Thursday, April 28, 2011
Z-axis homing & opto-isolation board
I designed a 5-channel opto-isolator board to keep the ugly outside world away from my HobbyCNC controller board. I designed the board to run from around 12 volts, but I added a constant-current source to each input line. This ensures 20ma flowing through the opto isolator LED. The added benefit is I can add extra LED's to the 'chain' without affecting the signal and I can safely run the limit switch wiring through the same wiring harness as the stepper motor cables with no fear of interference.
I added a yellow LED on the board for each of the 5 inputs - this allows me to verify an input is working without needing to grab a voltmeter. The constant current source also allows me to put another LED out where the work is done.
To complete the board is a green power LED and a fuse. In the images, the opto chips are actually surface-mounted on the underside of the board - the one lit yellow LED is for the limit switches - this tells me all 6 limit switches are closed (good).
I created a special rig to set the z-axis home. It uses an alligator clip and a precision brass 1/2" square bar. I added an LED in the bar so I can confirm all is good before I press "home". Any problem with this setup and the system will drive the tool into the workpiece - a real big issue if using small drills for PCBs. The short video shows the Z axis home setup being tested to confirm continuity, then the video shows the opto board during the same test.