[PIC] Soil moisture detection using microcontroller

[Netra Agadi]

hi guys..
Im doing the project which automatically turn on and off the water pump depending on the moisture content in soil. Im using Microcontroller 8051, programming done in c.
I have designed a circuit for soil moisture detection which I have attached below. the sensor output is given to adc. the sensor works such a way that voltage across sensor is proportional to moisture content in soil. The problem im facing is adc output value is increasing with decrease in voltage at sensor.
help me with this asap plz...
I have attached my code(it just display the adc o/p) too...check it.

#include<reg51.h>
sbit rs=P2^0;
sbit rw=P2^1;
sbit en=P2^2;
sbit cs=P2^4;
sbit rd=P2^5;
sbit wr=P2^6;
sbit intr=P2^7;
void delay(unsigned int time)
{
unsigned int i,j;
for(i=0;i<time;i++)
for(j=0;j<1275;j++);
}
void lcddata(unsigned char val)
{
rs=1;
rw=0;
P0=val;
en=1;
delay(5);
en=0;
}
void lcdcmd(unsigned char val)
{
rs=0;
rw=0;
P0=val;
en=1;
delay(5);
en=0;
}
void lcd_init()
{
	lcdcmd(0x38);
	lcdcmd(0x0e);
	lcdcmd(0x06);
	lcdcmd(0x01);
	lcdcmd(0x80);
}
unsigned char adc()
{
unsigned char adc;
cs=0;
//intr=1;
wr=0;
wr=1;
while(intr==1);
rd=0;
adc=P1;
rd=1;
return adc;
}
void main()
{
unsigned char a;
lcd_init();
P0=0x00;
P1=0xff;
cs=0;
rd=1;
wr=1;
intr=1;
while(1)
{
lcdcmd(0x83);
a=adc();
lcddata((a/100)+48);
lcddata(((a/10)%10)+48);
lcddata((a%10)+48);
delay(30);
}
}

 

[jayanth.devarayanadurga]

Your image is not visible.

- - - Updated - - -

Your circuit is wrong. The transistor BE junction is not forward biased.

What is the voltage you get in DMM when the soil is dry and when soil is moist?

 

[WimRFP]

You only showed part of the circuit, where is the output that goes to the ADC?

If you only need to distinguish between dry ground and fully soaked ground, your sensor idea could work. When it is for some serious (agricultural) application, DC moisture measurement will not give you sufficient accuracy and repeatability over time.

In case of a serious application that needs to run for long time without human intervention, you need an AC source and very likely a 4 electrode arrangement. In such an arrangement AC current is injected into the ground via two electrodes. The other two are used to measure the voltage across the soil.

Such arrangement enables you to detect when something goes wrong (for example because of electrode corrosion).

 

[jayanth.devarayanadurga]

Moisture Sensor. Adjust the OpAmp circuit depending upon the voltage range you get for dry soil and moist soil.

 

[Netra Agadi]

the problem is with ADC because voltage across sensor is showing correctly.., for wet soil v/g is 2.07v and for dry soil 0.7v. Im getting 000 as adc o/p for 0.7 volts. . what is the possible changes i can make to adc.
thnks all for your response.

 

[jayanth.devarayanadurga]

Amplify 2.07 V to 5V so that 0.7V will be 1.67 V.


Method 2. Provide 1.25 V to Vref/2 pin of ADC using a voltage divider so that for 2.5V adc input adc value will be 255.