It took me about 4 different circuit designs to get it working. I destroyed my router the day I got it as I had no idea what was inside the potted control box. After a autopsy I found out that there was no microcontroller inside. There was a Telefunken U211B3 chip instead, which is designed in 1996 by the way. I think I killed the chip as I tried to connect it straight to my Arduino which was powered by an earthed USB. I got a new control box from Ebay and for some reason its voltage was 5.2V and my first one’s was 3.6V. Maybe it has completely different chip inside it.
Next I tried my circuit with an opto-isolator. The problem was that the control box could supply next to no current at all. 0.5mA of current collapsed the controllers voltage and limited the RPM range I could get from it. The solution was to add a double insulated power supply and a linear regulator that I used to fine adjust the voltage. This way I did not have to worry about current consumption of the digital potentiometer and the isolator. Now I can even widen the RPM range on the router by making the voltage just a bit higher.
I attached a schematic below. I have only tested it on the bench as I have not yet built my CNC. I have no idea if the router has enough torque on the low RPMs.
I take no responsibility so use this circuit at your own risk and only if you understand what is going on with it. If you have questions I can try to answer them but I am not an expert on electronics.
Makita_speed_controller_MCP4161.pdf (66.8 KB)