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Humidity sensor using 8051

Humidity sensor using 8051  (Hygrometer)

This project is about a simple humidity sensor based on 8051 microcontroller. Humidity sensor is also called hygrometer. This circuit can sense relative humidity (RH)  from 20% to 95% at an accuracy of 5%. The humidity information is displayed on a 16×2 LCD display. A relay is also provided which is set to be active when the humidity crosses a certain trip point. The circuit is mains operated and it is very easy to install. DHT11 is the humidity sensor used here. The details and working of the DHT11 humidity sensor is given below.

DHT11 humidity sensor.

Humidity sensor using 8051

DHT11 is a low cost humidity cum temperature sensor which has a digital output. Capacitive method is used for sensing the humidity and a thermistor is used for measuring the temperature. The sensor can sense relative humidity from 20% to 95% at a resolution of 5%. Temperature measurement is up to 50°C at a resolution of 2°C. The communication with the microcontroller is through a single wire. The basic communication scheme is given in the image below.

Scheme Humidity sensor

The to and fro communication with DHT11 sensor is very easy. Pin 2 of the  DHT11 is connected to the port pin of the micrcontroller. The connection scheme is shown in the image below. The data pin (pin2) of the DHT11 requires an external 10K pull-up resistor.

The communication protocol is explained as follows. The MCU (microcontroller unit) first sends a low signal of width 18mS to the DHT11. After this signal, the MCU pulls up the communication line and waits for the response from DHT11. It make take up to 2. to 40uS. Then the DHT11 pulls down the communication line and keeps it low for 80uS. Then DHT11 pulls up the line and keeps it high for 80uS. Then the DHT pulls down the line for 50uS and the next high pulse will be the first bit of the data. The data is send in bursts of 8 bits. Each high pulse of the burst indicates a data signal. The 50uS low signals between the data bits are just spacers. The logic of the data bit is identified by measuring the width of it. A 26 to 28uS wide pulse indicates a “LOW” and 70uS wide pulse indicates a “HIGH”.  In simple words, an pulse narrower than 50uS can be taken as a “LOW”  and wider than 50us can be taken as a “HIGH”. The first 8 bits of the data burst represents the integral value of the relative humidity, second 8 bits represent the decimal value of the  relative humidity, third 8 bits represent the integral value of the temperature data, and the last 8 bits represent the decimal value of the temperature data, For DHT11 the decimal values are always zero and we are measuring the relative humidity only in this project. So we need to just concern about the first 8 bits of data, that is the integral part of the relative humidity data. Circuit diagram of the humidity sensor is shown in the image below.

Schematic Humidity sensor using 8051

The humidity sensor DHT11 is connected to P3.1 of the 8051 microcontroller. R8 pulls up the communication line between DHT11 and 8051. The relay is driven using P2.0 of the microcontroller. Transistor Q1 switches the relay. R0 is a pull up resistor and R7 limits the base current of Q1. D5 is just a free-wheeling diode. Data lines of the LCD display is interfaced to Port 0 of the microcontroller. Control lines RS, R/E and E are connected to P2.7, P2.6 and P2.5 pins of the microcontroller respectively. R4 sets the contrast of the display. R5 limits the current through the back light LED. C9 is a by-pass capacitor. C8, C10 and X1 are associated with the clock circuitry. C11, R6 and S2 forms the reset circuit.

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