Implementing a simple menu interface on OLED display

While working on a project to automate environmental control in our greenhouse, I needed to implement a menu interface on a small OLED display. In this sub-project, meant to test the concept, I’ve used a Teensy 3.1, a small I2C-driven 0.96" monochrome OLED display and a rotary encoder.

Bill of materials:

Description

This project is a proof-of-concept for using a rotary encoder to manipulate an on-screen menu of options. A number of electronics design concepts are used here.

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Carl Sagan’s “Baloney detection kit” is arguably more important now than ever. His 9 rules for critical thinking work for science and they can work to detect political baloney, too.

Arguments from authority carry little weight — “authorities” have made mistakes in the past. They will do so again in the future. Perhaps a better way to say it is that in science there are no authorities; at most, there are experts.

Question: when was the last time you heard Trump provide an evidence-based argument for anything? He relies on the argument from authority in every sphere. He deliberately seeks out people on the basis of charisma rather than expertise - the mark of an amateur.

Resetting the Syncthing index

I use Syncthing[1] to keep my laptop, desktop, and workshop computers in sync.[2] At least 99.9% of the time it works perfectly. Rarely, it seems to choke because of some edge case that I’ve never been able to sort out. But it never recovers on its own. Instead, it continues to report that a remote is 99% done syncing.

The workaround that I’ve learned is to simply reset the index. When the index gets rebuilt everything automagically works. You can’t do it via the GUI; you have to execute a REST call against the server. It took me a while to find it.

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curl -X POST -H "X-API-Key: abc123" http://localhost:8384/rest/system/reset?folder=default

If you want to erase the entire index, execute the call without the folder parameter. Otherwise, provide the name of the folder. The API key isn’t abc123; it’s actually found in Actions > Settings > API key. Before executing the call, I pause syncing on both sides, rebuild the index, then start it up and let them go at it.

Reference


  1. No, there's no iOS client. I'm OK with that.

  2. Why don't I just use Dropbox like everyone else? I trust peer-to-peer syncing because I'm in control. I don't know what Dropbox is up to.

Displaying Cyrillic fonts on a 128x64 OLED display

Recently I picked up a couple inexpensive 128x64 pixel OLED displays with an I2C interface. It turns out that displaying Russian text on these displays is not difficult. But it’s non-obvious. This is a brief description of how to make it work.

First, there’s a variety of these little displays and they’re all seemingly configured a little differently. I used this device for this test.

There are two options for libraries to simplify communicating with SSD1306 boards:

The u8g2 library has a much more robust mechanism for selecting fonts, so that’s what I used.

Here’s the code in full:

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#include <Arduino.h>
#include <U8g2lib.h>

#ifdef U8X8_HAVE_HW_SPI
#include <SPI.h>
#endif
#ifdef U8X8_HAVE_HW_I2C
#include <Wire.h>
#endif

/*
Illustrating the use of cyrillic text on the 128x64 OLED display
*/
U8G2_SSD1306_128X64_NONAME_F_SW_I2C u8g2(U8G2_R0, SCL, SDA, U8X8_PIN_NONE);

void setup(void) {
u8g2.begin();
u8g2.enableUTF8Print(); // enable UTF8 support for the Arduino print() function
}

void loop(void) {
// select a font with 11px height
u8g2.setFont(u8g2_font_cu12_t_cyrillic);
u8g2.firstPage();
do {
u8g2.setCursor(0, 40);
u8g2.print("Всем привет!");
u8g2.setCursor(0,12);
u8g2.print("Как у вас?");
} while ( u8g2.nextPage() );
}

References

Reading data from Si7021 temperature and humidity sensor using Raspberry Pi

The Si7021 is an excellent little device for measuring temperature and humidity, communicating with the host controller over the I2C bus. This is a quick tutorial on using the Raspberry Pi to talk to this device. If you are unfamiliar with the conceptual framework of I2C or how to enable I2C access on the Raspberry Pi, I suggest starting here. Otherwise, let’s jump in.

You are probably working with the device mounted on a breakout board. I used this one from Adafruit. There are no surprises on the pins that it breaks out - Vin, 3v out, GND, SCL and SDA. One the 40-pin P1 header of the Raspberry Pi, SDA and SCL for I2C bus 1 occupy pins 2 and 3.

