Category Archives: Raspberry Pi

Running Python CGI Scripts on the Raspberry Pi

Python is the language of choice for controlling the Raspberry Pi’s GPIO pins. It seems only natural then that to interact with your Pi over the web it should run a web server able to handle Python CGI scripts. Following the steps below will get the lightweight nginx web server running on your Pi, handing Python requests off to a uwsgi helper process.

  1. Install nginx
  2. Add a location to the /etc/nginx/sites-available/default to pass Python requests on to uwsgi. This needs to be placed inside the “server” block of the configuration, for example right after one of the existing “location” sections.
  3. Create a Python CGI script at /usr/share/nginx/www/hello.py
  4. Start nginx
  5. Build and install the uwsgi with the cgi plugin
  6. Create the file /etc/uwsgi.ini
  7. Create the file /usr/share/nginx/www/hello.py
  8. Create an init script for uwsgi at /etc/init.d/uwsgi
  9. Start uwsgi and configure it to start on boot
  10. Open up your web browser and go to http://{your pi’s ip address}/hello.py
    If you’re using the browser on your Pi then you could instead go to http://localhost/hello.py
    If you see the message “Hello World” then everything is working.
    Hello_World

Controlling the MCP4151 Digital Potentiometer with the Raspberry Pi

We’re going to use the Raspberry Pi’s SPI bus to control Microchip’s MCP4151 8-bit digital potentiometer.  The MCP4151 is an 8 pin SPI device that can be used to programmatically control output voltage. The GPIO pins on the pi run at 3.3 volts meaning that we can command the pot to output between 0 and 3.3 volts. However, if we instead power the pot with 5 volts then we can control a voltage between 0 and 5 volts. Note that PWM is a possible alternative to a digital pot that doesn’t require an extra chip. However, this can add  noise to the signal that wasn’t acceptable for my project.

Microchip’s datasheet is recommended reading before starting this project.

Parts List

Step 1: Configure SPI on the Raspberry Pi

Follow the first two steps in Controlling an SPI device with the Raspberry Pi.

Step 2: Wire up the components

Being a pin-limited device, the SPI input and output on the MCP4151 shares one pin. That’s not a problem in our case since we’re only interested in writing to the device.

You’ll notice that we’re powering the pot with 5 volts despite the Raspberry Pi being a 3.3 volt device. This is fine because 1) The Pi sends commands to the pot but receives nothing back. Therefore, there is no risk of overvoltage on the Pi’s pins. And 2) The pot recognizes any SPI input over 0.7 volts as a high signal. This means the 3.3 volts from the Pi is plenty to communicate with the pot.

digital pot

As you can see, the power, ground and the SPI signals take up most of the pins. The actual potentiometer terminals are pins 5, 6 and 7.

Pin Description
1 SPI chip select input
2 SPI clock input
3 SPI serial data input/output
4 Ground
5 Potentiometer terminal A
6 Potentiometer wiper terminal
7 Potentiometer terminal B
8 Positive power supply input

IMG_20150315_140946

IMG_20150315_140905

Step 3: Create the python script

To test out the device, we’re going to continuously loop through all the possible values for the potentiometer. This will cause the voltage at the wiper terminal grow and shrink between 0 and 5 volts. Create the following script on your Pi.

#!/usr/bin/python

import spidev
import time

spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 976000

def write_pot(input):
    msb = input >> 8
    lsb = input & 0xFF
    spi.xfer([msb, lsb])

while True:
    for i in range(0x00, 0x1FF, 1):
        write_pot(i)
        time.sleep(.005)
    for i in range(0x1FF, 0x00, -1):
        write_pot(i)
        time.sleep(.005)

Step 4: Run the script

Make the script executable and run it with the following commands.

If all goes well, you should see the LED continuously fade to full brightness then off again.

Controlling an SPI device with the Raspberry Pi

The Raspberry Pi has a Broadcom BCM 2835 chip allowing it to interface with SPI devices on its GPIO pins. There are two chip select pins meaning that the Pi can control two devices simultaneously.

P1 Header Pin Function
19 MOSI – master output slave input
21 MISO – master input slave output
23 SCLK – clock
24 CE0 – chip enable 0
26 CE1 – chip enable 1

Step 1: Enable SPI on the Raspberry Pi

  1. In your Pi’s terminal, run
  2. Go to Advanced Options > SPI
  3. Choose “Yes” for both questions then select Finish to exit raspi-config
  4. Either reboot your Pi or run this command to load the kernel module

Step 2: Install spidev

Spidev is a python module that allows us to interface with the Pi’s SPI bus.Watch movie online The Transporter Refueled (2015)

Step 3: Python script

Finally, we can write and run a python script to control the SPI device.

  1. Create a file called spi-test.py in your favorite editor
  2. Make the file executable and run it

Notes on spidev

Unless the spi.max_speed_hz field is a value accepted by the driver, the script will fail when you run it. The field can be set to these values on the raspberry pi:

Speed spi.max_speed_hz value
125.0 MHz 125000000
62.5 MHz 62500000
31.2 MHz 31200000
15.6 MHz 15600000
7.8 MHz 7800000
3.9 MHz 3900000
1953 kHz 1953000
976 kHz 976000
488 kHz 488000
244 kHz 244000
122 kHz 122000
61 kHz 61000
30.5 kHz 30500
15.2 kHz 15200
7629 Hz 7629

Two SPI devices can be controlled in python by creating two SpiDev objects, one for each device.