TemperatureAndHumidityWithDHT11 < IoT_Course

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<h2 align="center">Lecture 4: DHT11 Temperature and Humidity Sensor</h2> <h2 align="center">CSC 321: Embedded Sysytem</h2> <h2 align="center">First Semester 2020/2021</h2>
---+ The DHT11
| <p> __PINOUT:__ </p> <p> __The DHT11 has only__ </p> <p>3 pins:<br /><br /></p> <ul> <li>Vcc</li> </ul><ul> <li>Ground</li> </ul><ul> <li>one data pin</li> </ul><p> </p> | <img alt="dht11.jpg" height="300" src="%ATTACHURL%/dht11.jpg" title="dht11.jpg" width="300" /> |

<div title="Page 1">

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.

</div>
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---+ 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.<br /><br />Can one do more with a single pin?

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---+ Serial Protocol

How can we, with a single pin<br /><br />
   * initiate a measurement
   * read humidity and temperature data
   * make sure the data are correct?
The answer is: <b>a serial protocol</b><br />and the DHT11 implements its own serial protocol

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---+ Reading and understanding the data sheet

Let’s have a look at the [[https://iotworkshop.africa/pub/UCC_Course_2021/TheDHT11TemperatureAndHumiditySensor/DHT11.pdf][DHT11 data sheet]]:

Most of the following information is just a copy from the data sheet.
---++++ <img alt="dht11_features.png" height="246" src="%ATTACHURL%/dht11_features.png" style="background-color: transparent; color: #000000; font-size: small;" title="dht11_features.png" width="400" />
---++ 
---+ How does a resistive humidity measurement work?

<img alt="resistiveHumiMeas.png" height="191" src="https://iotworkshop.africa/pub/UCC_Course/Lecture8:DHT11TemperatureAndHumiditySensor/resistiveHumiMeas.png" title="resistiveHumiMeas.png" width="319" />

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---+ Resistive Humidity Measurement(2)
   * Variations in relative humidity produce changes variations<br />of resistivity,which is measured.
   * The resistance of certain hygroscopic materials such as<br />hygroscopic salts have such a property
   * These materials are mounted as thin films on<br />the substrate with metal contacts
   * The devices can be very small
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---+ The NTC Thermistor

<img alt="ntcThermistor.png" height="398" src="https://iotworkshop.africa/pub/UCC_Course/Lecture8:DHT11TemperatureAndHumiditySensor/ntcThermistor.png" title="ntcThermistor.png" width="728" />

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---+ 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

---++ 
---+ 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.
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---+ Measurement Precision

<img alt="dht11Specs.png" height="133" src="https://iotworkshop.africa/pub/UCC_Course/Lecture8:DHT11TemperatureAndHumiditySensor/dht11Specs.png" title="dht11Specs.png" width="774" />

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---+ How to connect the device

<img alt="connections.png" height="368" src="https://iotworkshop.africa/pub/UCC_Course/Lecture8:DHT11TemperatureAndHumiditySensor/connections.png" title="connections.png" width="650" />

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---+ 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

<img alt="dht11_power.png" height="57" src="%ATTACHURL%/dht11_power.png" title="dht11_power.png" width="600" />

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---+ Single Wire two way interface

<img alt="dht11_comm.png" height="153" src="%ATTACHURL%/dht11_comm.png" title="dht11_comm.png" width="700" />

---++ 
---+ Overall Communication Process

Communication Format can be seperated into three stages

1) Request<br /> 2) Response<br /> 3) Data Reading
---++ 
---+ Request Reading

<img alt="dht11_request.png" height="186" src="%ATTACHURL%/dht11_request.png" title="dht11_request.png" width="700" />

---++ 
---+ 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<br />keep it low for a minimum of 18 ms
   * We must pull it high again and wait for another 20-40 &mu;s
   * Finally we must re-program the pin as input and wait for the DHT11 to respond.
---++ 
---+ Response from DHT11

<img alt="dht11_response.png" height="189" src="%ATTACHURL%/dht11_response.png" title="dht11_response.png" width="700" />

---++ 
---+ 
---+ Data Reading

<img alt="dht11_datareading.png" height="236" src="%ATTACHURL%/dht11_datareading.png" title="dht11_datareading.png" width="700" />
---++ 
---+ Logic Analyzer

<img alt="dht11_logic.png" height="554" src="%ATTACHURL%/dht11_logic.png" title="dht11_logic.png" width="700" />
---++ 
---+ 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<br /><br />
   * 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
   * <div title="Page 6"> <p>If the CheckSum is correct, the values are correct so we can use them. If CheckSum is wrong we discard the packet.</p> <p>To read the data bits can use a counter and start count uSeconds of High level. For counts &gt; 24uS we replace with bit '1'. For counts &lt;=24 we replace with bit'0'</p> </div>
---++ 
---+ 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:
   * <verbatim>measure()</verbatim>
   * <verbatim>temperature()</verbatim>
   * <verbatim>humidity() </verbatim>
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---+ Starting the measurement

The data pin has to be programmed as output
<verbatim style="padding-left: 30px;">d = dht.DHT11(machine.Pin(16))</verbatim>

And read data as
<verbatim style="padding-left: 30px;">d.measure()
d.temperature() # eg. 23.6 (°C)
d.humidity()    # eg. 41.3 (% RH)
 </verbatim>
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-- %USERSIG{IsaacArmahMensah - 2021-05-27}%

---++ Comments

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Topic attachments
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

Topic revision: r1 - 2021-05-27 - IsaacArmahMensah