Once you’ve wired it all up (don’t forget common ground connections to the Pi,) then we’re ready to write some code. First, we need to study the device a little bit.

Si7021 Humidity and temperature sensor

The device is quite accurate for temperature, typically ±0.4 degrees C. The humidity is ±3%. It can operate down to -40 degrees C, which is important in Canada where I live!

The I2C implementation on the device is straightforward. It has a fixed I2C hardware address of 0b01000000 (0x40). The instruction set is not large, but there’s some nuance which we’ll explain. First, let’s get some #define statements out of the way in our code:

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#define SI7021_ADDR 0x40

// I2C COMMANDS
#define SI7021_MRH_HOLD 0xE5
#define SI7021_MRH_NOHOLD 0xF5
#define SI7021_MT_HOLD 0xE3 // measure temp, hold master
#define SI7021_MT_NOHOLD 0xF3 // measure temp, no hold master
#define SI7021_RT_PREV 0xE0 // read temp from last RH measurement
#define SI7021_RESET 0xFE // reset
#define SI7021_WR_USER1 0xE6 // write RH/T user register 1
#define SI7021_RD_USER1 0xE7 // read RH/T user register 1
#define SI7021_WR_HCTL 0x51 // write heater control register
#define SI7021_RD_HCTL 0x11 // read heater control register
#define SI7021_RD_ID1 0xFA 0x0F // read electronic ID 1st byte
#define SI7021_RD_ID2 0xFC 0xC9 // read electronic ID 2nd byte
#define SI7021_RD_REV 0x84 0xB8 // read firmware revision

Read More

RF communication between Arduino Nanos using nRF24L01

In this tutorial I’ll go through a simple example of how to get two Arduino Nano devices to talk to one another.

Materials

You’ll need the following materials. I’ve posted Amazon links just so that you can see the items, but they can be purchased in a variety of locations.

  • Arduino Nano 5V/16 MHz, or equivalent (Amazon)
  • Kuman rRF24L01+PA+LNA, or equivalent (Amazon)

About the nRF24L01+

The nRF24L01+ is an appealing device to work with because it packs a lot of functionality on-chip as opposed to having to do it all in software. There is still a lot of work to be done in code; but it’s a good balance between simplicity and functionality. It’s also inexpensive.

What follows is a lengthy description of the nRF24L01+ device. If you just want to connect up your devices, then you can skip to the device hookup section.

nRF24L01+ theory of operation

There are several libraries for the nRF24L01 in the public domain that seek to simplify interactions with a variety of MCU’s. While they are fine (and we’ll make use of one here) you should understand how the device works so that when you inevitably branch out from the basic demonstration projects, you know how to achieve what you want. Read the nRF24L01 datasheet. I’ll start out here by reviewing it at a high level.

More than likely you are working with a breakout board for this surface-mount device. So you will concern yourself only with the following pins: Vcc, GND, CE, CSN, IRQ, MISO, MOSI, SCK. For the purposes of this example, we won’t be using the interrupt line IRQ so you can leave it disconnected.

The nRFL01 has a relatively simple instruction set for the SPI interface.

Read More

Using the Raspberry Pi to communicate over the I2C bus using C

I recently wrote about using the excellent bcm2835 library to communicate with peripheral devices over the SPI bus using C. In this post, I’ll talk about using the same library to communicate over the I2C bus. Nothing particularly fancy, but you’ll need to pay careful attention to the datasheet of the device we’re using. TheTSL2561 is a sophisticated little light sensor that has a very high dynamic range and is available on a breakout board from Adafruit. I’m not going to delve into the hookup of this device as you can take a look at the Adafruit tutorial for that. Note that we’re not going to use their library. (Well, I borrowed a bunch of their #define statements for device constants.)

TSL2561 functions

The TSL2561 has two analog-digitial (ADC) channels. Channel 0 responds to broad spectrum visible and IR wavelengths, whereas channel 1 responds to IR only. For most applications, you’ll address channel 0.

