Lecture8:DHT11TemperatureAndHumiditySensor < Embedded

DHT11 Temperature and Humidity Sensor

Lecture 8

Uli Raich

UCC semester 2017/2018

Slide 1 of 21

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 21

DHT11 pinout

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

The DHT11 has only
3 pins:

Ground

one data pin

Slide 3 of 21

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 4 of 21

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 5 of 21

How does a resistive humidity measurement work?

Slide 6 of 21

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 7 of 21

The NTC Thermistor

Slide 8 of 21

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 9 of 21

Text from the data sheet

Slide 10 of 21

Measurement Precision

Slide 11 of 21

How to connect the device

Slide 12 of 21

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 13 of 21

Single Wire two way interface

Slide 14 of 21

Overall Communication Process

Slide 15 of 21

How to initiate a Measurement?

Slide 16 of 21

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 17 of 21

Response from DHT11

Slide 18 of 21

A zero bit

Slide 19 of 21

A one bit

Slide 20 of 21

Complicated?

Wow, this looks complicated. How can we write a program to do all this?

Let's start slowly:

Initiating a measurement seems do-able
Why not just read the data from the device every 5 µs
and print out the information?

The zeros and ones do not look too convincing.
Let’s save those onto a file and look at it with gnuplot

Slide 21 of 21

-- Uli Raich - 2017-10-20

Comments

I	Attachment	History	Action	Size	Date	Who
png	connections.png	r1	manage	42.6 K	2017-10-20 - 16:27	UnknownUser
png	dht11.png	r1	manage	597.9 K	2017-10-20 - 16:34	UnknownUser
png	dht11Intro.png	r1	manage	26.5 K	2017-10-20 - 16:27	UnknownUser
png	dht11Specs.png	r1	manage	11.5 K	2017-10-20 - 16:27	UnknownUser
png	humiCalib.png	r1	manage	24.3 K	2017-10-20 - 16:27	UnknownUser
png	initiate.png	r1	manage	153.8 K	2017-10-20 - 16:27	UnknownUser
png	ntcThermistor.png	r1	manage	80.6 K	2017-10-20 - 16:27	UnknownUser
png	one.png	r1	manage	149.6 K	2017-10-20 - 16:27	UnknownUser
png	overall.png	r1	manage	180.3 K	2017-10-20 - 16:27	UnknownUser
png	power.png	r1	manage	14.5 K	2017-10-20 - 16:27	UnknownUser
png	resistiveHumiMeas.png	r1	manage	54.9 K	2017-10-20 - 16:27	UnknownUser
png	response.png	r1	manage	28.8 K	2017-10-20 - 16:28	UnknownUser
png	twoWay.png	r1	manage	32.0 K	2017-10-20 - 16:28	UnknownUser
png	zero.png	r1	manage	181.6 K	2017-10-20 - 16:28	UnknownUser

This topic: Embedded_Systems > WebHome > LectureSlides > Lecture8:DHT11TemperatureAndHumiditySensor
Topic revision: r2 - 2017-10-20 - uli