Read from an ADC channel on a Raspberry Pi. Shamelessly lifted from Adafruit.

readadc.py

import RPi.GPIO as GPIO


def readadc(adcnum, clockpin=18, mosipin=23, misopin=24, cspin=25):
    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