PDB for n00bs

PDB is the python debugger, which is very handy for debugging scripts. I use it two ways.

If I’m having a problem with the script, I’ll put in the line

import pdb; pdb.set_trace()

just before where the problem occurs. Once the pdb line is hit, I get the interactive debugger and can start stepping through the program and seeing where it blows up, and what variables are getting set to before that happens.

However, I recently found a very handy second way. I was debugging a script with a curses interface, which cleans up when it exits. Unfortunately, that cleanup means that my terminal gets wiped when something crashes, so instead of a stack trace, I just get dumped back to the terminal when something goes wrong, with no information at all left on the screen.

Invoking the script with

python -m pdb ./my_script.py

gets me the postmortem debugger, so when something goes wrong, the program halts and I get the interactive debugger and some amount of stack trace. It’s messy looking because of curses, but I can at least see what is going on.

New Swarm Controllers

I’ve ordered the second version of the swarm control boards. If you want some, you can get them here, but I advise against doing so until after a post shows up here saying either that they work, or that they’re busted.

In the mean time, I’ve been realizing that the boards are good for all sorts of stupid tricks. For instance, you can control people using galvanic vestibular stimulation, which uses 1-1.5mA at pretty low voltages (More academic version, more hacking). Since the swarm control boards already use a 3.7v lithium cell, additional voltage regulation isn’t needed (if anything, they may be too weak), and PWM can be used to control the current. A resistor in series might also be good, in case of… errors.

The same board could also be connected to a door latch, or magnetic strike, which would let a user connect to a web page (the ESP8266 can serve web pages and act as an AP) and put in a password to open the door. Lockitron appears to be making a business out of selling this, but the mechanics are cheaper.

Given that there’s also an I2C bus on the device, IO expanders, sensors, and other goofiness could be added to make wearables that respond to the environment, smart dust sensors, IoT nodes for home automation, scales that tweet about how much you weigh, etc. IoT is the new black! It’s a floor wax! It’s a dessert topping!

 

 

 

ESP8266, Serial Adapters, and Resets

As detailed in the previous post, I’ve been having some trouble getting the Arduino development environment to automatically reset my ESP8266 board using the DTR and CTS lines of the serial adapter. Part of my problem may still have been the cheap serial adapter, but today I found a new part.

The ESP8266 is extremely sensitive to noise on the CH_PD line, and I was using a 9″ long jumper to connect CH_PD to RTS. I confirmed with my O-scope that RTS was pulsing as it should, but the first pulse threw the ESP8266 into some weird state where it spewed noise on a bunch of pins (GPIO0 seemed to be the worst), and uploading would, naturally, fail.

Switching to 3″ jumpers cleared up the problem and let my Arduino IDE reset the ESP8266 as it should.

I’ve changed the schematic in the Github repo for the project to reflect the new reset wiring, but I still have to add a 5V input connection for charging the battery. Once that’s done, I can design a new PCB.

 

A Word of Warning

My PhD work (TinyRobo) uses a USB-Serial converter to talk to the ESP-8266 modules in the tiny robots. Normal FTDI cables have a cable that ends in a 0.1″ 6-pin header with this pinout:

  1. Black – Ground
  2. Brown – CTS
  3. Red – VCC
  4. Orange – TX
  5. Yellow – RX
  6. Green – RTS

It turns out that esptool can manipulate the DTR and RTS lines to reset the chip in bootloader mode, which is great for uploading code to it. It also means I can get away with not having any parts on the TinyRobo boards to handle the reset, which is great because it lets me keep the board small. Unfortunately, the FTDI cable I have doesn’t expose the RTS line, so I got a converter module for cheap off Amazon. The particular module I got is this one:

2016-04-15 14.10.13

I added that red wire and cut a trace so that the pins would be:

  1. DTR
  2. RX
  3. TX
  4. VCC
  5. CTS
  6. Ground

So far, so good, but I can’t upload with it. I threw a scope on the lines, and it looks like instead of swinging from VCC to ground like well-behaved TTL serial lines, they swing from VCC to VCC minus some tiny voltage, less than a volt. Adding pull-downs on the lines doesn’t seem to have helped. It could be that the timing is of, but I suspect that somewhere, some cheapskate saved some fraction of a cent on this board, at the expense of it doing the one thing it was supposed to do (YOU HAD ONE JOB).

Lightstone Repairs

I have a Lightstone, and a copy of the game “Journey to Wild Divine”, despite not being a whifty hippie. My friend Ne0nRa1n gave them to me years ago for biofeedback hacking, but pointed out that the finger sensor connections were broken.

The Lightstone is actually a USB interface to GSR and pulse sensors. Internally, the board uses a M430F133 chip from TI to call the shots, along with a ST72F623F2M1. Why there are two microcontrollers in there, I’m not sure. It could be that one is handling USB communication and the other is dealing with the analog signals, which is supported by the ST part being connected directly to the USB port (with a pair of test points along the traces) and the TI part having a lot of traces running to the analog section of the board.

Overall, the design of the hardware is solid. The Lightstone is easy to open up, the board is well-assembled and has what I’d consider good looking PCB design. There are lots of test points in the analog and digital sections. Spare GPIOs on the MSP430 are broken out to little pads for possible hackery. There are even populated headers that probably were used for programming the microcontrollers.

However, there’s one point where the hardware falls down. The finger contacts for the GSR and pulse sensors are on thin wires with minimal strain relief. Two nice microcontrollers, sweet board design, and it falls apart because wires break.

