[sudo-discuss] vending machine homing switch mystery solved

so i've been working on the black sudoroom vending machine (vending1): https://sudoroom.org/wiki/hack-o-mat which had previously been hacked by students at UC Berkeley (i think) I believe it's a Snacktron like Emma and Ed's machine. (unlike our brown vending machine which is a snackshop II) pictures of their machine can be seen here: http://spaz.org/~jake/pix/vend/ they had ripped out the original control unit, and wired it up with a bunch of reed relays and an arduino. The motors of the vending machine are wired as a grid of rows and columns, so the controller needs to be able to enable a single row and column in order to power one product motor. But it also needs to be able to detect when the product motor has rotated a full turn, and is back at the "home" position where it needs to stop. There's a "home" sensor on the back of the motor, but the way it's wired was a mystery for a while. Here's some pictures of the homing circuit: http://spaz.org/~jake/pix/vend/IMG_20160401_015211.jpg http://spaz.org/~jake/pix/vend/IMG_20160401_015700.jpg as you can see, the only difference between the button being pressed or not pressed, is that the 420nF capacitor is in parallel with the motor or not. Since the whole thing is in series with a diode (to protect against trying to turn a motor backwards) we were stumped as to how to detect whether a motor was homed or not. Thanks to Zach who donated some test equipment including this awesome signal generator (along with a nice power supply and bench multimeter): http://imgur.com/LTSFfOe I was able to connect an AC waveform in series with a 12VDC supply to one of the motors, and figure out how to detect whether the home switch is closed or not. Basically, you make a 31KHz (or similar frequency) squarewave in series with the power supply, and watch the current through the motor. If the button is open and the capacitor is not in-circuit, the motor's inductance absorbs the AC and the current draw is relatively smooth. But when the motor arrives at the home position (when you need to stop delivering product!) the capacitor gets switched in, and now you'll see a lot of the AC on the current path of the system. Here is a video showing the voltage across a 22-ohm series resistor while the motor is rotating with 12VDC: https://www.youtube.com/watch?v=xHIGDw5HRCg apparently it takes that motor about 11 seconds to dispense a product and finish returning to home position, assuming the product weighs nothing. by the way, you'll notice that i'm using the old analog oscilloscope. Unfortunately the LeCroy 9510 has started crashing, basically bluescreening. It probably needs to be opened up and cleaned, as it got wet after a fire before it was donated to sudoroom. Or maybe someone wants to donate a nicer oscilloscope to sudoroom? anyway, now I know how to detect the motor homing signal, and I just need to wire up this machine to its arduino to detect the signal and dispense product more accurately. If anyone wants to help, please let me know. by the way, if you are wondering how the UC students detected when the motors were finished dispensing product, the answer is absurd: they put a laser beam across the path where product would fall when dispensed, and they watched for the laser beam to be interrupted. did it work? I don't know. probably the semester ended and they got their grade and moved on, and then they donated the vending machine to us. -jake