This is the latest (main) BeagleBoard documentation. If you are looking for stable releases, use the drop-down menu on the bottom-left and select the desired version.

Sensors

In this chapter, you will learn how to sense the physical world with BeagleBone Black. Various types of electronic sensors, such as cameras and microphones, can be connected to the Bone using one or more interfaces provided by the standard USB 2.0 host port, as shown in The USB 2.0 host port.

Note

All the examples in the book assume you have cloned the Cookbook repository on git.beagleboard.org. Go here Cloning the Cookbook Repository for instructions.

Fig. 362 The USB 2.0 host port

The two 46-pin cape headers (called P8 and P9) along the long edges of the board (Cape Headers P8 and P9) provide connections for cape add-on boards, digital and analog sensors, and more.

Fig. 363 Cape Headers P8 and P9

The simplest kind of sensor provides a single digital status, such as off or on, and can be handled by an input mode of one of the Bone’s 65 general-purpose input/output (GPIO) pins. More complex sensors can be connected by using one of the Bone’s seven analog-to-digital converter (ADC) inputs or several I²C buses.

Displays and Other Outputs discusses some of the output mode usages of the GPIO pins.

All these examples assume that you know how to edit a file (Editing Code Using Visual Studio Code) and run it, either within the Visual Studio Code (VSC) integrated development environment (IDE) or from the command line (Getting to the Command Shell via SSH).

Choosing a Method to Connect Your Sensor

Problem

You want to acquire and attach a sensor and need to understand your basic options.

Solution

Some of the many sensor connection options on the Bone shows many of the possibilities for connecting a sensor.

Fig. 364 Some of the many sensor connection options on the Bone

Choosing the simplest solution available enables you to move on quickly to addressing other system aspects. By exploring each connection type, you can make more informed decisions as you seek to optimize and troubleshoot your design.

Input and Run a Python or JavaScript Application for Talking to Sensors

Problem

You have your sensors all wired up and your Bone booted up, and you need to know how to enter and run your code.

Solution

You are just a few simple steps from running any of the recipes in this book.

• Plug your Bone into a host computer via the USB cable (Getting Started, Out of the Box).

• Start Visual Studio Code (Editing Code Using Visual Studio Code).

• In the bash tab (as shown in Entering commands in the VSC bash tab), run the following commands:

bone$ cd
bone$ cd beaglebone-cookbook-code/02sensors

Fig. 365 Entering commands in the VSC bash tab

Here, we issued the change directory (cd) command without specifying a target directory. By default, it takes you to your home directory. Notice that the prompt has changed to reflect the change.

Note

If you log in as debian, your home is /home/debian. If you were to create a new user called newuser, that user’s home would be /home/newuser. By default, all non-root (non-superuser) users have their home directories in /home.

Note

All the examples in the book assume you have cloned the Cookbook repository on git.beagleboard.org. Go here Cloning the Cookbook Repository for instructions.

• Double-click the pushbutton.py file to open it.

• Press ^S (Ctrl-S) to save the file. (You can also go to the File menu in VSC and select Save to save the file, but Ctrl-S is easier.) Even easier, VSC can be configured to autosave every so many seconds.

• In the bash tab, enter the following commands:

debian@beaglebone:beaglebone-cookbook/code/02sensors$ ./pushbutton.py
data= 0
data= 0
data= 1
data= 1
^C

This process will work for any script in this book.

Reading the Status of a Pushbutton or Magnetic Switch (Passive On/Off Sensor)

Problem

You want to read a pushbutton, a magnetic switch, or other sensor that is electrically open or closed.

Solution

Connect the switch to a GPIO pin and read from the proper place in /sys/class/gpio.

To make this recipe, you will need:

• Breadboard and jumper wires.

• Pushbutton switch.

• Magnetic reed switch. (optional)

You can wire up either a pushbutton, a magnetic reed switch, or both on the Bone, as shown in Diagram for wiring a pushbutton and magnetic reed switch input.

