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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.

DHT11 pinout

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.

DHT11 timing diagram

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.

DHT11 connection dagram

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.

Circuit diagram.

hygrometer circuit

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.

Program.

RS EQU P2.7                    RW EQU P2.6                    E  EQU P2.5                    ORG 000H  MOV DPTR,#LUT  SETB P3.5  CLR P2.0  MOV TMOD,#00100001B  MOV TL1,#00D  ACALL DINT  ACALL TEXT1  MAIN: MOV R1,#8D        SETB P3.5        CLR P3.5        ACALL DELAY1        SETB P3.5  HERE:JB P3.5,HERE  HERE1:JNB P3.5,HERE1  HERE2:JB P3.5,HERE2  LOOP:JNB P3.5,LOOP       RL A       MOV R0,A       SETB TR1  HERE4:JB P3.5,HERE4        CLR TR1        MOV A,TL1        SUBB A,#50D        MOV A,R0        JB PSW.7, NEXT        SETB ACC.0        SJMP ESC  NEXT:CLR ACC.0  ESC: MOV TL1,#00D       CLR PSW.7       DJNZ R1,LOOP       ACALL DINT       ACALL TEXT1       ACALL LINE2       ACALL TEXT2       ACALL HMDTY       ACALL CHECK       ACALL DELAY2       LJMP MAIN                       DELAY1: MOV TH0,#0B9H              MOV TL0,#0B0H              SETB TR0                    HERE5: JNB TF0,HERE5                      CLR TR0                               CLR TF0                             RET     DELAY2:MOV R1,#112D    BACK:ACALL DELAY1         DJNZ R1,BACK         RET      CHECK:MOV A,R0         MOV B,#65D         SUBB A,B         JB PSW.7,NEXT1         ACALL TEXT3         SETB P2.0         SJMP ESC1    NEXT1:ACALL TEXT4          CLR P2.0    ESC1:CLR PSW.7         RET                    CMD: MOV P0,A      CLR RS      CLR RW      SETB E      CLR E      ACALL DELAY      RET    DISPLAY:MOV P0,A      SETB RS      CLR RW      SETB E      CLR E      ACALL DELAY      RET        HMDTY:MOV A,R0        MOV B,#10D        DIV AB        MOV R2,B        MOV B,#10D        DIV AB        ACALL ASCII        ACALL DISPLAY        MOV A,B        ACALL ASCII        ACALL DISPLAY        MOV A,R2        ACALL ASCII        ACALL DISPLAY        MOV A,#"%"        ACALL DISPLAY        RET                  TEXT1: MOV A,#"H"         ACALL DISPLAY         MOV A,#"y"         ACALL DISPLAY         MOV A,#"g"         ACALL DISPLAY         MOV A,#"r"         ACALL DISPLAY         MOV A,#"o"         ACALL DISPLAY         MOV A,#"m"         ACALL DISPLAY         MOV A,#"e"         ACALL DISPLAY         MOV A,#"t"         ACALL DISPLAY         MOV A,#"e"         ACALL DISPLAY         MOV A,#"r"         ACALL DISPLAY         RET                 TEXT2: MOV A,#"R"         ACALL DISPLAY         MOV A,#"H"         ACALL DISPLAY         MOV A,#" "         ACALL DISPLAY         MOV A,#"="         ACALL DISPLAY         MOV A,#" "         ACALL DISPLAY         RET    TEXT3: MOV A,#" "         ACALL DISPLAY         MOV A,#" "         ACALL DISPLAY         MOV A,#"O"         ACALL DISPLAY         MOV A,#"N"         ACALL DISPLAY         RET      TEXT4:MOV A,#" "         ACALL DISPLAY         MOV A,#"O"         ACALL DISPLAY         MOV A,#"F"         ACALL DISPLAY         MOV A,#"F"         ACALL DISPLAY         RET      DINT:MOV A,#0CH       ACALL CMD      MOV A,#01H       ACALL CMD      MOV A,#06H       ACALL CMD      MOV A,#83H       ACALL CMD      MOV A,#3CH       ACALL CMD      RET        LINE2:MOV A,#0C0H         ACALL CMD        RET                          DELAY: CLR E      CLR RS      SETB RW      MOV P0,#0FFH      SETB E      MOV A,P0      JB ACC.7,DELAY      CLR E      CLR RW      RET    ASCII: MOVC A,@A+DPTR         RET        LUT: DB  48D       DB  49D       DB  50D       DB  51D       DB  52D       DB  53D       DB  54D       DB  55D       DB  56D       DB  57D      END             

About the program.

The communication protocol with the sensor and the microcontroller is already explained. Data out pin of the DHT11 is connected to P3.5 of the microcontroller. At the start of the MAIN loop, P3.5 is held high. Then it is made low and an 18mS delay routine (DELAY1) is called. Then P3.5 is made high. This forms the first 18mS wide start signal for the DHT11. Now the communication line is high and the microcontroller polls the status of this line and waits there until it is low. It becomes low when the DHT11 sends back the response signal. Then the microcontroller waits for the second response signal which is a high signal. When this high signal is received the microcontroller waits for the next low signal and after this low signal, data transmission starts.

I have already mentioned that, each data bit is represented by a high signal and the width of it determines the logic. So when ever the microcontroller receives a data pulse, Timer1 is started and the program waits there until the data pulse vanishes. Then the timer is stopped. Now the TL1 ie Timer1 low register contains the width of the data pulse. Then the width is compared with 50 by substracting 50 from the TL1 count. If carry flag (PSW.7) is set it means that the width is less than 50uS and if carry flag is not set it means that the width is higher than 50uS.

If width is less than 50uS it indicates a low signal and ACC.0 is cleared. If the width is higher than 50uS it indicates a high signal and ACC.0 is set.Then TL1 register and PSW.7 bit are cleared. Then the LOOP is iterated 8 times and accumulator is rotated left during the begining of each iteration. The rotate left (RL) instruction is used beacause during each iteration you updates ACC.0 only and you have to save that bit before the next update.This is achieved by shifting it to left each time.

The last update of ACC.0 doesnot require a position shift because it is in the right place it is supposed to be.That is the reason behind placing the RL A instruction at the begining of the LOOP. After the 8 th iteration the LOOP is exited without rotating the accumulator. Now the content of the accumulator is equal to the integral value of the current relative humidity in percentage.

Then subroutine DINT is called for initializing the display.Then subroutine TEXT1 is called which displays “Hygrometer”. Then subroutine LINE2 is called for shifting the cursor to 2nd line. Then subroutine HMDTY is called for displaying the humidity value in percentage.Then subroutine CHECK is called for checking whether the humidity value is above or below 65%. If humidity is above 65%, relay is activated and else the relay is deactivated. The the 2S delay subroutine DELAY2 is called. 2 second delay is given because you can take radings from the DHT11 once in every 2 seconds only.This also makes the display stable. Then the program jumps to MAIN label and the entire process is repeated.

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