Shallow Thoughts : : hardware

How to Re-initialize a Stuck ESP32 (in CircuitPython)

I designed my particulate air quality sensor project around Adafruit's PyPortal. It uses a ESP32 coprocessor for networking.

Unfortunately, the ESP32 is a little flaky. It tends to lose track of the network after an hour or so:

ESP32 not responding
Traceback (most recent call last):
  File "code.py", line 182, in 
  File "adafruit_requests.py", line 725, in post
  File "adafruit_requests.py", line 649, in request
  File "adafruit_connection_manager.py", line 331, in get_socket
  File "adafruit_connection_manager.py", line 248, in _get_connected_socket
  File "adafruit_connection_manager.py", line 61, in connect
  File "adafruit_esp32spi/adafruit_esp32spi_socketpool.py", line 114, in connect
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 899, in socket_connect
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 801, in socket_open
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 422, in _send_command_get_response
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 378, in _wait_response_cmd
  File "adafruit_esp32spi/adafruit_esp32spi.py", line 292, in _wait_for_ready
TimeoutError: ESP32 not responding

I tried re-initializing the network, but it didn't help: re-initializing always died with Timed out waiting for SPI char and SCK in use.

There are lots of people asking about this on the net, but I couldn't find a discussion that actually had a solution for how to re-initialize a stuck ESP32. So I asked Claude. I know, AI, eww ... but Claude seems to have access to CircuitPython code and discussions that Google doesn't index, so sometimes it's the best way to find out how to solve CircuitPython problems. It took a couple of iterations (each requiring a few hours of testing, since it typically takes an hour or so before the network stops working), but we got there. Here's what seems to work for me.

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Particular: A Particulate Air Quality Sensor

We're thinking about replacing our ancient fireplace with a modern wood stove. There are lots of reasons, but one is that the house smells smoky when we use the fireplace (which is pretty much every night in winter), and I can't help wondering what all that smoke is doing to my lungs.

Dave insists that the smoke all gets sucked up the chimney and I shouldn't worry about it. I tried to look it up, but it seems like there's hardly any published research on that (or maybe I was just choosing the wrong search terms).

But why not actually measure it? I've occasionally wanted a particulate matter sensor anyway; we get a lot of wildfire smoke here in New Mexico most summers (sometimes from local fires, sometimes from as far away as California or Canada) and sometimes the air quality can get pretty bad.

Of course you can buy ready-to-go air quality sensors. But what's the fun in that, when you can make your own for about half the price? (If you don't count the value of your time, that is.)

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A Homebuilt CO2 Meter as a Virus Risk Proxy

Despite most of the world deciding that COVID is over, I continue to be cautious about it. (My one bout of COVID resulted in congestive heart failure which I'm still dealing with, so I'm fairly anxious not to get it again.)

That means that I'm cautious about indoor gatherings. Some places say they've upgraded their ventilation, but can you believe them? I've long read about people using CO2 meters as a proxy, to tell you how well the air is circulating and how high the virus load might be in a crowd, and I've wanted to get one of my own.

You can buy CO2 meters, of course. But making a custom one sounds so much more fun! Reading Wired's story about New Zealand's Kawaiicon cybersecurity convention that provided CO2 trackers inspired me to finally order some parts.

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Crimping JST PH Connectors

I've written about the several neat boards I recently ordered from Adafruit.

But when I ordered, I was confused about which connectors were which, and didn't end up ordering all the connectors I needed.

Adafruit calls the connectors they use "Stemma", and (I realized too late) they have a helpful page called What is Stemma? explaining the different connectors. I had ordered several of the small ones, "Stemma QT", more technically a 4 Pin JST SH, which were perfect for connecting a Feather board to a CO2 meter. But I hadn't realized that the bigger board, the PyPortal, needs a larger connector also called Stemma, more technically a JST PH.

It turned out to be hard to find JST PH connectors with wires already attached ("pigtails") and what I found were impressively expensive in lots of two or three. I imagine I might want a fair number of JST PH, especially the 2-connector type used for batteries. So I ordered a boxed assortment of 2, 3 and 4-pin JST PH connectors and a crimp tool.

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Installing CircuitPython on a Gemma M0

I wrote previously about my difficulties installing CircuitPython on an ESP32 Feather.

When I ordered the Feather, I ordered a bunch of other stuff too, including a tiny wearable microcontroller that's sold specifically for MicroPython: a Gemma M0.

Again, I had trouble getting MicroPython working, but the Gemma's problem was quite different.

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Installing CircuitPython on an ESP32 Reverse Feather TFT

I've been wanting to play around with CircuitPython for ages. I like Python, I like microcontrollers, what's not to like? Quite a while back, I even ordered a Feather M0 for that — but I didn't do my research, ordered the wi-fi version and it turned out that's the one Feather M0 that can't run CircuitPython.

This time I checked more carefully before ordering, and got a processor that for sure claimed to run CircuitPython.

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Automatic Plant Watering with a Raspberry Pi

An automatic plant watering system is a project that's been on my back burner for years. I'd like to be able to go on vacation and not worry about whatever houseplant I'm fruitlessly nursing at the moment. (I have the opposite of a green thumb -- I have very little luck growing plants -- but I keep trying, and if nothing else, I can make sure lack of watering isn't the problem.)