Fig. 366 Diagram for wiring a pushbutton and magnetic reed switch input

The code in Monitoring a pushbutton (pushbutton.py) reads GPIO port P9_42, which is attached to the pushbutton.

Pythonc

Listing 15 Monitoring a pushbutton (pushbutton.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	pushbutton.py
# //      Reads P9_42 and prints its value.
# //	Wiring:	Connect a switch between P9_42 and 3.3V
# //	Setup:	
# //	See:	
# ////////////////////////////////////////
import time
import gpiod
import os

ms = 100    # Read time in ms
CHIP = 'gpiochip0'
LINE_OFFSET = [7] #  P9_42 is gpio 7
chip = gpiod.Chip(CHIP)
lines = chip.get_lines(LINE_OFFSET)
lines.request(consumer='pushbutton.py', type=gpiod.LINE_REQ_DIR_IN)

while True:
    data = lines.get_values()
    print('data = ' + str(data[0]))
    time.sleep(ms/1000)

pushbutton.py

Put this code in a file called pushbutton.py following the steps in Input and Run a Python or JavaScript Application for Talking to Sensors. In the VSC bash tab, run it by using the following commands:

bone$ ./pushbutton.py
data = 0
data = 0
data = 1
data = 1
^C

The command runs it. Try pushing the button. The code reads the pin and prints its current value.

You will have to press ^C (Ctrl-C) to stop the code.

If you want to run the C version do:

bone$ gcc -o pushbutton pushbutton.c -lgpiod
bone$ ./pushbutton
data = 0
data = 0
data = 1
data = 1
^C

If you want to use the magnetic reed switch wired as shown in Diagram for wiring a pushbutton and magnetic reed switch input, change P9_42 to P9_26 which is gpio 14.

Mapping Header Numbers to gpio Numbers

Problem

You have a sensor attached to the P8 or P9 header and need to know which gpio pin it is using.

Solution

The gpioinfo command displays information about all the P8 and P9 header pins. To see the info for just one pin, use grep.

bone$ gpioinfo | grep -e chip -e P9.42
gpiochip0 - 32 lines:
        line   7: "P8_42A [ecappwm0]" "P9_42" input active-high [used]
gpiochip1 - 32 lines:
gpiochip2 - 32 lines:
gpiochip3 - 32 lines:

This shows P9_42 is on chip 0 and pin 7. To find the gpio number multiply the chip number by 32 and add it to the pin number. This gives 0*32+7=7.

For P9_26 you get:

bone$ gpioinfo | grep -e chip -e P9.26
gpiochip0 - 32 lines:
        line  14: "P9_26 [uart1_rxd]" "P9_26" input active-high [used]
gpiochip1 - 32 lines:
gpiochip2 - 32 lines:
gpiochip3 - 32 lines:

0*32+14=14, so the P9_26 pin is gpio 14.

Reading a Position, Light, or Force Sensor (Variable Resistance Sensor)

Problem

You have a variable resistor, force-sensitive resistor, flex sensor, or any of a number of other sensors that output their value as a variable resistance, and you want to read their value with the Bone.

Solution

Use the Bone’s analog-to-digital converters (ADCs) and a resistor divider circuit to detect the resistance in the sensor.

The Bone has seven built-in analog inputs that can easily read a resistive value. Seven analog inputs on P9 header shows them on the lower part of the P9 header.

Fig. 367 Seven analog inputs on P9 header

To make this recipe, you will need:

• Breadboard and jumper wires.

• 10k trimpot or

• Flex resistor (optional)

• 22 kΩ resistor

A variable resistor with three terminals

Wiring a 10 kΩ variable resistor (trimpot) to an ADC port shows a simple variable resistor (trimpot) wired to the Bone. One end terminal is wired to the ADC 1.8 V power supply on pin P9_32, and the other end terminal is attached to the ADC ground (P9_34). The middle terminal is wired to one of the seven analog-in ports (P9_36).

Fig. 368 Wiring a 10 kΩ variable resistor (trimpot) to an ADC port

Reading an analog voltage (analogIn.py) shows the code used to read the variable resistor. Add the code to a file called analogIn.py and run it; then change the resistor and run it again. The voltage read will change.