The sensors are on a 6-pin DIN connector, with red, black, orange, green, yellow, and white wires. The red and black wires each go to a GSR contact. The GSR contacts are silver or silver alloy buttons, so I want to keep those. The other four wires go to the pulse sensor, which is a three pin device. Looking into the front of the device, the left lead gets the yellow wire, the center lead gets the green and orange wires, and the right lead gets the white wire.

2016-01-01 21.31.12

To repair the fingertip sensors, I had to pull out the hinge pins that hold the sensor case together. I used very fine-tipped pliers for this, starting from the hinge and then grasping the tip once I had pushed the pin out enough. Then I took the sensors apart, and cut out the bad section of wire. In the image below, the spot to grab the hinge pin is just to the right of the small spring.

2016-01-01 21.23.35

I stripped the original cable, and wrapped the stripped sections in heat shrink. I drilled out the molded strain reliefs so I could thread the wires back through them more easily, threaded the wires, and used more heat shrink to improve the strain relief. My new wires are not as flexible as the old ones, but should be more durable. Finally, I soldered the sensors to the ends of the new wires, and put the sensor cases back together.

2016-01-01 23.05.01

If you do this repair, be careful soldering to the silver-alloy sensor buttons for the GSR sensor. The silver part is surprisingly easy to soften and distort with heat from a soldering iron. I slightly damaged one of mine, but managed to do the other one with no problems.

2016-01-01 23.59.10

I’m using liblightstone to get the information from the device. So far, it seems to be working fine.

Web Development with Flask and Python

I haven’t done anything that could properly be called web development since about 2002, when I took a college course in it. There have been a few developments in the field since then, and I’m a little rusty.

I chose Flask, because I like python and because Django seems like overkill for what I’m doing. There are literally dozens of frameworks out there, and I imagine some people know and care about the differences between them. If I had comments enabled, they’d be yelling at me to switch to Django right now. Hence, no comments.

Installing Flask is easy on Ubuntu:

sudo apt-get install python-flask

I copied the Flask “hello world” from the Flask page, ran it with

python snowflake.py

which got me the expected result, a web server running on port 5000 with a “Hello World” message.

My plan for this web app is to have users be able to visit some page, and the page will contain an image of a snowflake, generated from the url they used to visit. That’s it, but over on Facebook, my post about generating snowflakes from people’s names made people go just about nuts asking for them. Rather than generate them myself, I figured I’d write a web app.

Flask is a joy to work with. In debug mode, it detects changes to the file that contains the currently running app, and restarts when that file changes. It also presents a traceback and interactive debugger if something goes wrong in your app (it goes without saying that this needs to NEVER reach production, since it’s an interactive python shell on your server).

At this point, I have the core functionality of the app together, and I’m not even done with my beer. I can visit a URL, and a snowflake image gets generated from that URL. Everything else is details, and then deployment.

A couple of downloads later, the image now gets converted to png, and served back to the user as an image in a page. Soon, deployment!

KiCad is Still a Shitshow

Six months ago, I designed a schematic and circuit board in KiCad. Yesterday, I tried to open it and got a list of errors so long that it didn’t fit on my screen from top to bottom. The reason is that KiCad loads the footprints of the parts you use from Github, so when (not if) the KiCad devs change how they store footprints, your install of KiCad breaks. In other words, installing KiCad gives the KiCad devs permission to break your workflow for no reason.

I fixed my KiCad package list by writing a little python script that goes through the KiCad github repo and generates a new footprint list based on however they happen to be organized and whatever they happen to be called today. Copy that over my previously working footprint list, and the error message went away.

However, since the last time I used it, my install of KiCad has stopped rendering in CVPCB. No error message or anything, just doesn’t render the copper layers, parts, etc. I got part of the ratsnest, and that’s it.

Clearly, the thing to do is to install the newest version of KiCad from the developer’s PPA.

Nope, that returns a 404.

Clearly, the thing to do is to install using the script the devs wrote to install and build KiCad.

Nope, the script fails because they use a version of WXWidgets that like no one else on earth uses.

So let’s recap: No method of installing KiCad works, and just LEAVING YOUR WORKING INSTALL ALONE is not sufficient to ensure that it continues to work.

Since I’m designing a small PCB, CadSoft’s Eagle is looking more and more tempting. Limited PCB sizes aren’t a problem if you’re making something small.

Structured Light Scanner

2015-10-05 23.15.31

I got the projector back in the spring, planning to do some projection mapping. I got distracted by some stuff for a festival, but some co-workers and I were talking about structured light scanning recently, so I threw this together.

For software, I’m probably going to do the calibration stuff in Python and OpenCV. I’d like the first demo to be perspective correction, to project on non-flat surfaces. Second will probably be virtual lighting.

Serious MOSFETs

2015-08-26 23.44.43

I’m designing a simple H-bridge for simple but large projects. These are 300A 40V MOSFETS. The board also has a driver for the MOSFETs. I hope to find a driver that uses I2C or some other interface, rather than PWM.

2015-10-12 22.13.25

The board overall is pretty small, but I haven’t figured out a good way to heat sink it. The unpopulated round footprints are for capacitors, and when the caps are installed, they block any easy installation of a heat sink over the MOSFETs. I may design the second iteration of the board around thermal management, and have holes for mounting a commodity CPU heat sink over the FETs.

The current design of the board is available here.

I’ve tested a prototype of the current design, and it does work, but I didn’t stress it very hard.

2015-09-13 17.07.01

ToyBrain Rides Again

Now with WiFi, current monitoring, and the ever-worrying li-poly battery chemistry.

image

I haven’t tested the WiFi module or motor drivers yet, but the board is fully populated and the battery regulation circuit works.

There are a few design changes in the pipeline, but I’m going to go for a full hardware test and try to find more bugs before I create version 2 of the boards.

The current design is in github, along with all the docs.