Slide 1: DHT11 Temperature and Humidity Sensor

Lecture 8

Uli Raich

UCC semester 2017/2018

Slide 2: 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 3: DHT11 pinout

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

Slide 4: 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 5: 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 6: How does a resistive humidity measurement work?

Slide 7: 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 8: The NTC Thermistor

Slide 9: 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 10: Text from the data sheet

Slide 11: Measurement Precision

Slide 12: How to connect the device

Slide 13: 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 14: Single Wire two way interface

Slide 15: Overall Communication Process

Slide 16: How to initiate a Measurement?

Slide 17: 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 18: Response from DHT11

Slide 19: A zero bit

Slide 20: A one bit

Slide 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

-- Uli Raich - 2017-10-20

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