PythonJavaScript

Listing 17 Reading an analog voltage (analogIn.py)

#!/usr/bin/env python3
#//////////////////////////////////////
#	analogin.py
# 	Reads the analog value of the light sensor.
#//////////////////////////////////////
import time
import os

pin = "2"        # light sensor, A2, P9_37

IIOPATH='/sys/bus/iio/devices/iio:device0/in_voltage'+pin+'_raw'

print('Hit ^C to stop')

f = open(IIOPATH, "r")

while True:
    f.seek(0)
    x = float(f.read())/4096
    print('{}: {:.1f}%, {:.3f} V'.format(pin, 100*x, 1.8*x), end = '\r')
    time.sleep(0.1)

# // Bone  | Pocket | AIN
# // ----- | ------ | --- 
# // P9_39 | P1_19  | 0
# // P9_40 | P1_21  | 1
# // P9_37 | P1_23  | 2
# // P9_38 | P1_25  | 3
# // P9_33 | P1_27  | 4
# // P9_36 | P2_35  | 5
# // P9_35 | P1_02  | 6

analogIn.py

Note

The code in Reading an analog voltage (analogIn.js) outputs a value between 0 and 4096.

A variable resistor with two terminals

Some resistive sensors have only two terminals, such as the flex sensor in Reading a two-terminal flex resistor The resistance between its two terminals changes when it is flexed. In this case, we need to add a fixed resistor in series with the flex sensor. Reading a two-terminal flex resistor shows how to wire in a 22 kΩ resistor to give a voltage to measure across the flex sensor.

Fig. 369 Reading a two-terminal flex resistor

The code in Reading an analog voltage (analogIn.py) and Reading an analog voltage (analogIn.js) also works for this setup.

Reading a Distance Sensor (Analog or Variable Voltage Sensor)

Problem

You want to measure distance with a LV-MaxSonar-EZ1 Sonar Range Finder, which outputs a voltage in proportion to the distance.

Solution

To make this recipe, you will need:

• Breadboard and jumper wires.

• LV-MaxSonar-EZ1 Sonar Range Finder

All you have to do is wire the EZ1 to one of the Bone’s analog-in pins, as shown in Wiring the LV-MaxSonar-EZ1 Sonar Range Finder to the P9_33 analog-in port. The device outputs ~6.4 mV/in when powered from 3.3 V.

Warning

Make sure not to apply more than 1.8 V to the Bone’s analog-in pins, or you will likely damage them. In practice, this circuit should follow that rule.

Fig. 370 Wiring the LV-MaxSonar-EZ1 Sonar Range Finder to the P9_33 analog-in port

Reading an analog voltage (ultrasonicRange.py) shows the code that reads the sensor at a fixed interval.

PythonJavaScript

Listing 19 Reading an analog voltage (ultrasonicRange.py)

#!/usr/bin/env python
# //////////////////////////////////////
# //	ultrasonicRange.js
# // 	Reads the analog value of the sensor.
# //////////////////////////////////////
import time
ms = 250;  # Time in milliseconds

pin = "0"        # sensor, A0, P9_39

IIOPATH='/sys/bus/iio/devices/iio:device0/in_voltage'+pin+'_raw'

print('Hit ^C to stop');

f = open(IIOPATH, "r")
while True:
    f.seek(0)
    data = f.read()[:-1]
    print('data= ' + data)
    time.sleep(ms/1000)

# // Bone  | Pocket | AIN
# // ----- | ------ | --- 
# // P9_39 | P1_19  | 0
# // P9_40 | P1_21  | 1
# // P9_37 | P1_23  | 2
# // P9_38 | P1_25  | 3
# // P9_33 | P1_27  | 4
# // P9_36 | P2_35  | 5
# // P9_35 | P1_02  | 6

ultrasonicRange.py

Reading a Distance Sensor (Variable Pulse Width Sensor)

Problem

You want to use a HC-SR04 Ultrasonic Range Sensor with BeagleBone Black.