I've had all the parts sitting around for quite some time, and had tried them all individually, but never seemed to make the time to put them all together. Today's "Raspberry Pi Jam" at Los Alamos Makers seemed like the ideal excuse.

Sensing Soil Moisture

First step: the moisture sensor. I used a little moisture sensor that I found on eBay. It says "YL-38" on it. It has the typical forked thingie you stick into the soil, connected to a little sensor board.

The board has four pins: power, ground, analog and digital outputs. The digital output would be the easiest: there's a potentiometer on the board that you can turn to adjust sensitivity, then you can read the digital output pin directly from the Raspberry Pi.

But I had bigger plans: in addition to watering, I wanted to keep track of how fast the soil dries out, and update a web page so that I could check my plant's status from anywhere. For that, I needed to read the analog pin.

Raspberry Pis don't have a way to read an analog input. (An Arduino would have made this easier, but then reporting to a web page would have been much more difficult.) So I used an ADS1115 16-bit I²sup>C Analog to Digital Converter board from Adafruit, along with Adafruit's ADS1x15 library. It's written for CircuitPython, but it works fine in normal Python on Raspbian.

It's simple to use. Wire power, ground, SDA and SDC to the appropriate Raspberry Pi pins (1, 6, 3 and 5 respectively). Connect the soil sensor's analog output pin with A0 on the ADC. Then

# Initialize the ADC
i2c = busio.I2C(board.SCL, board.SDA)
ads = ADS.ADS1015(i2c)
adc0 = AnalogIn(ads, ADS.P0)

# Read a value
value = adc0.value
voltage = adc0.voltage

With the probe stuck into dry soil, it read around 26,500 value, 3.3 volts. Damp soil was more like 14528, 1.816V. Suspended in water, it was more like 11,000 value, 1.3V.

Driving a Water Pump

The pump also came from eBay. They're under $5; search for terms like "Mini Submersible Water Pump 5V to 12V DC Aquarium Fountain Pump Micro Pump". As far as driving it is concerned, treat it as a motor. Which means you can't drive it directly from a Raspberry Pi pin: they don't generate enough current to run a motor, and you risk damaging the Pi with back-EMF when the motor stops.

Instead, my go-to motor driver for small microcontroller projects is a SN754410 SN754410 H-bridge chip. I've used them before for driving little cars with a Raspberry Pi or with an Arduino. In this case the wiring would be much simpler, because there's only one motor and I only need to drive it in one direction. That means I could hardwire the two motor direction pins, and the only pin I needed to control from the Pi was the PWM motor speed pin. The chip also needs a bunch of ground wires (which it uses as heat sinks), a line to logic voltage (the Pi's 3.3V pin) and motor voltage (since it's such a tiny motor, I'm driving it from the Pi's 5v power pin).

Here's the full wiring diagram.

Driving a single PWM pin is a lot simpler than the dual bidirectional motor controllers I've used in other motor projects.

GPIO.setmode(GPIO.BCM)
GPIO.setup(23, GPIO.OUT)
pump = GPIO.PWM(PUMP_PIN, 50)
pump.start(0)

# Run the motor at 30% for 2 seconds, then stop.
pump.ChangeDutyCycle(30)
time.sleep(2)
pump.ChangeDutyCycle(0)

The rest was just putting together some logic: check the sensor, and if it's too dry, pump some water -- but only a little, then wait a while for the water to soak in -- and repeat. Here's the full plantwater.py script. I haven't added the website part yet, but the basic plant waterer is ready to use -- and ready to demo at tonight's Raspberry Pi Jam.

Easy DIY Cellphone Stand

Over the years I've picked up a couple of cellphone stands as conference giveaways. A stand is a nice idea, especially if you like to read news articles during mealtime, but the stands I've tried never seem to be quite what I want. Either they're not adjustable, or they're too bulky or heavy to want to carry them around all the time.

A while back, I was browsing on ebay looking for something better than the ones I have. I saw a few that looked like they might be worth trying, but then it occurred to me: I could make one pretty easily that would work better than anything I'd found for sale.

I started with plans that involved wire and a hinge -- the hinge so the two sides of the stand would fold together to fit in a purse or pocket -- and spent a few hours trying different hinge options.I wasn't satisfied, though. And then I realized: all I had to do was bend the wire into the shape I needed. Voilà -- instant lightweight adjustable cellphone stand.

And it has worked great. I've been using it for months and it's much better than any of the prefab stands I had before.

Bend a piece of wire

I don't know where this wire came from: it was in my spare-metal-parts collection. You want something a little thinner than coathanger wire, so you can bend it relatively easily; "baling wire" or "rebar wire" is probably about right.

Bend the tips around

Adjust the curve so it's big enough that your cellphone will fit in the crook of the wires.

Bend the back end down, and spread the two halves apart

Adjust so it fits your phone

Coat the finished stand with rubberized coating (available at your local hardware store in either dip or spray-on varieties) so it won't slide around on tables and won't scratch anything. The finished product is adjustable to any angle you need -- so you can adjust it based on the lighting in any room -- and you can fold the two halves together to make it easy to carry.