Category Archives: Prototyping

NeoPomodorino: A Pomodoro style timer

This is a simple Pomodoro style timer I made using a Proto Shield, an Arduino clone, a NeoPixel 24 LED ring clone, and various bits and pieces from my parts drawer.

The NeoPomodorino in action

The NeoPomodorino in action

The single button is wired to pin 3 of the Arduino using a simple RC debounce circuit featured some time ago on Hackaday. It’s probably not needed, but what the hell.

Debounce circuit

Debounce circuit

The LED ring is wired to pin 6 (like it is in every NeoPixel code example I’ve seen so far). As suggested a few places, I have a 470 ohm resistor in series with the data pin, and a electrolytic capacitor across VCC and GND. Like the debounce circuit, these are also probably not needed.

Usage

  • Long button presses (> 500ms) will turn the device on and off. This also resets the timer.
  • Short button presses will add 1 minute to the current timer.
  • Each pomodoro consists of 25 minutes of work (red color) followed by a short 5 minute pause (yellow color).
  • Every fourth pause is a longer one of 25 minutes (green color).
  • Between each pomodoro, the device will briefly flash which number is (blue color). 1 quarter of the ring = 1 pomodoro.

Get the code on GitHub.

Happy Hacking! 🙂

RPIWeather: My wireless monitoring system

Some of the first posts on this blog were about how to interact with the famous nRF24L01 2.4GHz radio chipset from both an Arduino and a Raspberry Pi. Well.. Much have happened in the mean time, but at least I have managed to put together something using what I learned back then. I decided to make a wireless monitoring system for environmental data in and around our house with at least one sensor node in each room as well as a couple of sensors outside. It is called RPIWeather and you can get all the code and documentation by clicking the name. This post is just meant as a short summary.

RPIWeather consists of a bunch of wireless sensor nodes, a base station that collects the data packets, a database server, and one or more frontends. My goal has been to collect data at 5 minute intervals and keep it indefinitely for analysis, comparison, and visualization. This creates some demands for power efficiency and reliability, which I find interesting.

The sensor nodes

My first prototype was based around an Arduino and some proto-board and is described here, here, and here. It looked like this:

Arduino with nRF24L01 and DHT22

Arduino with nRF24L01 and DHT22

To keep things simple, clean, and small, I decided to ditch the Arduino platform and code in straight avr-libc for the ATtiny84 chip from Atmel. So I made another prototype, but this time with an ATtiny84 instead.

Partial prototype without the DHT22 sensor

Partial prototype without the DHT22 sensor

Little by little a schematic emerged. Besides the radio, the sensor and the MCU, I also needed some sort of power supply. As the sensor nodes are to be distributed throughout the house, this implied battery power, but it couldn’t just be any old battery. The thing is that the radio chip can only tolerate up to 3.6v and the DHT22 sensor can only operate down to ~3v, which leaves a narrow band of usable voltages. A traditional voltage regulator would have burned though the batteries quite quickly, so I had to put in a switching boost converter. This way 2 AA NiMH batteries can deliver a reasonably stable 3.3v supply voltage for the circuit. Pro tip: when buying NiMH batteries, do yourself a favor and use the low self-discharge kind like Eneloop. In this use-case they will go for several months between charges.

Schematic for the sensor nodes

Schematic for the sensor nodes

With the schematic in place, I made a PCB layout and sent it to ITEAD for manufacturing. Here are a couple of pictures showing various stages of assembly. The funny shape of the boards was needed to make them fit snugly into some cool boxes from New Age Enclosures.

Sensor node PCBs

Sensor node PCBs

The first 10 finished boards

The first 10 finished boards

Inside a single sensor node

Inside a single sensor node

A single sensor node

A single sensor node

The back of a sensor node

The back of a sensor node

The 10 first sensor nodes

The 10 first sensor nodes

In addition to the DHT22 sensors, I have also made firmware for collecting data from a wind vane and a rain gauge. But I haven’t deployed any of that yet. Maybe next spring…

All in all the main results for the sensors are:

  • They work! I have tested concurrent operation of 10 units without major problems.
  • Firmware optimized for ATtiny84 @ 1 MHz (8MHz crystal with the CKDV8 fuse set)
  • Temperature and humidity measurement using a DHT22 sensor
  • Wind speed and wind direction measurement using a La Crosse TX23 anemometer
  • Rainfall measurement using a WS-2300-16 rain gauge
  • Wireless operation using the very popular nRF24L01+ 2.4 GHz radio chip
  • Very compact CRC32 implementation
  • Battery powered operation using a very efficient 3.3v DC/DC converter
  • Power management by sleeping the CPU and turning off unneeded hardware
  • Battery voltage measurement included in data packet
  • Typical battery draw while sleeping has been measured to around 17uA
  • Total firmware size from 3.0 kiB to 3.5 kiB (depending on sensor type)

The base station

I use a Raspberry Pi as base station. To make it more appliance-like than a typical Raspbian installation, I have instead installed a variant of Tiny Core Linux called piCore. This is basically a very small Linux distribution that loads all programs into memory on boot and leaves the SD card alone after this. This eliminates the usual risk of corrupting the file system when doing a power-cycle.

