As interesting as the Raspberry Pi may be, it does not have a way to read analog inputs directly, which makes it difficult to use in some DIY projects. Luckily, working with an ADC on the RPi is really simple and easy thanks to this lesson from Adafruit.

In that tutorial, you’ll learn how to setup the MCP3008 ADC with the RPi by using SPI communication to use a potentiometer to adjust the volume of an MP3 that is currently playing. I followed the tutorial and it worked nicely but I had other ideas for this ADC.

For a long time now, I wanted to build a home-monitoring device with a bunch of analog sensors and the RPi, combined with this ADC, seems to be just the right computer for it. So, this required a small test and I decided to use two simple analog sensors to measure temperature (TMP36) and humidity (HIH4030). I followed the ADC connection diagram in the Adafruit tutorial and then connected the TMP36 and the HIH4030 to analog channels 0 and 1, respectively.

The following is the python code (adapted from the Adafruit tutorial code) that checks the analog sensors every second and calculates the right values for temperature and humidity:

#!/usr/bin/env python
import time
import os
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
DEBUG = 0

# read SPI data from MCP3008 chip, 8 possible adc's (0 thru 7)
def readadc(adcnum, clockpin, mosipin, misopin, cspin):
        if ( (adcnum > 7) or (adcnum < 0) ):
                return -1
        GPIO.output(cspin, True)

        GPIO.output(clockpin, False)  # start clock low
        GPIO.output(cspin, False)     # bring CS low

        commandout = adcnum
        commandout |= 0x18  # start bit + single-ended bit
        commandout <<= 3    # we only need to send 5 bits here
        for i in range(5):
                if (commandout & 0x80):
                        GPIO.output(mosipin, True)
                else:
                        GPIO.output(mosipin, False)
                commandout <<= 1
                GPIO.output(clockpin, True)
                GPIO.output(clockpin, False)

        adcout = 0
        # read in one empty bit, one null bit and 10 ADC bits
        for i in range(12):
                GPIO.output(clockpin, True)
                GPIO.output(clockpin, False)
                adcout <<= 1
                if (GPIO.input(misopin)):
                        adcout |= 0x1

        GPIO.output(cspin, True)

        adcout >>= 1       # first bit is 'null' so drop it
        return adcout

# change these as desired - they're the pins connected from the
# SPI port on the ADC to the Cobbler
SPICLK = 18
SPIMISO = 23
SPIMOSI = 24
SPICS = 25

# set up the SPI interface pins
GPIO.setup(SPIMOSI, GPIO.OUT)
GPIO.setup(SPIMISO, GPIO.IN)
GPIO.setup(SPICLK, GPIO.OUT)
GPIO.setup(SPICS, GPIO.OUT)

# temperature sensor (TMP36) connected to adc #0
temperature_adc = 0;
humidity_adc = 1;

while True:
        # read the analog pin
        analog_temp_value = readadc(temperature_adc, SPICLK, SPIMOSI, SPIMISO, SPICS)
        analog_hum_value = readadc(humidity_adc, SPICLK, SPIMOSI, SPIMISO, SPICS)

        temp_voltage = (analog_temp_value * 3.3 / 1024.0) * 1000.0;
        temp = (temp_voltage - 500)/10;

        humidity = ((analog_hum_value * 12) / 75) - 26;

        if DEBUG:
                print "Temperature analog value: ", analog_temp_value
                print "Temperature voltage: ", temp_voltage
                print "Humidity analog value: ", analog_hum_value

        print "Temperature: ", temp
        print "Humidity: ", humidity
        print ""

        # hang out and do nothing for a second
        time.sleep(1)

Running this python script (it must be run as root, otherwise you won’t get access to the GPIO) should output something like:

Temperature:  21.865234375
Humidity:  72

Temperature:  22.1875
Humidity:  73

In case you’re wondering where those calculations for temperature and humidity came from: for temperature check this post, for humidity check this post.

UPDATE: CodingCat has left a very valuable set of comments regarding the process of obtaining exact values from the humidity sensor. Check the comments below.

I’d like to thank Filipe Valpereiro from Inmotion for providing the MCP3008 ADC for me to test. If you’re a “do-it-yourself-er” in Portugal, check their website for all kinds of components.