Meetup report: 10 May 2018

TL;DR: Interesting evening, and we’ll do it again this Thursday.

The tables

Earlier in the week, I managed to redesign the pendulum ball holders to snugly fit the magnets in place, as well as being a bit more springy an nicer-looking. My mission for the evening was to print out a few examples and tweak until happy. I did get a few prints, but the bottom layers are squished and misaligned, leading to a misshapen print that did not fit that great. I think I’m overextruding.

Some prints

I also tried a newer version of Cura, but that resulted in skipped steps which messed up the last print I did.

Otto's rfid indicator project

Otto brought a very interesting project he’s doing for a friend: An ESP32 with OLED screen and RFID reader integrated into a handheld tool, with a Mutitoyo indicator attached. The idea is that this is used to make measurements easier. The user would position the indicator at a specific point and then scan an RFID tag near the point to capture the reading. The tag also stores the allowable range of readings to indicate a pass or fail. The ESP32 also serves as a web server to automatically transfer the readings to a central database. This eliminated manual entry errors and makes the whole process faster.

We discussed placing of a possible RFID sensor for home entry. Putting it into the mains power meter box seems a bit risky and uninsurable. However, the front gate has an extra access plate which is accessible and probably has 12V power already, so it should be pretty easy to wire in there.

Bob the Biped

Tom and Steph worked some more on the Alphabot. Not sure what they managed to figure out.


Ruan brought a disassembled smoothie maker. He is hoping to use the motor to make a wood lathe. We figured out how it’s controlled and came to the conclusion that the motor was probably too fast and had too little torque for a direct drive.

Triac for speed control

He then worked on the square tube layout for the lathe.

Lathe design

All in all, a nice evening.

Meetup report: 3 May 2018

We had an interesting evening. My phone camera is fuzzy, and I forgot to take lots of photos again...

Otto showed us his 3D printer battery holder for an ancient Festo drill/driver. Great design, with space for 4 18650 cells and a voltmeter to provide manual discharge cutoff. A really solid way to upcycle a rugged tool with modern batteries. No photos, aaargh.

Otto also brought his Fluke scopemeter, and we hooked it up to the pendulum wave op-amp output and the FET gate. There’s a lot of noise, but the signals from the coil picking up the magnet (amplified by the op-amp) comes through nicely. We got a 500mV swing either way of the average.

coil op-amp signals

You can also clearly see the asymmetry in the signal caused by the coil being offset from the center position. The output of the op-amp shows a clear grouping of 2 pulses with a longer pause between them, exactly as planned.

Driving signal, interference and pulse length

We measured the op-amp pulse length, and it’s about 50ms from the start of the rise to the zero-crossing. This corresponds well to my observations that a driving pulse of longer than about 45ms did nothing to drive the pendulum faster.

We picked up no interference from the other pendulums. However, we saw some big spurious signals when the other coils were firing.

Steph and Tom got the Arduino on the Alphabot working, and with some help from a spare lipo battery I had lying around, got the wheels turning. Needs batteries.

I also got the 3D printer printing, but the filament broke inside the extruder, where I could not see it, so I only printed a part once everyone was gone.

Altogether a nice evening. Doing it again this Thursday 6:30 pm.