On the base station I have installed a small python script that continually polls the nRF24L01 radio using the Raspberry Pi’s built-in SPI interface pins. When a data packet arrives, it is buffered (in case of network problems) and forwarded to the database.

The database

With my limited data requirements, I could probably use any old database out there without issues. However, given the nature of the data, it was natural to select a so-called timeseries database. For this project I chose InfluxDB, which was very, very easy to set up on my Ubuntu-based file server. It automatically exposes a simple HTTP interface for storing and querying data, which is just what I need for this project.

The frontend(s)

Well, I’m not quite there yet… So far the only frontend is the admin interface that is built in to InfluxDB. Not very user friendly (not to mention the Wife Acceptance Factor)! The next step now is to make some sort of nice frontend for this. I have several ideas floating around my head:

  • An intranet website. The easiest way.
  • An Android widget. Both the wife and I have Android smartphones.
  • Dedicated hardware devices. This would be a great use-case for my new ESP8266 wifi modules and some 84×48 Nokia LCDs I have lying around.

Stay tuned…

Making prototype perf board hardware

In my last post, I described the various electronic components in my soon-to-be wireless thermo- and hygrometer. In this post, we’ll look at how to wire up some prototype perf boards.

Connecting an nRF24L01 module and a DHT22 sensor to an Arduino

An nRF24L01 radio module communicates with its host using SPI. Luckily for us, the Arduino (as well as most of the Atmel AVR chips in general) supports SPI directly in the hardware. The hardware SPI pins on the Arduino UNO are digital pins 11 (MOSI), 12 (MISO), and 13 (SCK). Furthermore, we need to wire up the radio module’s pins for CSN (Chip Select), CE (Chip Enable), VCC (3.3V), and GND. There’s also a pin called IRQ, but it seems to be used less commonly. All I/O pins are 5V tolerant, but VCC must not be above 3.6V according to the datasheet. This means that we don’t need to have a level-converter chip in order to use the radio module with an Arduino.

nRF24L01

nRF24L01

I connected the module to my Arduino board like this:

nRF24L01 pin: Arduino pin:
1 (GND) GND
2 (VCC) 3.3V
3 (CE) Digital pin 9
4 (CSN) Digital pin 10
5 (SCK) Digital pin 13
6 (MOSI) Digital pin 11
7 (MISO) Digital pin 12
8 (IRQ) Digital pin 8

Please note that the SPI pins are totally different on Arduino Leonardo boards, where they can only be accessed through the ICSP pin header. Backward compatibility FTW…

The DHT22 sensor is much simpler to hook up. It has four pins, but we only need three of them. I would have liked to use one of the remaining “traditional” digital pins on the Arduino, but due to the infamous pin header alignment bug, it’s much easier to reconfigure one of the analog input pins as a digital pin when using perf board. I have used pin A5, which is also known as digital pin 19.

DHT22 pin: Arduino pin:
1 (VCC) 5V (3.3V would probably also work)
2 (DATA) Digital pin 19
3 (N/C)
4 (GND) GND

According to the datasheet, there’s supposed to be a pull-up resistor from the data pin to VCC. In practice it seems that the weak internal pull-up in the ATMEGA chip is enough. The sensor uses a weird 1-wire protocol on the data-pin, but fortunately we have the library provided by Adafruit.

With this knowledge it is just a matter of finding a suitable piece of perf board and the various needed bits and pieces. I must say that I never thought I would find a purpose for this exact piece of scrap perf board, but in this case it’s perfect.

Arduino shield

Arduino shield

When mounted it looks like this:

Arduino with nRF24L01 and DHT22

Arduino with nRF24L01 and DHT22

Connecting an nRF24L01 module to a Raspberry PI

Now we are ready for part two of this post. Again, we are going to use the hardware SPI port provided by the hardware. All the pins we need are located in the big 26-pin P1 header. There’s a nice write-up about P1 (and the other headers) on eLinux.org. I have connected the nRF24L01 module like this:

nRF24L01 pin: Raspberry PI pin (P1 header):
1 (GND) 20 (GND)
2 (VCC) 17 (3.3V)
3 (CE) 16 (GPIO 23)
4 (CSN) 18 (GPIO 24)
5 (SCK) 23 (SCLK / GPIO 11)
6 (MOSI) 19 (MOSI / GPIO 10)
7 (MISO) 21 (MISO / GPIO 9)
8 (IRQ)

This is mostly based on the documentation for this code I found while researching how to connect the parts. It is a Raspberry Pi port of a Beagleboard port of Maniacbug’s nice RF24 library for Arduino. The author writes that he doesn’t know if it works or not. Well it doesn’t. Not right away at least. It took some work to get running, but I’ll get back to that in the next post.

My adaptor board:

nRF24L01 adaptor board for Raspberry Pi

nRF24L01 adaptor board for Raspberry Pi

And mounted on the Pi with a radio module:

Mounted adaptor board

Mounted nRF24L01 adaptor board on Raspberry Pi

That’s it! Our prototype hardware is finished. In the next post I’ll explain how to get a simple ping/pong test working.