Solution

The HC-SR04 Ultrasonic Range Sensor (shown in HC-SR04 Ultrasonic range sensor) works by sending a trigger pulse to the Trigger input and then measuring the pulse width on the Echo output. The width of the pulse tells you the distance.

Fig. 371 HC-SR04 Ultrasonic range sensor

To make this recipe, you will need:

• Breadboard and jumper wires.

• 10 kΩ and 20 kΩ resistors

• HC-SR04 Ultrsonic Range Sensor.

Wire the sensor as shown in Wiring an HC-SR04 Ultrasonic Sensor. Note that the HC-SR04 is a 5 V device, so the banded wire (running from P9_7 on the Bone to VCC on the range finder) attaches the HC-SR04 to the Bone’s 5 V power supply.

Fig. 372 Wiring an HC-SR04 Ultrasonic Sensor

Driving a HC-SR04 ultrasound sensor (hc-sr04-ultraSonic.js) shows BoneScript code used to drive the HC-SR04.

Listing 21 Driving a HC-SR04 ultrasound sensor (hc-sr04-ultraSonic.js)

#!/usr/bin/env node

// This is an example of reading HC-SR04 Ultrasonic Range Finder
// This version measures from the fall of the Trigger pulse 
//   to the end of the Echo pulse

var b = require('bonescript');

var trigger = 'P9_16',  // Pin to trigger the ultrasonic pulse
    echo    = 'P9_41',  // Pin to measure to pulse width related to the distance
    ms = 250;           // Trigger period in ms
    
var startTime, pulseTime;
    
b.pinMode(echo,   b.INPUT, 7, 'pulldown', 'fast', doAttach);
function doAttach(x) {
    if(x.err) {
        console.log('x.err = ' + x.err);
        return;
    }
    // Call pingEnd when the pulse ends
    b.attachInterrupt(echo, true, b.FALLING, pingEnd);
}

b.pinMode(trigger, b.OUTPUT);

b.digitalWrite(trigger, 1);     // Unit triggers on a falling edge.
                                // Set trigger to high so we call pull it low later

// Pull the trigger low at a regular interval.
setInterval(ping, ms);

// Pull trigger low and start timing.
function ping() {
    // console.log('ping');
    b.digitalWrite(trigger, 0);
    startTime = process.hrtime();
}

// Compute the total time and get ready to trigger again.
function pingEnd(x) {
    if(x.attached) {
        console.log("Interrupt handler attached");
        return;
    }
    if(startTime) {
        pulseTime = process.hrtime(startTime);
        b.digitalWrite(trigger, 1);
        console.log('pulseTime = ' + (pulseTime[1]/1000000-0.8).toFixed(3));
    }
}

hc-sr04-ultraSonic.js

This code is more complex than others in this chapter, because we have to tell the device when to start measuring and time the return pulse.

Accurately Reading the Position of a Motor or Dial

Problem

You have a motor or dial and want to detect rotation using a rotary encoder.

Solution

Use a rotary encoder (also called a quadrature encoder) connected to one of the Bone’s eQEP ports, as shown in Wiring a rotary encoder using eQEP2.

Fig. 373 Wiring a rotary encoder using eQEP2

Table 129 On the BeagleBone and PocketBeage the three encoders are:

eQEP0	P9.27 and P9.42 OR P1_33 and P2_34
eQEP1	P9.33 and P9.35
eQEP2	P8.11 and P8.12 OR P2_24 and P2_33

Table 130 On the AI it’s:

eQEP1	P8.33 and P8.35
eQEP2	P8.11 and P8.12 or P9.19 and P9.41
eQEP3	P8.24 and P8.25 or P9.27 and P9.42

To make this recipe, you will need:

• Breadboard and jumper wires.

• Rotary encoder.

We are using a quadrature rotary encoder, which has two switches inside that open and close in such a manner that you can tell which way the shaft is turning. In this particular encoder, the two switches have a common lead, which is wired to ground. It also has a pushbutton switch wired to the other side of the device, which we aren’t using.

