The following features are added using a small PCB:

1. Safety features

  • Interlock switches for the three doors
  • Over temperature input for the coolant temperature
  • Flowmeter input for the coolant
  • Status LED (Laser active / inactive)

2. Comfort features

  • 4 relay outputs (coolant pump, exhaust fan, air compressor, AUX)
  • 12V nozzle blower
  • 12V cooling fans (e.g. for closed loop cooling system)

Before starting:

Working on the K40 laser is dangerous. The power supply has output voltages of 25kV touching this during operation is will cause death. Also the mains power can be dangerous for you life. If you don't know what you are doing consult an expert.

BEFORE modifying the Lasercutter REMOVE power cord. Never touch the high voltage output of the power supply even if the power cord was removed.

Wiring of the signals

Safety Assist PCB

Laser Power Supply

L-Laser Tube Cathode
FGChassis Ground
ACPhase AC 115/230V
ACNeutral AC 115/230V
PWater protection (normal operation when LOW, Laser off if OPEN)
LLaser Switch (active LOW)
INLaser Current Control (PWM / analog 0V-5V)
5VOutput +5V
24VOutput +24V

How it works

1. Safety features

All interlock inputs are connected as a chain. This means all of them must be closed to switch on the laser. If not, the laser remains off. If some of the features (e.g. flow meter) are not used the connector pins must be shorted.

2. Comfort features

The comfort features all relay on the presence of the M2 nano EX output (connector 5). This is available from board revision 7 or higher. The M2 nano outputs a high level on that pin if the stepper motors are anabled. It also provides an afterrun feature because the pin stays high for some time after cutting has been finished.

Additional material

Things required to complete the project

Here some Aliexpress links to temperature protection switches, flow meter, micro switch.

Flow metertemperature switch, microswitch

3D printed acessories

Door switch mount


This project is released under the Attribution-NonCommercial ShareAlike 4.0 (CC BY-NC-SA 4.0)

Pictures of my K40

Mounting of the PCB

Microswitches as door interlock

Closed loop water cooling