TSL2561 I2C interface

The TSL2561 datasheet is a little confusing because the device family also uses the SMBus and the format differences get lost between the text and the figures. The bottom line with the TSL2561 is that if you want to read a register, you write to the COMMAND register, then read a byte. It’s important to understand how the COMMAND register is configured so that you can read and write to the appropriate registers. Here is the COMMAND register format:

Note that the CMD bit (7) must always be set. For ordinary read/write operations, we’ll leave the CLEAR, WORD, and BLOCK bits unset. The remaining 3:0 ADDRESS bits specify the register that we are addressing. The registers are found in Table 2, reproduced below:

Editorial note: don’t be tempted to figure out the bits and encode the command yourself. Always use symbolic references for bit positions. By using symbolic references to bit positions and register addresses you will make your code much more readable. If you configure the COMMAND register as 0x8A, then I have convert the hex to binary and refer back to the datasheet to understand what you’re trying to do. On the other hand, if you configure the command as TSL2561_COMMAND_BIT | TSL2561_REGISTER_ID then I can immediately see you are addressing the ID register.

Sample code

I will go through a working example section by section and provide a github link at the end where you can grab the entire code.

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char buf[3];
uint8_t err;

printf("Running ... \n");

if (!bcm2835_init())
{
printf("bcm2835_init failed. Are you running as root??\n");
return 1;
}

if (!bcm2835_i2c_begin())
{
printf("bcm2835_i2c_begin failed. Are you running as root??\n");
return 1;
}

In our main function, we begin by declaring variables we’ll need later and call two important functions on the bcm2835 library: bcm2835_init() and bcm2835_i2c_begin(). The former sets up our library and from the documentation:

Initialises the library by opening /dev/mem (if you are root) or /dev/gpiomem (if you are not) and getting pointers to the internal memory for BCM 2835 device registers. You must call this (successfully) before calling any other functions in this library (except bcm2835_set_debug). If bcm2835_init() fails by returning 0, calling any other function may result in crashes or other failures. If bcm2835_init() succeeds but you are not running as root, then only gpio operations are permitted, and calling any other functions may result in crashes or other failures.

bcm2835 library I2C module

The latter starts I2C operations by forcing P1-03 (SDA) and P1-05 (SCL) to their alternate function ALT0 thereby enabling them for I2C use. After all I2C operations are done, the program should call bcm2835_i2c_end() to return those pins to their regular functions. Note that for the purposes of this demonstration, I check all of the return codes and printf an informative messages. In a robust application we would want to deal with this in a more fault-tolerant way.

Next we’ll set up some features of the bus:

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bcm2835_i2c_setSlaveAddress(TSL2561_ADDR_FLOAT);
bcm2835_i2c_setClockDivider(BCM2835_I2C_CLOCK_DIVIDER_150);

After that, we ready to work with the device. Let’s begin with a simple reading of the ID register. To simplify matters, we’ll create a reusable function readRegister():

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uint8_t readRegister(uint8_t reg, uint8_t *fail) {
uint8_t b[2];
b[0] = TSL2561_COMMAND_BIT | reg;
int err = bcm2835_i2c_write(b,1);
if( err != BCM2835_I2C_REASON_OK ) {
printf("Unable to write command register %02x\n",err);
*fail = 1; return 1;
}
err = bcm2835_i2c_read(b,1);
if( err != BCM2835_I2C_REASON_OK ) {
printf("Unable to read last command response %02x\n",err);
*fail = 1; return 1;
}
*fail = 0;
return b[0];
}

When we want to read a register, we just need to pass the address of the register and a pointer to a uint8_t in which we’ll return the status (0 for success and 1 for failure.) Why don’t we just return a status? It’s becuase we’re already returning the results of the read. When the caller passes the address of a status variable, we can fill it, and the caller just looks at it afterwards.

In lines 2-3, we are building the COMMAND “register” value to send. Because the datasheet says to set the CMD bit, we do that. Then we logical OR the address into bits 3:0. Then we write the COMMAND register to the device and read a byte. Remember that we’ve already set the hardware address previously.

So calling readRegister() to read the hardware ID will look like:

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//	Read the ID register

uint8_t id = readRegister(TSL2561_REGISTER_ID, &err);
if( err == 1) {
printf("Check ID register failed.\n"); return 1;
}
printf("The ID is %02x.\n",id);

We can do something similar to read another register, such as the TIMING register 0x01h:

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//	Read the timing register

uint8_t tr = readRegister(TSL2561_REGISTER_TIMING,&err);
if(err == 1) {
printf("Check timing register failed.\n");
return 1;
}
printf("The timing register is %02x.\n",tr);

On my device I get a value of 0x03 which is the default power-up value according to the datasheet.

