TemperatureAndHumidityWithDHT11 < IoT_Course

Lecture 4: DHT11 Temperature and Humidity Sensor

CSC 321: Embedded Sysytem

First Semester 2020/2021

The DHT11

*PINOUT:* *The DHT11 has only* 3 pins: Vcc Ground one data pin

PINOUT:

The DHT11 has only

3 pins:

Ground

one data pin

The DHT11 is a basic, low-cost digital temperature and humidity sensor.

It uses a capacitive humidity sensor and a thermistor to measure the surrounding air, and

spits out a digital signal on the data pin (no analog input pins needed).

Its fairly simple to use, but requires careful timing to grab data.

The only real downside of this sensor is you can only get new data from it once every 2 seconds.

Slide 1 of 25

A single GPIO pin

We have seen how we can drive an LED from a single GPIO pin

programmed as output pin or how we can read its state

through another GPIO pin, programmed as input pin.

Can one do more with a single pin?

Slide 2 of 25

Serial Protocol

How can we, with a single pin

initiate a measurement
read humidity and temperature data
make sure the data are correct?

The answer is: a serial protocol
and the DHT11 implements its own serial protocol

Slide 3 of 25

Reading and understanding the data sheet

Let’s have a look at the DHT11 data sheet:

Most of the following information is just a copy from the data sheet.

Slide 4 of 25

How does a resistive humidity measurement work?

Slide 5 of 25

Resistive Humidity Measurement(2)

Variations in relative humidity produce changes variations
of resistivity,which is measured.
The resistance of certain hygroscopic materials such as
hygroscopic salts have such a property
These materials are mounted as thin films on
the substrate with metal contacts
The devices can be very small

Slide 6 of 25

The NTC Thermistor

Slide 7 of 25

A processor on chip

In order to convert these measurements into numeric values

and send them to the end user through a serial protocol,

a preprogrammed micro-controller must be implemented on the chip.

In the case of the DHT11 this is an 8 bit micro-controller,

which does the conversion into binary and which creates the serial protocol

Slide 8 of 25

Text from the data sheet

This sensor includes a resistive-type humidity measurement component and an NTC temperature measurement component,

and connects to a high-performance 8-bit microcontroller, offering excellent quality, fast response, anti-interference ability

and cost-effectiveness. Each DHT11 element is strictly calibrated in the laboratory that is extremely accurate on humidity calibration.

The calibration coefficients are stored as programmes in the OTP memory, which are used by the sensor’s internal signal detecting process.

The single-wire serial interface makes system integration quick and easy. Its small size, low power consumption and

up-to-20 meter signal transmission making it the best choice for various applications, including those most demanding ones.

The component is 4-pin single row pin package.

Slide 9 of 25

Measurement Precision

Slide 10 of 25

How to connect the device

Slide 11 of 25

Powering the device

As we can see from the specs below, the DHT11 power line

can be directly connected to the cobbler 3.3V (or the 5V) line

Slide 12 of 25

Single Wire two way interface

Slide 13 of 25

Overall Communication Process

Communication Format can be seperated into three stages

1) Request
2) Response
3) Data Reading

Slide 14 of 25

Request Reading

Slide 15 of 25

What does this mean for our program?

We must:

Program the GPIO pin onto which we connected our DHT11 as output
We must pull this line done (send a login level zero) and
keep it low for a minimum of 18 ms
We must pull it high again and wait for another 20-40 μs
Finally we must re-program the pin as input and wait for the DHT11 to respond.

Slide 16 of 25

Response from DHT11

Slide 17 of 25

Slide 18 of 25

Data Reading

Slide 19 of 25

Logic Analyzer

Slide 20 of 25

What the user of the device wants

The user of the device would like to have a library which hides

all these details. He wants functions to

Send request
Read response
Read each data segment and save it to a buffer
Sum the segments and check if the result is the same as CheckSum
If the CheckSum is correct, the values are correct so we can use them. If CheckSum is wrong we discard the packet.

To read the data bits can use a counter and start count uSeconds of High level. For counts > 24uS we replace with bit '1'. For counts <=24 we replace with bit'0'

Slide 21 of 25

Reading the final data

The dht11 library uses a custom 1-wire protocol to get the measurements from the sensor.

The payload consists of a humidity value, a temperature value and a checksum.

Then measure and read their values with:

```
measure()
```
```
temperature()
```
```
humidity() 
```

Slide 22 of 25

Starting the measurement

The data pin has to be programmed as output

d = dht.DHT11(machine.Pin(16))

And read data as

d.measure()
d.temperature() # eg. 23.6 (°C)
d.humidity()    # eg. 41.3 (% RH)

Slide 23 of 25

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-- Isaac Armah-Mensah - 2021-05-27

Comments

I	Attachment	History	Action	Size	Date	Who
jpg	dht11.jpg	r1	manage	55.2 K	2021-05-27 - 04:43	IsaacArmahMensah
png	dht11_bitidentify.png	r1	manage	314.1 K	2021-05-27 - 05:16	IsaacArmahMensah
png	dht11_comm.png	r1	manage	147.9 K	2021-05-27 - 05:01	IsaacArmahMensah
png	dht11_datareading.png	r1	manage	213.5 K	2021-05-27 - 05:15	IsaacArmahMensah
png	dht11_endframe.png	r1	manage	334.8 K	2021-05-27 - 05:16	IsaacArmahMensah
png	dht11_features.png	r1	manage	102.8 K	2021-05-27 - 05:01	IsaacArmahMensah
png	dht11_logic.png	r1	manage	701.8 K	2021-05-27 - 05:15	IsaacArmahMensah
png	dht11_power.png	r1	manage	64.7 K	2021-05-27 - 05:01	IsaacArmahMensah
png	dht11_request.png	r1	manage	469.7 K	2021-05-27 - 05:16	IsaacArmahMensah
png	dht11_response.png	r1	manage	436.0 K	2021-05-27 - 05:16	IsaacArmahMensah

This topic: IoT_Course_English > WebHome > Slides > TemperatureAndHumidityWithDHT11
Topic revision: r1 - 2021-05-27 - IsaacArmahMensah