turning it on the laser was always 100% and adjusting FAN1 speed from the LCD did not do anything. Is it supposed to be ON like this at full S255 or 100%? I have a small display on the laser that tells me as well that it is indeed getting 100% from FAN1 port without me sending any GCODE.

My firmware currently is the default ryan provides for the mini rambo plus the shop LCD controller.

Laser accepts TTL/PWM 3.3v-12V.

The TTL pin is coming out of this board and going into mini Rambo FAN1 port. Am I doing something wrong?


The fan port is lind of funny. It is easier to toggle the negative side than toggle the positive side. So in reality, the positive side is always tied to 12V. Thr negative side is either ground (when the fan is on) or floating (well, tied through an LED to 12V).

You can connect the ttl to the negative side of the fan. Don’t connect anything to the positive side. But it will be backwards. That will probably also start with it powered on.

Cool. Would fan2 behave same way then? And if it works could I still just use it with m106 and 107 or do I need to modify firmware? If I do that I might as well map a PWM pin and hopefully that doesn’t behave like always High Fan port.

Thanks for chiming in.

The M106 S0 will turn it all the way on and M106 S255 will turn it off. Other than that, it should work.

On my SKR Pro board using a logic pin, not fan pin (I reassigned the fan pin to a logic pin), with a banggood laser, I have to use a pulldown resistor (2.4K between the laser input and ground) otherwise the thing kicks on full blast when power is applied, no matter if the pin is at logic zero.

I’m not using one of those boards you linked.

If your laser comes on when applying power and the logic pin is disconnected (floating) I’d put in a pulldown resistor just for safety sake. It’s possible that board, like my SKR pro, doesn’t bring the voltage down hard enough to turn the thing off.

These silly things are dangerous, can’t believe they don’t put a pulldown resistor on the laser itself, stay safe!

Next digikey/mouser order I’ll get a 2.2K surface mount resistor and solder the thing on the laser itself.

Thanks for your input. So the board I linked above is a pretty cool thing. It is for NEJE lasers which reviews point to currently being the best China laser company. Pretty decent priced too.

The small board takes 3.3v to 12v and then talks to the laser driver itself. There is a switch on the board that lets you either control the laser manually or switch over to TTL mode and only then the SKR or mini Rambo will be able to control it.

So its like another switch you must press before laser turns on, which I like over direct connection and GCode mistake making it go crazy for something this dangerous.

Should not be any need for resistor as this $11 board lets you put in 3.3v-12V so FAN port or a PIN both would work fine.

Challenge is as per above, when I move over to TTL the mini RAMBO even when not running GCODE is sending ON state or 100% TTL/PWM or that is what the board is thinking it is getting from the mini Rambo FAN port, normal connections.

EDIT: Hmm I think I see what you mean. I could try the resistor. One would have hoped this mini board with its added logic took care of this.

Ok so the Fan port 1 and 2 both work TTL 12v as intended plug and play just like jtech modules with 12v ttl/pwm but you just can’t use same ground or power supply for both.

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That makes sense. I was over thinking it. It may depend on the kind of power supply (the 2 pin LED/laptop plastic ones will probably work and the meanwell/metal ones might not). Glad it’s working for you.

Yep. It was a PC power supply.

Thinking on it. Driving from two different moles wires might just solve it. Or as you say just try one of those laptop ones.

My power supply laptop one is the shop 6A one and laser wants 3A by itself so thought that’s not a bright idea using shop one alone.


I think to have the same issue, but I do not understand the cause and, more importantly, your solution.

I am trying to control a laser by means of exactly your TTL-control-board using the PWM-fan-output (of a duet 2 wifi board), but when plugged the fan output into the TTL input of the laser control board it is always at maximum power. I am not understanding the reason for that as well as your solution. Can you give me more details about both?

My best regards

Hi mate, this goes back a year but what I recall is that if you use the same power supply for both the laser and the board then laser will always stay full on. Atleast on mini Rambo. Try different sockets as well.

Oh yes!

You are right, only now I remember that the first test using a different PSU for the laser worked as expected. Thank you very much.


This may be due to the RAMPS (and by extension RAMBO) boards switching the negative, rather than the positive, side of the circuit.

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