Wire the encoder to P8_11 and P8_12, as shown in Wiring a rotary encoder using eQEP2.

BeagleBone Black has built-in hardware for reading up to three encoders. Here, we’ll use the eQEP2 encoder via the Linux count subsystem.

Then run the following commands:

bone$ config-pin P8_11 qep
bone$ config-pin P8_12 qep
bone$ show-pins | grep qep
P8.12        12 fast rx  up  4 qep 2 in A    ocp/P8_12_pinmux (pinmux_P8_12_qep_pin)
P8.11        13 fast rx  up  4 qep 2 in B    ocp/P8_11_pinmux (pinmux_P8_11_qep_pin)

This will enable eQEP2 on pins P8_11 and P8_12. The 2 after the qep returned by show-pins shows it’s eQEP2.

Finally, add the code in Reading a rotary encoder (rotaryEncoder.py) to a file named rotaryEncoder.py and run it.

PythonJavaScript

Listing 22 Reading a rotary encoder (rotaryEncoder.py)

#!/usr/bin/env python
# // This uses the eQEP hardware to read a rotary encoder
# // bone$ config-pin P8_11 eqep
# // bone$ config-pin P8_12 eqep
import time
    
eQEP = '2'
COUNTERPATH = '/dev/bone/counter/counter'+eQEP+'/count0'
	
ms = 100 	# Time between samples in ms
maxCount = '1000000'

# Set the eEQP maximum count
f = open(COUNTERPATH+'/ceiling', 'w')
f.write(maxCount)
f.close()

# Enable
f = open(COUNTERPATH+'/enable', 'w')
f.write('1')
f.close()

f = open(COUNTERPATH+'/count', 'r')

olddata = -1
while True:
	f.seek(0)
	data = f.read()[:-1]
	# Print only if data changes
	if data != olddata:
		olddata = data
		print("data = " + data)
	time.sleep(ms/1000)

# Black OR Pocket
# eQEP0:	P9.27 and P9.42 OR P1_33 and P2_34
# eQEP1:	P9.33 and P9.35
# eQEP2:	P8.11 and P8.12 OR P2_24 and P2_33

# AI
# eQEP1:	P8.33 and P8.35
# eQEP2:	P8.11 and P8.12 or P9.19 and P9.41
# eQEP3:	P8.24 and P8.25 or P9.27 and P9.42

rotaryEncoder.py

Try rotating the encoder clockwise and counter-clockwise. You’ll see an output like this:

data = 32
data = 40
data = 44
data = 48
data = 39
data = 22
data = 0
data = 999989
data = 999973
data = 999972
^C

The values you get for data will depend on which way you are turning the device and how quickly. You will need to press ^C (Ctrl-C) to end.

See Also

You can also measure rotation by using a variable resistor (see Wiring a 10 kΩ variable resistor (trimpot) to an ADC port).

Acquiring Data by Using a Smart Sensor over a Serial Connection

Problem

You want to connect a smart sensor that uses a built-in microcontroller to stream data, such as a global positioning system (GPS), to the Bone and read the data from it.

Solution

The Bone has several serial ports (UARTs) that you can use to read data from an external microcontroller included in smart sensors, such as a GPS. Just wire one up, and you’ll soon be gathering useful data, such as your own location.

Here’s what you’ll need:

• Breadboard and jumper wires.

• GPS receiver

Wire your GPS, as shown in Wiring a GPS to UART 4.

Fig. 374 Wiring a GPS to UART 4

The GPS will produce raw National Marine Electronics Association (NMEA) data that’s easy for a computer to read, but not for a human. There are many utilities to help convert such sensor data into a human-readable form. For this GPS, run the following command to load a NMEA parser:

bone$ npm install -g nmea

Running the code in Talking to a GPS with UART 4 (GPS.js) will print the current location every time the GPS outputs it.