Now we need to get down to the business of writing to a register. Since we have to explicitly turn on the ADC, we’ll have to write to a control register. A generic writeRegister() should help with this. Again our design uses a pointer to a uint8_t to return the status. We don’t have to do this because a write operation has no useful return, but for API symmetry, I wrote the function the same way.

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void writeRegister(uint8_t reg, uint8_t val, uint8_t *fail) {
uint8_t b[2];
b[0] = TSL2561_COMMAND_BIT | reg;
int err = bcm2835_i2c_write(b,1);
if( err != BCM2835_I2C_REASON_OK ) {
printf("Unable to write command register %02x\n",err);
*fail = 1; return;
}
b[0] = val;
err = bcm2835_i2c_write(b,1);
if( err != BCM2835_I2C_REASON_OK ) {
printf("Unable to write command register %02x\n",err);
*fail = 1; return;
}
err = bcm2835_i2c_read(b,1);
if( err != BCM2835_I2C_REASON_OK ) {
printf("Unable to read following write command register %02x\n",err);
*fail = 1; return;
}
*fail = 0;
return;
}

Writing to a register is similar to reading except that after addressing the register, we have to send it some data in a subsequent write operation. Following those two operations, we have an obligatory read and move on.

Lines 3-9 address the COMMAND register as we did before. Lines 9-14 write the caller’s specified value to the address specified in the preceding COMMAND call. Then a read that we can disregard and return to the caller.

Turn on the ADC

Turning on the ADC couldn’t be easier; we just need to address the CONTROL register 0x00. The CONTROL register documentation tells us that we simply need to set the POWER bits (1:0) to 0x03 to power up the device or 0x00 to power it down.

Doing that in code using our generic write function couldn’t be simpler:

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writeRegister(TSL2561_REGISTER_TIMING,TSL2561_CONTROL_POWERON, &err );
if( err == 1 ) {
printf("Unable to power on the TSL2561.\n"); return 1;
}

Take a broad spectrum reading on Channel 0

Now we come to the reason we started working with the device, to take a light measurement. We’re going to focus on the visible + IR channel (Channel 0) but the same principles apply to either channel. We’re just going to do sequential reads from the two channel 0 registers and assemble the result:

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uint8_t LSB0 = readRegister(TSL2561_REGISTER_CHAN0_LOW, &err);
if( err == 1 ) {
printf("Unable to read LSB0\n"); return 1;
}
uint8_t MSB0 = readRegister(TSL2561_REGISTER_CHAN0_HIGH, &err);
if( err == 1 ) {
printf("Unable to read MSB0\n"); return 1;
}
int lux = (int)(MSB0 << 8) | (int)LSB0;
printf("Light value is %d lux.\n",lux);

There’s a lot more that we could cover, both about the operation of the device and about using I2C on the Raspberry Pi in general, but this should be enough to get you started with luminosity measurement using the TSL2561 or in beginning to code your own I2C interfaces using the BCM2835 library on the Raspberry Pi.

References

Implementing ADC using Raspberry Pi and MCP3008

Several years ago I wrote about adding analog-to-digital capabilities to the Raspberry Pi. At that time, I used an ATtinyx61 series MCU to provide ADC capabilities, communicating with the RPi via an I2C interface. In retrospect it was much more complicated than necessary. What follows is an attempt to re-do that project using an MCP3008, a 10 bit ADC that communicates on the SPI bus.

MCP3008 device

The MCP3008 is an 8-channel 10-bit ADC with an SPI interface[1]. It has a 4 channel cousin, the MCP3004 that has similar operating characteristics. The device is capable of performing single-ended or differential measurements. For the purposes of this write-up, we’ll only concern ourselves with single-ended measurement. A few pertinent details about the MCP3008:

  • It is capable of conversion rates of around 200 kilosamples per second.
  • It operates on SPI modes 0,0 or 1,1[2]

If you have done any work with SPI, you’ll know that there are 4 signals. MOSI stands for “master out, slave in” whereas MISO stands for “master in, slave out”. The two other signals are the clock which provides a time standard for the digital transaction and the SS (slave select), also called CE (chip enable) or CS (chip select.)

