Nwazet Key Lime Pi - Reading Analog Inputs

By The Pi HutJan 18, 20170 comments

Before you start, ensure that the SPI driver is loaded on your Raspberry Pi distribution.

The following Python sample reads from a TMP36 analog temperature sensor every second. It converts the raw temperature data into millivolts, degrees Celsius and Fahrenheit. For reliable and efficient SPI communication (no bit-banging), the sample depends on the py-spidev Python module. To install py-spidev, do the following, assuming that the 'git' and 'python-dev' packages are already installed on the Raspberry Pi:

git clone https://github.com/doceme/py-spidev.git cd /py-spidev python setup.py install

Then, using the 'nano' editor, do the following:

nano tmp36.py

Paste the sample code below into the editor, save it and exit.

Then, make tmp36.py executable and run it.

chmod +x tmp36.py ./tmp36.py

The output of the script should look like this:

raw temp= 206 mV= 663.8671875 Celsius= 16.38671875 Fahrenheit= 61.49609375

Yeah, my office is a tad nippy at the moment ;)

#!/usr/bin/python

import spidev
import time

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

# read SPI data from one of the MCP3008's eight possible inputs (0 thru 7)
def readadc(adcnum):
        if ((adcnum > 7) or (adcnum < 0)):
                return -1
        r = spi.xfer2([1,(8+adcnum)<<4,0])
        adcout = ((r[1]&3) << 8) + r[2]
        return adcout

# TMP36 temperature sensor connected to ADC input 0
TempSensor = 0

while True:
        rawTemp = readadc(TempSensor)
        # Convert the raw ADC input to milliVolts, degrees Celsius and Fahrenheit
        milliVolts = rawTemp * (3300.0 / 1024.0)
        tempCelsius = ((milliVolts - 100.0) / 10.0) - 40.0
        tempFahrenheit = (tempCelsius * 9.0 / 5.0 ) + 32

        print "raw temp=", rawTemp
        print "mV=", milliVolts
        print "Celsius=", tempCelsius
        print "Fahrenheit=", tempFahrenheit

        time.sleep(1)