Listing 24 Talking to a GPS with UART 4 (GPS.js)

#!/usr/bin/env node
// Install with: npm install nmea

// Need to add exports.serialParsers = m.module.parsers;
// to the end of /usr/local/lib/node_modules/bonescript/serial.js

var b = require('bonescript');
var nmea = require('nmea');

var port = '/dev/ttyO4';
var options = {
    baudrate: 9600,
    parser: b.serialParsers.readline("\n")
};

b.serialOpen(port, options, onSerial);

function onSerial(x) {
    if (x.err) {
        console.log('***ERROR*** ' + JSON.stringify(x));
    }
    if (x.event == 'open') {
       console.log('***OPENED***');
    }
    if (x.event == 'data') {
        console.log(String(x.data));
        console.log(nmea.parse(x.data));
    }
}

GPS.js

If you don’t need the NMEA formatting, you can skip the npm part and remove the lines in the code that refer to it.

Note

If you get an error like this TypeError: Cannot call method ‘readline’ of undefined

add this line to the end of file /usr/local/lib/node_modules/bonescript/serial.js:

exports.serialParsers = m.module.parsers;

Fig. 375 Table of UART outputs

Measuring a Temperature

Problem

You want to measure a temperature using a digital temperature sensor.

Solution

The TMP101 sensor is a common digital temperature sensor that uses a standard I²C-based serial protocol.

To make this recipe, you will need:

• Breadboard and jumper wires.

• Two 4.7 kΩ resistors.

• TMP101 temperature sensor.

Wire the TMP101, as shown in Wiring an I2C TMP101 temperature sensor.

Fig. 376 Wiring an I²C TMP101 temperature sensor

There are two I²C buses brought out to the headers. Table of I2C outputs shows that you have wired your device to I²C bus 2.

Fig. 377 Table of I²C outputs

Once the I²C device is wired up, you can use a couple handy I²C tools to test the device. Because these are Linux command-line tools, you have to use 2 as the bus number. i2cdetect, shown in I2C tools, shows which I²C devices are on the bus. The -r flag indicates which bus to use. Our TMP101 is appearing at address 0x49. You can use the i2cget command to read the value. It returns the temperature in hexadecimal and degrees C. In this example, 0x18 = 24{deg}C, which is 75.2{deg}F. (Hmmm, the office is a bit warm today.) Try warming up the TMP101 with your finger and running i2cget again.

I²C tools

bone$ i2cdetect -y -r 2
  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
  00:          -- -- -- -- -- -- -- -- -- -- -- -- --
  10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
  20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
  30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
  40: -- -- -- -- -- -- -- -- -- 49 -- -- -- -- -- --
  50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- --
  60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
  70: -- -- -- -- -- -- -- --

bone$ i2cget -y 2 0x49
  0x18

Reading the temperature via the kernel driver

The cleanest way to read the temperature from at TMP101 sensor is to use the kernel driver.

Assuming the TMP101 is on bus 2 (the last digit is the bus number)

I²C TMP101 via Kernel

bone$ cd /sys/class/i2c-adapter/
bone$ ls
i2c-0  i2c-1  i2c-2                   # Three i2c buses (bus 0 is internal)
bone$ cd i2c-2        # Pick bus 2
bone$ ls -ls
0 --w--w---- 1 root gpio 4096 Jul  1 09:24 delete_device
0 lrwxrwxrwx 1 root gpio    0 Jun 30 16:25 device -> ../../4819c000.i2c
0 drwxrwxr-x 3 root gpio    0 Dec 31  1999 i2c-dev
0 -r--r--r-- 1 root gpio 4096 Dec 31  1999 name
0 --w--w---- 1 root gpio 4096 Jul  1 09:24 new_device
0 lrwxrwxrwx 1 root gpio    0 Jun 30 16:25 of_node -> ../../../../../../../../firmware/devicetree/base/ocp/interconnect@48000000/segment@100000/target-module@9c000/i2c@0
0 drwxrwxr-x 2 root gpio    0 Dec 31  1999 power
0 lrwxrwxrwx 1 root gpio    0 Jun 30 16:25 subsystem -> ../../../../../../../../bus/i2c
0 -rw-rw-r-- 1 root gpio 4096 Dec 31  1999 uevent