SPI communication in 8-bit read/write frames

In this example, we are going to use an SPI library to communicate with the MCP3008 in 8-bit frames, so the pertinent section of the datasheet is on page 21, section 6.1 Using the MCP3004/MCP3008 with Microcontroller (MCU) SPI Ports. The Figure 6-1 (reproduced below) shows how we will go about communicating with the device over the SPI bus.

From the communication diagram above, we get an excellent overview of the entire transaction. First, we must drop CS to initiate the transaction. With the CS low, we begin clocking in and out data. Figure 6-1 shows that we must clock in a single start bit (0x01) followed by mode and channel select bits. Table 5-2 shows the configuration bits that we must clock-in to return an ADC reading.

For example, if we wish to make a single-ended reading on channel 0, we must clock in the bits 1000. Note from figure 6-1, we must shift the bits by 4 binary places, so that for a single-ended reading from channel 0, we would clock in 0b1000000 or 0x80.

Software implementation

I chose to implement this in C rather than Python this time. There are a handful of libraries for the BCM2835. I used the bcm2835 library which is excellent. It is low-level enough that I can what’s going on, but not completely “bare metal” programming. You can find out more about the spi module of this library.

I will start with the code section-by-section then provide a link to the entire source code. First, of course, you’ll need to install the library. You can find a version-agnostic install script here. I used it; it works.

First, we’ll include a couple libraries that we need, and set up three constants. The first is the 0b00000001 that we need to transfer as the start bit. The second is the end bits 0b00000000 that we clock in to the MCP3008 so that we can clock out 8 bits of the ADC value. Finally, since I set up my test circuit to measure on channel 0, I just define a constant for that.

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#include <stdio.h>
#include <bcm2835.h>

uint8_t start = 0x01;
uint8_t end = 0x00;
uint8_t chan = 0x00;

Next I declare my function prototypes. Just C business as usual.

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int readADC(uint8_t chan);
float volts_adc(int adc);

In the body of main, I start by testing whether I can initiate the SPI interface on the Pi:

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if (!bcm2835_init())
{
printf("bcm2835_init failed. Are you running as root??\n");
return 1;
}

if (!bcm2835_spi_begin())
{
printf("bcm2835_spi_begin failed. Are you running as root??\n");
return 1;
}

If we pass those tests, we’re ready to go. Let’s set up the interface.

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bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST);      // The default
bcm2835_spi_setDataMode(BCM2835_SPI_MODE0); // The default
bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_65536); // The default
bcm2835_spi_chipSelect(BCM2835_SPI_CS0); // The default
bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW); // the default

To read the ADC value, we have to prepare the bytes that we’ll clock in first. All of that is done in a function readADC.

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int readADC(uint8_t chan){
char buf[] = {start, (0x08|chan)<<4,end};
char readBuf[3];
bcm2835_spi_transfernb(buf,readBuf,3);
return ((int)readBuf[1] & 0x03) << 8 | (int) readBuf[2];
}

It looks like there’s a lot going on here, but basically we are performing bit manipulations to get the input bits in the right order and the same for the output bits. First we declare an output buffer buf[] whose contents are three bytes. The first is the start bit 0b00000001, followed by the mode selections bytes, and terminated by a junk byte so that we can finish clocking out the resulting data. How do we interpret the value of (0x08|chan)<<4? Start from the inside of the parenthesis. 0x08 is 0b00001000 where the 1 bit here represents the selection of single-ended mode on the ADC. We logical OR that with the channel that we want to read. Finally, outside the parenthesis, we shift it over by 4 bits so these bits are in the upper nibble. Remember we have to clock in the data MSB first?

Next we declare an input buffer readBuf[3] to hold the data we’re reading in. Then we perform a 3 byte transfer. Now, what do we do with the results? Ouch. Well, remember we reading in 3 bytes. The first lines up with our start bit, so it’s junk and we’ll just ignore readBuf[0]. What about the next byte readBuf[1]? From Figure 6 of the datasheet, you can see that we only care about the 2 lower bits of the first byte which will become the upper two bits of the 10-bit ADC result. First we logical AND those with 0x03 (0b00000011) to get rid of anything above the first two bits. Then we shift it over by 8 bits, so that when we logical OR it with the lower 8 bits in readBuf[2] it coheres into a single 16 bit int. The casts just keep everything in 16 bits along the way.

