Question 12.8: You want to connect a triple-axis accelerometer to a Raspber......
You want to connect a triple-axis accelerometer to a Raspberry Pi.
Use an analog accelerometer with a MCP3008 ADC chip to measure the X, Y, and Z analog outputs.
To try this recipe, you will need:
• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• MCP3008 eight-channel ADC IC (see “Integrated Circuits” on page 381)
• ADXL335 triple-axis accelerometer (see “Modules” on page 381)
Figure 12-14 shows the arrangement for this, using a breadboard. It uses three channels of the ADC to measure the X, Y, and Z acceleration forces.
You will need to set up SPI on your Raspberry Pi, so if you have not already done so, follow Recipe 8.6.
Open an editor (nano or IDLE) and paste in the following code. As with all the program examples in this book, you can also download the program from the Code section of the Raspberry Pi Cookbook website, where it is called adc_accelerometer.py.
| import spidev, time spi = spidev.SpiDev() spi.open(0,0) def analog_read(channel): r = spi.xfer2([1, (8 + channel) << 4, 0]) adc_out = ((r[1]&3) << 8) + r[2] return adc_out while True: x = analog_read(0) y = analog_read(1) z = analog_read(2) print(“X=%d\tY=%d\tZ=%d” % (x, y, z)) time.sleep(1) |
The program simply reads the three forces and prints them out:
| $ sudo python adc_accelerometer.py X=508 Y=503 Z=626 X=508 Y=504 Z=624 X=506 Y=505 Z=627 X=423 Y=517 Z=579 X=411 Y=513 Z=548 X=532 Y=510 Z=623 X=609 Y=518 Z=495 X=607 Y=521 Z=496 X=610 Y=513 Z=499 |
The first three readings took place with the accelerometer level. In the next three, the whole breadboard was tipped to one side. You can see that the X reading has decreased. Tipping the breadboard the other way causes the X reading to increase.
Discussion
The most common use for an accelerometer is to detect tilt. This works because the Z axis force is dominated by the pull of gravity (Figure 12-15).
When the accelerometer is tilted in one direction, some of that vertical force of gravity becomes active on another axis of the accelerometer.
We can use this principle to detect when the tilt is past a certain threshold. The following program (tilt.py) illustrates this point:
| import spidev, time spi = spidev.SpiDev() spi.open(0,0) |
When you run the program, you will start to see direction messages. You could use this to control a roving robot or a motorized pan-tilt head with a webcam attached:
| $ sudo python tilt.py Left Left Right Forward Forward Back Back |
See Also
See the datasheet for the accelerometer chip used in the module.
There are many other analog accelerometer modules available. You may find that they give different readings. Make sure that the analog outputs do not exceed 3.3V.
| Table A-3. Prototyping equipment | |
| Description | Suppliers |
| M-M jumper wires | SparkFun: PRT-08431, Adafruit: 759 |
| M-F jumper wires | SparkFun: PRT-09140, Adafruit: 825 |
| F-F jumper wires | SparkFun: PRT-08430, Adafruit: 794 |
| Half-sized breadboard | SparkFun: PRT-09567 Adafruit: 64 |
| Pi Cobbler | Adafruit: 1105 |
| Table A-6. Integrated circuits | |
| 7805 voltage regulator | SparkFun: COM-00107 |
| L293D motor driver | SparkFun: COM-00315, Adafruit: 807 |
| ULN2803 Darlington driver IC | SparkFun: COM-00312, Adafruit: 970 |
| DS18B20 temperature sensor | SparkFun: SEN-00245, Adafruit: 374 |
| MCP3008 eight-channel ADC IC | Adafruit: 856 |
| TMP36 temperature sensor | SparkFun: SEN-10988, Adafruit: 165 |
| Table A-8. Modules | |
| Raspberry Pi camera module | Adafruit: 1367, MCM: 28-17733, CPC: SC13023 |
| Arduino Uno | SparkFun: DEV-11021, Adafruit: 50, CPC: A000066 |
| Level converter, four-way | SparkFun: BOB-11978, Adafruit: 757 |
| Level converter eight-way | Adafruit: 395 |
| LiPo boost converter/charger | SparkFun: PRT-11231 |
| PowerSwitch tail | Adafruit: 268 |
| 16-channel servo controller | Adafruit: 815 |
| Motor driver 1A dual | SparkFun: ROB-09457 |
| RaspiRobot board | Sparkfun: KIT-11561, raspirobot.com |
| PiFace digital interface board | MCM: 83-14472, CPC: SC12827 |
| Humble Pi | MCM: 83-14637, CPC: SC12871 |
| Pi Plate | Adafruit: 801 |
| Gertboard | MCM: 83-14460, CPC: SC12828 |
| Breakout board with paddle terminals | MCM: 83-14876, CPC: SC12885 |
| PIR motion detector | Adafruit: 189 |
| Venus GPS module | SparkFun: GPS-11058 |
| Methane sensor | SparkFun: SEN-09404 |
| Gas sensor breakout board | SparkFun: BOB-08891 |
| ADXL335 triple-axis accelerometer | Adafruit: 163 |
| 4×7-segment LED with I2C backpack | Adafruit: 878 |
| Bicolor LED square-pixel matrix with I2C backpack | Adafruit: 902 |
| PiLite interface board | Ciseco, CPC: SC13018 |
| aLaMode interface board | Makershed: MKWY1, Seeedstudio: ARD10251P |
| Freetronics Arduino LCD shield | www.freetronics.com |
| RTC module | Adafruit: 264 |
| 16 x 2 HD44780 compatible LCD module | SparkFun: LCD-00255, Adafruit: 181 |
