I have a schematic diagram here.. i was wondering if some of the experienced guys over here could help me verify if this circuit would work if i were to build it. Its a rather simple circuit, here is the part list for the circuit and also the diagram... hope someone could give me some comments... thanks alot

C1 10nF ceramic disc capacitor

C2 100nF ceramic disc capacitor

C3 1000uF electrolytic capacitor

D1 1N4148 signal diode

D2 LED

Q1 BC327 TO-92 transistor

R1 10K 0.25W variable resistor

R2 1K 0.25W variable resistor

R3, R4 470R 0.5W resistor

U1 TL072 Op-amp



This circuit basically consists of P1 and P2 which are measurement rods driven into soil.. and a voltage will be indicated at the voltmeter attached at P3 and P4, potentiometer R1 is to adjust the range of the voltage so that it will be between the 15 v voltmeter. The op amp basically compares the value.. R2 is adjusted till the motor starts ( motor outputs are P6 and P5) the transistor amplifies the current to the motor, while the diode avoids the circuit from getting burnt from the motors inductance while the capacitors help smoother the circuits output

Its a soil resistance sensor... P1 and P2 are the sensing rods

this is the idea of the project... this is how it basically work's

Basically there will be a grounding electrode between the measuring rods.. My objective is to lower the soil resistivity in order to maintain the resistance between the grounding electrode to soil.. This is important in order to maintain grounding electrodes resistance at an acceptable range.
So the measuring rods gives me an indication of the soil resistance (this is not a valid measurement since its not the 4 pole testing arrangement) but it gives me an indication.. Meaning that at this point the grounding resistance is at an acceptable value. So when ever the resistance between the rods goes above this value.. my valve need to be open and sprinkle the solution on the test area.. And it should close when the resistance between the rods decreases to the earlier acceptable value. So my control circuit only powers the valve to open and close.. Meaning the water pump is powered by a separate power supply.

I have a better version of the circuit... wat do u guys think about this..are the components easy to obtain? where should i put the voltmeter in between the probes would be fine right?

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Added on March 6, 2009, 5:00 pm

I tried making this circuit on multisim... can someone verify that its similar to the original circuit above...

http://www.fairchildsemi.com/ds/LM%2FLM311.pdf here is the data sheet for the comparator.. are they way i connected the pins right? im unsure coz im still a newbie in electronics

This post has been edited by dello: Mar 6 2009, 05:03 PM

i tested it in multisim..it works.. the soil can be assumed as just a resistance.. so can realize this using a resistor.. thats how i tested it

i hv done it already.. it works... the output are also working..

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Added on March 8, 2009, 11:47 pmnow need to build the circuit in real... and need to do some measuring for the soil.. how long will it take me to interface the reading to a pc? is it complicated? any comments

This post has been edited by dello: Mar 8 2009, 11:47 PM