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The pot plant monitor

This pot plant is connected to my computer. Every five minutes or so, the resistance of the soil is measured. The resistance varies indirectly with the dampness of the soil, as you can see in the graph.

I water the plant every few days, as indicated. It would be fun to extend this into a full-blown irrigation system by building some kind of motorized watering contraption, but I don't want to risk the wooden floor. =)

How the sensor works

This is the world's cheapest ohm-meter:

The microcontroller drains the capacitor by providing a 0V output on the pin. It then re-configures the pin into a high-impedance input pin and resets an internal clock. Now the capacitor is slowly charged through the resistor (soil), and when the voltage level has become so high that it corresponds to a logical 1, the microcontroller receives a pin change interrupt, reads the clock and reports this value to the computer.

Posted Wednesday 9-Jan-2008 17:20

Discuss this page

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Anonymous
Mon 31-Mar-2008 14:43
While I quite like the idea of the plant monitor, I wonder what the effects of the sensor will be on the system.
Current flows. Electrolysis will happen. The electrodes will dissolve over time. There's a magnetic field, etc.
Anonymous
Thu 3-Apr-2008 09:40
At University of Gent (molecular genetics department), we weigh the plants to determine if they need water. This way we can water them very precisely and there's no milliliter of water given to much or to little.

-femeref
Anonymous
Wed 9-Apr-2008 15:47
One interesting thing missing in the schematics is a suitable value for the capacitor.
Anonymous
Wed 9-Apr-2008 15:56
The amount of current would be very small. And the time it flows can be minimized by providing 5V on the sensing side when not measuring resistance, in effect giving very ner zero volts of difference between the electrodes.

Electrolysis would happen to a very small extent, but it can be minimized in the same way as the current.

Time to put that gold jewelry to use! It won't dissolve if used as electrodes.

The magnetic field would probably not be strong enough to be measurable with affordable instruments, given the small current.
Anonymous
Thu 10-Apr-2008 11:42
Plants tend to grow, thus gain weight. How do you calculate the amount of plant growth?
-webpresence04 at soulsurf se

At University of Gent (molecular genetics department), we weigh the plants to determine if they need water. This way we can water them very precisely and there's no milliliter of water given to much or to little.

-femeref
lft
Linus Åkesson
Thu 10-Apr-2008 20:25
One interesting thing missing in the schematics is a suitable value for the capacitor.

Good point. I'm using a 10 uF electrolytic capacitor.
Anonymous
Mon 14-Apr-2008 21:35

lft wrote:

10 uF electrolytic capacitor

Thanks!
Anonymous
Tue 22-Jul-2008 11:51
The amount of current would be very small. And the time it flows can be minimized by providing 5V on the sensing side when not measuring resistance, in effect giving very ner zero volts of difference between the electrodes.

Don't even need to do that, as the capacitor will charge up until there is close to zero volts across the resistor (in this case, the soil).