Real life

So, does the software work? We can test it by applying a logical probe instrument and find out. I used an Intronix logic analyzer to watch the conversion. Here’s the result:

Compare the logic analyzer image to the datasheet. Looks similar! On the MISO line, we can ignore the first byte 0x07. With the second byte, 0xFB (0b11111011) we only care about the bottom two bits (11). In the third byte, we use all 8 bits. Putting those 10 bits together we have 0b1111111111 or 0x3FF, 1023 decimal. That’s the largest number we can express in 10 bits. That’s because I tied channel 0 to the 3.3v out of the Raspberry Pi. Now we can calculate the voltage. Using the reference of 3.3v, the ADC value of 1023 represents 3.3v and we can compute an arbitrary value using a function:

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float volts_adc(int adc) {
return (float)adc*3.3f/1023.0f;
}

And that’s it - a working example of reading the MCP3008 using C on the Raspberry Pi. If you’d like the entire code for the example application, you can find the gist here..

References


  1. Datasheet can be found here.

  2. The SPI bus can operate in different ways depending on the clock polarity and phase and how the data relates to clock transitions. "Mode 0,0" means that the clock polarity is 0 and its phase is 0 whereas "mode 1,1" means that the clock polarity and phase are both 1.

2018: Experiment No. 1

2018 is my year of experiments (Why? TL;DR: New Year’s resolutions are over-rated and have a high failure rate. Anyone can run an experiment for a month.) My first experiment (No news for a month) is nearly done and I’ll declare it a success.

Background

The round-the-clock sensational news cycle exists in large part to create wealth for the already-too-wealthy. Little of it is actionable, leaving us at the same time both outraged and impotent. Mostly I decided to give up on the news because of Donald Trump, the demented psychopathic moron who managed to get elected president.[1] Since Trump took office, like others, I’ve found myself cycling repeatedly through the stages of grief. But mostly I’ve been stuck on anger. There’s something about willful ignorance that does that to me.

Experiment

The methodology was simple. I simply willed myself to avoid the news for an entire month. After briefly considering the use of tools that would block news websites, I decided to go cold-turkey.

Results

Some of the things that I noticed:

  • Airports are saturated with news. I travelled a bit during the month. With TV’s blaring the news in every terminal area, it’s impossible to avoid hearing the news. I learned that a book highly critical of Trump was published and that the man himself was displeased. I learned that Congressional Republicans are trying to stop Special Counsel Robert Mueller’s investigation without looking like that’s what they’re doing.
  • Social media can be a significant vector of news. The sidebar on Facebook likes to trumpet the latest bush crash, earthquake, and political twist. But I also discovered that you can resize your browser to make the sidebar go away. Presto!
  • I tended to want to look at the news when I was bored. If I had a moment of boredom, I’d think about the news. Given that the news is supposed to serve in large part the factual needs of an informed electorate, seeking it out of boredom is more in keeping with the values of the entertainment industry, not those of journalism.
  • Outsourcing the news to others slows down the cycle. It was impossible to avoid the news completely. I heard others talking about political happenings and other current events. In fact, I even asked about them. But by outsourcing the news-seeking to others, I was able to slow down the process and keep it at a distance in a way that made it seem more abstract. I didn’t feel as outraged.
  • I felt more productive Once I eliminated the desire to read the news, I was able to stay with purposeful tasks longer.

Conclusions

After a month of no news, I miss reading good journalism. I may go back to it. Or I may not. The experiment was such a success that it would be hard to go back. The real problem for most of us is that the overlap between our circle of interest (what’s going on in the world) and our circle of influence is very small. David Cain noticed the same thing when he quit the news: “Being concerned makes us feel like we’re doing something when we’re not.”

Now off to my next experiment - a month of practicing a secular technology “sabbath”.


  1. I use these terms very carefully. Many have speculated that he suffers from some form of dementia owing to events where he slurs his words and perseverates. His sociopathic or psychopathic behaviours are well-documented; he is man devoid of empathy. And finally, his lack of reading is well-known. For all I can tell, the man is a functional illiterate. In contrast, his predecessor is a bibliophile and read widely and voraciously throughout his tenure.