Assuming the TMP101 is at address 0x49

bone$ echo tmp101 0x49 > new_device

This tells the kernel you have a TMP101 sensor at address 0x49. Check the log to be sure.

bone$ dmesg -H | tail -3
[ +13.571823] i2c i2c-2: new_device: Instantiated device tmp101 at 0x49
[  +0.043362] lm75 2-0049: supply vs not found, using dummy regulator
[  +0.009976] lm75 2-0049: hwmon0: sensor 'tmp101'

Yes, it’s there, now see what happened.

bone$ ls
2-0049  delete_device  device  i2c-dev  name new_device  of_node  power  subsystem  uevent

Notice a new directory has appeared. It’s for i2c bus 2, address 0x49. Look into it.

bone$ cd 2-0049/hwmon/hwmon0
bone$ ls -F
device@  name  power/  subsystem@  temp1_input  temp1_max temp1_max_hyst  uevent  update_interval
bone$ cat temp1_input
24250

There is the temperature in milli-degrees C.

Other i2c devices are supported by the kernel. You can try the Linux Kernel Driver Database, https://cateee.net/lkddb/ to see them.

Once the driver is in place, you can read it via code. Reading an I2C device (i2cTemp.py) shows how to read the TMP101.

PythonJavaScript

Listing 25 Reading an I²C device (i2cTemp.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	i2cTemp.py
# //      Read a TMP101 sensor on i2c bus 2, address 0x49
# //	Wiring:	Attach to i2c as shown in text.
# //	Setup:	echo tmp101 0x49 > /sys/class/i2c-adapter/i2c-2/new_device
# //	See:	
# ////////////////////////////////////////
import time

ms = 1000   # Read time in ms
bus = '2'
addr = '49'
I2CPATH='/sys/class/i2c-adapter/i2c-'+bus+'/'+bus+'-00'+addr+'/hwmon/hwmon0';

f = open(I2CPATH+"/temp1_input", "r")

while True:
    f.seek(0)
    data = f.read()[:-1]    # returns mili-degrees C
    print("data (C) = " + str(int(data)/1000))
    time.sleep(ms/1000)

i2cTemp.py

Run the code by using the following command:

bone$ ./i2cTemp.js
data (C) = 25.625
data (C) = 27.312
data (C) = 28.187
data (C) = 28.375
^C

Notice using the kernel interface gets you more digits of accuracy.

Reading i2c device directly

The TMP102 sensor can be read directly with i2c commands rather than using the kernel driver. First you need to install the i2c module.

bone$ pip install smbus

Listing 27 Reading an I²C device (i2cTemp.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	i2ctmp101.py
# //      Read at TMP101 sensor on i2c bus 2, address 0x49
# //	Wiring:	Attach to i2c as shown in text.
# //	Setup:	pip install smbus
# //	See:	
# ////////////////////////////////////////
import smbus
import time

ms = 1000               # Read time in ms
bus = smbus.SMBus(2)    # Using i2c bus 2
addr = 0x49             # TMP101 is at address 0x49

while True:
    data = bus.read_byte_data(addr, 0)
    print("temp (C) = " + str(data))
    time.sleep(ms/1000)

i2ctmp101.py

This gets only 8 bits for the temperature. See the TMP101 datasheet (https://www.ti.com/product/TMP101) for details on how to get up to 12 bits.

Reading Temperature via a Dallas 1-Wire Device

Problem

You want to measure a temperature using a Dallas Semiconductor DS18B20 temperature sensor.

Solution

The DS18B20 is an interesting temperature sensor that uses Dallas Semiconductor’s 1-wire interface. The data communication requires only one wire! (However, you still need wires from ground and 3.3 V.) You can wire it to any GPIO port.

To make this recipe, you will need:

• Breadboard and jumper wires.

• 4.7 kΩ resistor

• DS18B20 1-wire temperature sensor.

