I recently picked up a second hand/defective 'Renkforce RF100' printer. A pretty obscure German brand. The problem with it was: broken hotend fan cable which caused clogging that the previous user was unable to fix. I have thoroughly cleaned the hotend, rewired the fan and now it works again.
The mainboard it used is not very well known, designated as MPX.3 which seems similar to this HICTOP brand RAMPS 1.4 board. Renkforce's documentation is very sparse. Through some digging I found a GitHub repo with some customizable Marlin firmware and this diagram:
No real circuit diagram to be found for this specific Renkforce board, however. It does look similar to the MKS Base 1.0 board. As you can see, it has support for up to two extruders, a fan and a heated bed ("EEFB"). Only one extruder (E0, labeled 5 on the picture above) is in use on this printer.
Here is where I need some advice:
I would like to add a part cooling fan to this printer. Right now, only the hotend fan (40 mm, 12 V 0.15 A) is connected to the 'Fan' (labeled 7 in the picture above) header on the board and it will run at 100 % as soon as the hotend reaches >50 °C. Now, I could of course wire the hotend and parts cooling fans together, but I would like to have separate control over the parts cooling fan, in order to only run it at specific times (e.g. ramp it up after the first few layers are printed).
My question: Could I, for instance, just screw the parts cooling fan leads into the spare extruder terminal (E1) and re-program the firmware to let it know that that pin (D7, labeled 6 on the picture above) is a FAN_PIN? Which I could then ideally address/control with the G-code
M106. In a sense I would repurpose the E1 terminal for a fan - do I need to worry about (fire) safety here? Since it's an extruder terminal? Or is that terminal "simply" a 12 V connection that I can connect to whatever? Do I need to add a MOSFET next to the terminal? Or am I overthinking this and should I use another position / pin on the board? This is really the limit of my electronics knowledge right now and safety is a primary concern. Any input is appreciated!
Edit for future reference: this GitHub repo has a modern (at the time of writing) version of Marlin suitable for this printer.