KLP1 3D Printer Hotend Fan Control Board

KLP1 3D printer has a problem that the hotend fan is constatly on, it's connected directly to +24 V rail. I've created a small board that's quite easy to assemble and gives control over hotend fan.

I've also created a gerber with PCB, so you could just order it.

• 5×5 field double sided 2.54 mm pitch perfboard (cut to size with a dremel)
• S2B-PH connector (JST-PH 2.0 mm pitch connector angled) is prefered but stock B2B-PH will also do, but you need to bend the pins and they can snap easily, so it's recomended to either have spare vertical PH connector if the stock one breaks.
• AO3400A (or similar SOT23 N-channel MOSFET, need Vdss of above 25 V and Vgs at 3.3 V)
• 10 kΩ 0805 smd resistor (5 kΩ to 100 kΩ should suffice, 0805 package fits nicely between SOT23 pins)
• 2 short insulated thin wires

This is what we are trying to build. A simple N-channel MOSFET low side switch. GPIO11 already has a 100 Ω gate resistor. "3D_Touch" is GPIO11 R42 is 100 Ω.

Solder the SOT23 MOSFET in shown position.

Solder a pulldown resistor between gate and source – 10 kΩ 0805 smd resistor (5 kΩ to 100 kΩ should suffice, 0805 package fits nicely between SOT23 pins).

Solder a short thin insulated wire, around 5 cm. There is plenty of space for the wires so can be longer.

Either bend the pins of the stock connector (warning: they are very fragile and can easily snap, better have a backup) or buy angled connector S2B-PH-KL.

!!!! If you end up using the original connector, be sure to swap the fan wires, as the pins can only be bend in the opposite direction, so that they will reach the PCB.

Solder a short insulated wire from +24 V of the PH connector. I did it from the bottom as it's easier.

At this point you can test the board if it's wired correctly.

Add wire to GND, plug in fan into the connector. Connect 12 V or 24 V power supply to +24 V connector, PSU ground to board GND the fan should not be spinning. Connect the GPIO11 pin to positive rail and the fan should start spinning.

Desolder the original connector from the THR board. In my case I've also desoldered unused pins for ease of access, but it's not necessary. Solder the GND and GPIO11

In klipper, change the MKS_THR.cfg, add the lines:

[heater_fan heatbreak_cooling_fan]
pin:MKS_THR:gpio11
heater_temp: 70.0