Wire up as shown in Wiring a Dallas 1-Wire temperature sensor.

Fig. 378 Wiring a Dallas 1-Wire temperature sensor

Edit the file /boot/uEnt.txt. Go to about line 19 and edit as shown:

17 ###
18 ###Additional custom capes
19 uboot_overlay_addr4=BB-W1-P9.12-00A0.dtbo
20 #uboot_overlay_addr5=<file5>.dtbo

Be sure to remove the # at the beginning of the line.

Reboot the bone:

bone$ reboot

Now run the following command to discover the serial number on your device:

bone$ ls /sys/bus/w1/devices/
28-00000114ef1b  28-00000128197d  w1_bus_master1

I have two devices wired in parallel on the same P9_12 input. This shows the serial numbers for all the devices.

Finally, add the code in Reading a temperature with a DS18B20 (w1.py) in to a file named w1.py, edit the path assigned to w1 so that the path points to your device, and then run it.

PythonJavaScript

Listing 28 Reading a temperature with a DS18B20 (w1.py)

#!/usr/bin/env python
# ////////////////////////////////////////
# //	w1.js
# //      Read a Dallas 1-wire device on P9_12
# //	Wiring:	Attach gnd and 3.3V and data to P9_12
# //	Setup:	Edit /boot/uEnv.txt to include:
# //              uboot_overlay_addr4=BB-W1-P9.12-00A0.dtbo
# //	See:	
# ////////////////////////////////////////
import time

ms = 500   # Read time in ms
#  Do ls /sys/bus/w1/devices and find the address of your device
addr = '28-00000d459c2c' # Must be changed for your device.
W1PATH ='/sys/bus/w1/devices/' + addr

f = open(W1PATH+'/temperature')

while True:
    f.seek(0)
    data = f.read()[:-1]
    print("temp (C) = " + str(int(data)/1000))
    time.sleep(ms/1000)

w1.py

bone$ ./w1.js
temp (C) = 28.625
temp (C) = 29.625
temp (C) = 30.5
temp (C) = 31.0
^C

Each temperature sensor has a unique serial number, so you can have several all sharing the same data line.

Playing and Recording Audio

Problem

BeagleBone doesn’t have audio built in, but you want to play and record files.

Solution

One approach is to buy an audio cape, but another, possibly cheaper approach is to buy a USB audio adapter, such as the one shown in A USB audio dongle.

Fig. 379 A USB audio dongle

Drivers for the Advanced Linux Sound Architecture (ALSA) are already installed on the Bone. You can list the recording and playing devices on your Bone by using aplay and arecord, as shown in Listing the ALSA audio output and input devices on the Bone. BeagleBone Black has audio-out on the HDMI interface. It’s listed as card 0 in Listing the ALSA audio output and input devices on the Bone. card 1 is my USB audio adapter’s audio out.

Listing the ALSA audio output and input devices on the Bone

bone$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Black [TI BeagleBone Black], device 0: HDMI nxp-hdmi-hifi-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: Device [C-Media USB Audio Device], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

bone$ arecord -l
**** List of CAPTURE Hardware Devices ****
card 1: Device [C-Media USB Audio Device], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

In the aplay output shown in Listing the ALSA audio output and input devices on the Bone, you can see the USB adapter’s audio out. By default, the Bone will send audio to the HDMI. You can change that default by creating a file in your home directory called ~/.asoundrc and adding the code in Change the default audio out by putting this in ~/.asoundrc (audio.asoundrc) to it.

Listing 30 Change the default audio out by putting this in ~/.asoundrc (audio.asoundrc)

pcm.!default {
  type plug
  slave {
    pcm "hw:1,0"
  }
}
ctl.!default {
  type hw
  card 1
}

audio.asoundrc

You can easily play .wav files with aplay:

bone$ aplay test.wav

You can play other files in other formats by installing mplayer:

bone$ sudo apt update
bone$ sudo apt install mplayer
bone$ mplayer test.mp3

Discussion

Adding the simple USB audio adapter opens up a world of audio I/O on the Bone.