Automation and Control

Every automatic system follows the same three steps, over and over:

1. Sense: read a sensor (temperature, light, moisture).
2. Decide: compare the reading with a rule, such as “colder than 0 °C (32°F)?”
3. Act: switch something: a light, a buzzer, a fan.

You’ve already built one: the Automatic Night Light senses darkness and switches on an LED, using a transistor instead of a program. A thermostat, a car’s automatic headlights and a garage-door safety sensor all work this way.

This program uses the DS18B20 sensor wiring from the Digital Thermometer build, plus an LED and 220 Ω resistor on GP15. Wire GP15 (physical pin 20, the last pin on the left side) through a 220 Ω resistor to the LED’s long leg, and the LED’s short leg to the − rail. The LED comes on when the temperature falls to 0 °C (32°F), and stays on until it’s back up to 1 °C (34°F). If the sensor goes missing or a reading fails, the LED also comes on, so a fault is never silent. To test it, change the numbers to ALARM_ON = 2.0 and ALARM_OFF = 3.0, put the probe in your ice bath (Lesson 29), then warm it in your hand. Change them back afterwards.

from machine import Pin
import onewire, ds18x20, time

sensor = ds18x20.DS18X20(onewire.OneWire(Pin(22)))
roms = sensor.scan()
led = Pin(15, Pin.OUT)       # alarm LED with a 220 ohm resistor

ALARM_ON = 0.0    # turn the alarm on at or below 0 C
ALARM_OFF = 1.0   # turn it off again only at 1 C or warmer
alarm = False

while True:
    if not roms:                      # no sensor: fail safe
        led.on()
        print("FAULT: no sensor found")
        time.sleep(5)
        roms = sensor.scan()          # keep looking
        continue
    try:
        sensor.convert_temp()
        time.sleep_ms(750)            # the sensor needs up to 750 ms
        t = sensor.read_temp(roms[0])
    except Exception:
        led.on()                      # failed reading: fail safe
        print("FAULT: reading failed")
        time.sleep(5)
        continue
    if not alarm and t <= ALARM_ON:
        alarm = True
    elif alarm and t >= ALARM_OFF:
        alarm = False
    led.value(1 if alarm else 0)
    print(round(t, 1), "ALARM" if alarm else "")
    time.sleep(5)

To sound a buzzer as well, drive it through a transistor (Lesson 10), not straight from a pin.

Good control systems plan for things going wrong. Ask: what happens if the sensor wire breaks, the reading is garbage, or the program crashes? That’s why this frost alarm lights its LED, rather than going quiet, if no sensor is found or a reading fails. This idea is called fail-safe design.

Real automation also has limits and overrides: a maximum on-time for a pump, a manual off switch, and a safe state when power returns. And whatever your program controls, it must stay low-voltage. Microcontroller projects in Electronics School never switch household power: relays and outlets on the mains need certified equipment and qualified people.

Sense, decide, act.

It stops a system from switching on and off rapidly when a reading sits right at the threshold.

Planning so that if something goes wrong, the system ends up in a safe state.