Meetup 20181206

The last meetup of the year was quiet: Only Bouwer arrived, so we experimented on his project.

Bouwer needed a circuit to buzz when pressure is applied to a sensor mounted on a stick, basically. He had some Velostat, which is a good choice in this case for building the sensor.

The idea is to build a circuit with an op-amp set up as a comparator to then drive the output buzzer.

The velostat sensor is put in series with a resistor that’s roughly the same resistance as the velostat at the desired force. This is mounted between VCC and ground, and the center point is tapped for the voltage.

The other input of the op-amp is simply connected to the center pin of a potentiometer, also connected from VCC to ground. That provides a fixed reference voltage divider that is settable to adjust the switching point.

He had a prototype with velostat, aluminium on either side, and some copper foil tape. We quickly found out that the copper foil adhesive is nonconductive, which is why this did not work.

We rebuilt the sensor, sticking the foil to some Depron foam, and sandwiching the velostat between these electrodes. Much more solid and repeatable.

We built the circuit and powered it with 2 coin cells. It worked perfectly!

Further improvements would be to add some resistors to increase the sensitivity of the potentiometer, and using a plain comparator instead of the op-amp.

It turned out to be a fun evening. It’s really nice to take a bunch of components and debug your way to a working setup.

Meetup 20181129

We had a very full garage this week! Lots of people, interesting projects, even though we had loadshedding from 8pm to 9pm.

Full Garage

I did a few labels on my masking tape labeller, after having found a way to make it run consistently without first having to recompile the Arduino code.

Steph cut out the wings for his pico stick (copied off my stick’s wings) and worked on his 7-segment display.

Albert and Stefaan (2 learners from Paul Roos) visited to talk about some projects they were thinking of doing. Later, they disassembled one of my old hard drives. Hopefully an enjoyable evening for them.

I helped Reuben go through the settings on his Marlin installation to get it to work with the Trinamic 2130 drivers with SPI. The extruder (non-spi) works now, but not the rest.

Bouwer (first-time visitor) wanted some help to design a circuit that would sound a buzzer if more than a certain amount of pressure is put on a mat made with Velostat (interesting material!). Otto talked him through a circuit and the tradeoffs he needs to make.

I talked with Jan a bit to sort out details for the belt tensioning on the Maslow, and he was busy with the hardware wiring looms.

Otto was working on getting his laptop set up with all the tools on Ubuntu. Looks like his Salae clone does not work on it yet. We tried looking at the SPI signals on the 3D printer board.

Otto also got an Itsy Bitsy set up with 3 serial ports, talking to an Arduino. This is for a project where he needs lots of serials. We discussed the pros and cons of Micropython.

Kim popped in for a few minutes. She had started growing cell cultures on a silicon mold she made from my 3D-printed surfaces, to see if they would follow the grooves caused by the layered build process. This should be interesting…

Altogether a really nice evening.