Relay Always Open?

[Nany]

Hi,

I'm working on a simple automatic watering circuit..
here the circuit...

one of the probe (sensor) is connected to the -ve side and the other is connected to the input of the variable resistor.

I connected 2 legs of the variable resistor. the middle as input with the amplifier, and the other with +ve side.

the common of the relay is connected with +ve, and the LED (as a pump) with the normal open.

the pump is supposed to work when soil is dry and stop if wet.

but .. when i put the probe in dry soil , the pump work and never stop,,, even if the soil is really wet,,

i hope you can help to know where is the problem.....

thank you.

 

[MikeMl]

Your pictures are truncated, so I cannot see the full schematic.

Is the probe supposed to be equivalent to a 10K resistor in dry soil? That is what the circuit is set up to do.

What is the supply voltage?

How is the 741 wired to drive the LED?

btw- this is a bad way of measuring soil moisture. The DC bias current through the probes will destroy them in a few days due to electrolysis. We had a recent thread about this here on ETO a few weeks ago. I'll see if I can find it...

Here is one of many previous threads on this topic.

 

[cowboybob]

Welcome to ETO, nany!

Goggle "carbon rod water probe". Using carbon rods for your purpose pretty much eliminates probe degradation. Although you may find the need to occasionally clean the probes to remove accumulated salt (or other) deposits.

Adjust probe depth to suit your moisture depth needs.

Also remember that the probes themselves will add an additional resistance to the circuit that you'll have to take into account. they'll also work equally well with DC or AC excitation.

 

[audioguru]

A lousy old 741 opamp was designed 46 years ago to work only when its supply is 30V. Many do not work with a 9V supply.
Its output might not go high enough or low enough to work properly in your circuit especially when the 9V battery drops to 6V when it is used.
Use a modern opamp instead that works perfectly with a low supply voltage.

Pure water DOES NOT CONDUCT electricity. The salts in water allow it to conduct. Is your soil salty enough?

 

[dr pepper]

Welcome to the forum.
Your circuit is very basic and probably will not be that reliable, however it sounds like its too sensitive, try a 5k potmeter instead of a 10k.
The 2 diodes are req because your using a 741, but theres nothing wrong with that.
Also you could try reducing the contact area of your probes, did you make them to a plan size?