Arduino ASCOM Focuser Pro2 DIY / Discussion / General Discussion: SCT implementation, USB only power supply, manual focus allowed

Hello folks, I want to thank Robert for his incredible work with the Pro2 focuser.

I used it to build this implementation for my 8" Edge HD SCT for which I wanted to power it just with the USB port (no 12V) and also be able to use the manual focus knob if needed without physically disconnecting the motor. I also wanted the whole thing in a single box attached to the scope and as small as possible. For this purpose, I did break out the circuit around three sides of the focuser, so didn't use the official PCB. The USB cable comes from the top. On the left side I have the Arduino Nano, on the back I have a small board to hold what's needed for buttons, buzzer and temp sensor, also a step up DC DC converter to 12V, then finally on the right side I have the drv8825.
I found the current limiting pot on the DRV8825 very imprecise at low currents, extrmely hard to set and not trusty to keep that value over time. I needed to keep the current low within USB 2.0 spec of 500mA hence I removed the pot and soldered the appropriate resistors to keep the current <350mA. I also noticed a significant inrush current (the DC-DC boost) of more than 4A for a few microseconds at power up, so I added a current limiting IC (XC8107) to slow down and limit the inrush current and I also added a beefy cap to the DC-DC boost input to make it stable. For the motor a picked a slim 14HM08-0504S Nema 14 0.5A motor that I drive from the 12V of the boost converter. The SCT has a much lower torque requirement than a rack and pinion focuser of course. For that I would have needed a motor with reduction gear. Last item I added is a 3.5mm audio socket to accommodate the cable for the two buttons focus remote. Note: add 0.1uF caps on both buttons!!

Thank you!!
Enzo