Two numbers, one pin
Behind the plastic grille are a humidity element, a temperature element and a small microcontroller that reads both for you. That last part is why one ordinary digital pin is enough, and why the number arrives already calibrated and already rounded.
What is behind the grille
Hold either board up and the part on top is a cage with square vents in it. Air gets in; the two things measuring it do not get touched.
The humidity element is a capacitor with a plastic film between its plates. The film soaks up water from the air and lets it go again, and its capacitance rises and falls with how much it is holding. The temperature element is a resistor whose resistance changes as it warms.
Neither of those is a number yet. Both are physical quantities that would need a reference, an amplifier and a calibration table to turn into anything you could print — which is the third thing in the cage.
Why the third part matters
The little microcontroller does all of that before anything reaches your board. It measures both elements, applies the calibration burned into it at the factory, and hands out a fixed pattern of forty bits.
That is the whole reason this sensor is a beginner's part. There is no analog wiring to get wrong, no reference voltage to supply, no resistor to work out, and nothing to calibrate. Three wires, one of which carries the answer.
It also decides what you cannot do. You do not get the raw capacitance, you cannot ask it to average, and you cannot ask for more precision than it offers: the DHT11 rounds to whole degrees and whole per cent, the DHT22 to tenths of both, and there is no setting anywhere that changes either.
What "humidity" means here
Both sensors report relative humidity — how close the air is to holding as much water as it can at its current temperature. It is a percentage of a limit that itself moves with temperature, which is why the same air can read 40 % in a warm room and 80 % in a cold one without a drop of water having gone anywhere.
That makes relative humidity hard to compare between rooms, and it is why a comfort reading turns it into a dew point before putting it on a display.
What comes out
One call to the library gives you two floating-point numbers. What varies between the two sensors in the box is how much of each number is real:
| DHT11 | DHT22 | |
|---|---|---|
| Temperature | whole degrees, ±2 °C | tenths, ±0.5 °C |
| Humidity | whole per cent, ±5 % | tenths, ±2 % |
| A new reading every | 1 second | 2 seconds |
Neither is fast. Both take a handful of milliseconds to answer and then refuse to answer again for a second or two, which is far more often than a room changes and is discussed properly in how often you may ask.
When it does not work
No, and there is nothing analog to read. The elements inside are wired to the sensor's own microcontroller, not to the header. What comes out of the DATA pin is a digital pattern, so an analogRead on it returns whatever the line happened to be sitting at.
No. Each part is calibrated at the factory and the coefficients are burned into memory inside the cage, which the sensor applies before it sends anything. You cannot adjust them, and a sensor reading consistently wrong is a placement problem rather than a calibration one.
That is a DHT11. Its resolution is one degree and one per cent, so a room warming slowly shows nothing at all and then jumps. A DHT22 reports tenths. Neither is a fault; both are the number the sensor is capable of producing.
No. Relative humidity is how full the air is compared with how much it could hold at its current temperature, so the same air reads differently as it warms and cools. Both sensors report the relative figure, which is why the dew point is often the more useful number to compute from it.
Three of each in the box, and a straightforward rule for which project gets which.
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