Reading sensors with Arduino: digital vs analogue inputs and the serial monitor

Once an LED blinks, the next step is making the board notice the world. That means inputs: buttons, light sensors, temperature sensors and the like. The two ideas to get right are the difference between digital and analogue inputs, and how to see what the board is reading using the serial monitor.

A digital input reads only two states, HIGH or LOW. A push button, a tilt switch, a reed switch near a magnet and many motion-sensor modules all give a digital signal. The catch is the floating pin. A pin connected to nothing does not read LOW; it picks up stray electrical noise and flickers between states. Fix it with a pull-up or pull-down resistor, which holds the pin at a known level until the button changes it. The Arduino has built-in pull-ups you can switch on in code: pinMode(2, INPUT_PULLUP); Wire the button between pin 2 and GND. Now the pin reads HIGH when the button is released and LOW when pressed. That reversed logic surprises everyone once; it is normal. Mechanical buttons also bounce: the contacts chatter for a few milliseconds, so one press can register as several. A short delay or a debounce routine (there is an official example) solves it.

Analogue inputs measure a voltage rather than just on or off. On the Uno these are the pins labelled A0 to A5. By default analogRead returns a number from 0 to 1023, where 0 is 0 V and 1023 is the reference voltage (normally the board's supply). Sensors that give an analogue voltage include:

Never feed an analogue pin more than the board's operating voltage. A sensor powered from a higher supply can do exactly that.

Some sensors send readings as data rather than a voltage, using buses called I2C, SPI or a one-wire protocol. Many digital temperature, humidity, distance and motion sensors work this way. You usually install a library from the IDE's library manager and call a function such as readTemperature. Check that a module runs at your board's logic voltage, since some are 3.3 V only.

The serial monitor shows text the board sends back over USB, which makes it the main debugging tool. In setup(): Serial.begin(9600); In loop(): Serial.println(analogRead(A0)); delay(200); Open the serial monitor in the IDE, set the same speed (9600 baud here), and you will see a stream of numbers. Turn the potentiometer and watch them change. The IDE's serial plotter draws the same numbers as a live graph, which is great for seeing noise or slow trends. If you see gibberish, the baud rate in the monitor does not match the one in Serial.begin.

Once you can see the numbers, act on them:

Calibrate in the place you will use the sensor. A light sensor's numbers at your desk will not match a window sill.

Which sensor are you trying to read? Post your wiring and readings in the comments and others can help you work out what is going on.