Composite Sensors - Part 2
DHT 11
For Part 2, the DH11 will be our focus. The DHT11 is a cost-effective digital temperature and humidity sensor. Using a capacitive humidity sensor and a thermistor to measure the surrounding air, a digital equivalent is returned on the data pin (no analog input pins needed.
The sensor is easily identifiable in its powder blue housing with 4 legs protruding. These four legs are VCC, GND, NC (Not Concerned), and Data. If you purchase the sensor all by its lonesome, then there is an extra PIN to ignore as shown below. If you purchase as a ready module, the manufacturer may have removed access to the pin. These two scenarios are illustrated below.
Owing to the construction of the sensor, it is not advisable to use it in highly moist or corrosive environments.
The DHT22 which looks like the DHT11 but has white or ivory packaging is similar in function to the DHT11, it's just more accurate. The DHT22 will also read a negative or below zero temperature, whereas the DHT11 is above zero only.
Please see DHT - DEEP Dive๏ปฟ for more engineering content.
Connection Diagram
When connecting or wiring the sensor, there are a few considerations. The sensor has a specific voltage operating range, and this makes it usable for low-power devices as well as devices operating at a regular voltage. There is a single pin that functions to return data from the sensor to the MCU interacting with it.
Power Supply
The device requires 5VDC but can also work at 3V3; this can be useful for devices that are K or J variants that operate at 3V3. The output of the device is a digital value, not an analog one which ensures a reliable return from the device.
Code Example
// LCD module connections
sbit LCD_RS at RB2_bit;
sbit LCD_EN at RB3_bit;
sbit LCD_D4 at RB4_bit;
sbit LCD_D5 at RB5_bit;
sbit LCD_D6 at RB6_bit;
sbit LCD_D7 at RB7_bit;
sbit LCD_RS_Direction at TRISB2_bit;
sbit LCD_EN_Direction at TRISB3_bit;
sbit LCD_D4_Direction at TRISB4_bit;
sbit LCD_D5_Direction at TRISB5_bit;
sbit LCD_D6_Direction at TRISB6_bit;
sbit LCD_D7_Direction at TRISB7_bit;
sbit Data at RA0_bit;
sbit DataDir at TRISA0_bit;
char message1[] = "Temp = 00.0 C";
char message2[] = "RH = 00.0 %";
unsigned short TOUT = 0, CheckSum, i;
unsigned short T_Byte1, T_Byte2, RH_Byte1, RH_Byte2;
void StartSignal(){
DataDir = 0; // Data port is output
Data = 0;
Delay_ms(25); // Low for at least 18us
Data = 1;
Delay_us(30); // High for 20-40 us
DataDir = 1; // Data port is input
}
unsigned short CheckResponse(){
TOUT = 0;
TMR2 = 0;
T2CON.TMR2ON = 1; // Start TMR2 while waiting for sensor response
while(!Data && !TOUT); // If there's no response within 256us, the Timer2 overflows
if (TOUT) return 0; // and exit
else {
TMR2 = 0;
while(Data && !TOUT);
if (TOUT) return 0;
else {
T2CON.TMR2ON = 0;
return 1;
}
}
}
unsigned short ReadByte(){
unsigned short num = 0, t;
DataDir = 1;
for (i=0; i<8; i++){
while(!Data);
TMR2 = 0;
T2CON.TMR2ON = 1; // Start TMR2 from 0 when a low to high data pulse
while(Data); // is detected, and wait until it falls low again.
T2CON.TMR2ON = 0; // Stop the TMR2 when the data pulse falls low.
if(TMR2 > 40) num |= 1<<(7-i); // If time > 40us, Data is 1
}
return num;
}
void interrupt(){
if(PIR1.TMR2IF){
TOUT = 1;
T2CON.TMR2ON = 0; // stop timer
PIR1.TMR2IF = 0; // Clear TMR0 interrupt flag
}
}
void main() {
unsigned short check;
TRISB = 0b00000000;
PORTB = 0;
TRISA = 0b00100001;
CMCON = 7;
INTCON.GIE = 1; //Enable global interrupt
INTCON.PEIE = 1; //Enable peripheral interrupt
// Configure Timer2 module
PIE1.TMR2IE = 1; // Enable Timer2 interrupt
T2CON = 0; // Prescaler 1:1, and Timer2 is off initially
PIR1.TMR2IF =0; // Clear TMR INT Flag bit
TMR2 = 0;
Lcd_Init();
Lcd_Cmd(_Lcd_Clear);
Lcd_Cmd(_LCD_CURSOR_OFF);
do {
Delay_ms(1000);
StartSignal();
check = CheckResponse();
if (!check) {
Lcd_Cmd(_Lcd_Clear);
Lcd_Out(1, 1, "No response");
Lcd_Out(2, 1, "from the sensor");
}
else{
RH_Byte1 = ReadByte();
RH_Byte2 = ReadByte();
T_Byte1 = ReadByte();
T_Byte2 = ReadByte();
CheckSum = ReadByte();
// Check for error in Data reception
if (CheckSum == ((RH_Byte1 + RH_Byte2 + T_Byte1 + T_Byte2) & 0xFF))
{
message1[7] = T_Byte1/10 + 48;
message1[8] = T_Byte1%10 + 48;
message1[10] = T_Byte2/10 + 48;
message2[7] = RH_Byte1/10 + 48;
message2[8] = RH_Byte1%10 + 48;
message2[10] = RH_Byte2/10 + 48;
message1[11] = 223; // Degree symbol
Lcd_Cmd(_Lcd_Clear);
Lcd_Out(1, 1, message1);
Lcd_Out(2, 1, message2);
}
else{
Lcd_Cmd(_Lcd_Clear);
Lcd_Out(1, 1, "Checksum Error!");
Lcd_Out(2, 1, "Trying Again ...");
}
}
}while(1);
}Moving on: Composite Sensors Part 3
๏ปฟComposite Sensors Part 3๏ปฟ๏ปฟ