MQ-2 gas sensor on a Pi with an MCP3008, on a Raspberry Pi 5

# An MQ-2 gas sensor on a Raspberry Pi 5, through an MCP3008.
# A Pi has no analog inputs, so the MCP3008 reads the MQ-2's AO on SPI0
# (CLK GPIO11, MISO GPIO9, MOSI GPIO10, CE0 GPIO8), from 3.3 V.
# The MQ-2's heater needs 5 V, so AO can reach 5 V: a 10 k and 20 k divider
# brings it down to two thirds before CH0. The alarm LED is on GPIO18.
from gpiozero import MCP3008, LED
from time import sleep

gas = MCP3008(channel=0)    # reads 0 to 1 of 3.3 V
alarm = LED(18)
DIVIDER = 1.5               # AO is 1.5 times what CH0 sees
LIMIT = 0.4                 # volts on AO: clean air is about 0.05

print("Warming up the heater...")
sleep(20)                   # a cold MQ-2 reads like a room full of gas

while True:
    ao = gas.voltage * DIVIDER
    if ao > LIMIT:
        alarm.on()
        print(f"AO {ao:.2f} V  GAS!")
    else:
        alarm.off()
        print(f"AO {ao:.2f} V  clean")
    sleep(1)