Question on the Plants Watering Watcher circuit

[audioguru2]

How can I reply when the moderator keeps deleting my posts!

The Plants Watering Watcher-2 circuit dims the LED with pulse-width-modulation. The LED doesn't have a threshold voltage, it dims smoothly from very bright to fully-off, depending on the AC impedance of the soil.

 

[dicky]

I've just downloaded the circuits for the watering watcher..I read the text where it says A square wave is used to avoid oxidisation..my knowledge as an "improver" allows me to think that electrolysis would be the enemy of the probes..can some one explain why a sine wave is US and why oxidation not electrolysis is the nemesis of electrodes [probes]..browsing down the responses on soil conductivity...the main component of vermiculite is mica a fantastic heat and electrical insulator.. thanks guru.....dicky

 

[audioguru2]

Hi Dicky,
The waveform between the probes is a fairly symmetrical signal to avoid the plating/deplating action that would occur with DC or asymmetry. I don't think that sine-wave or square-wave would make any difference, as long as it is symmetrical so that the average voltage is zero.

 

[dicky]

have seen ...thanks....dicky

 

[Codyhtml]

Pure water is actually very acidic a simiconductor factory in my town uses water that has been striped of ALL minerals and the water is so thirsty for minerals that it will actually work like an acid so if someone watered a plant with "pure" water it would probably disolve the probes and the pot lol

 

[audioguru2]

Hi Cody,
My tap-water has enough salts and minerals in it that it causes my Plants Watering Watcher-2  circuits to measure the same as a 47 ohm resistor between the probes. The water isn't acidic, it has dissolved conductive salts and minerals in it. "Pure" water doesn't conduct electricity and is made with distillation. Distilled water in dirt with plant fertilizer conducts fine for the circuit to indicate the amount of dryness.

My probes are just tinned copper and corrode a little. I sand and re-tin them each time (6 months to 1 year) that I replace the battery.

 

[sattyshack]

With your second circuit that has the blinking led, how would you modify it so you can have a lininer output with moisture vs voltage. would it be simmilar to the configuration of circuit one where the output does not blink. my objective is to try and connect this to the analog port on my microcontroller. Any help would be greatly appriciated

 

[audioguru2]

Hi Satty,
Welcome to our forum. ;D
The circuit of the Plants Watering Watcher-2 produces a logarithmic output so it shows a very wide range of moisture levels. It would be difficult to make it linear, so use your microcontroller to convert from logarithmic to linear.

I have removed its blinking oscillator and LED driver transistors, and filtered the PWM at its output for you:

View attachment 40152

 

[sattyshack]

thanks alot that will help me tons.

 

[sattyshack]

one more question in the new circuit that you just posted ic1-c the output is not being used is that i miss print or is it just not being used.

 

[audioguru2]

IC1c was the blinking oscillator. It isn't used so I disabled it by connecting both inputs high. Connect nothing to its output.

 

[sattyshack]

cool thanks for the help i think i am going to start gathering some parts and testing.

 

[sattyshack]

with this modification to the circuit what should i expect my output voltage range to be, i am not getting that good output. i have a range from 2.11V to 4.56V

 

[audioguru2]

If you short the probes together the output voltage should be very low, close to 0.00V. My damp soil measures the same as a 47 ohm resistor and probably would make the voltage of your circuit very low.
Try measuring the voltage with only a multimeter because maybe your microcontroller is pulling up the voltage.

 

[sattyshack]

i was weasuring it with a multimeter and when i shorted the leads i got the max voltage of 4.65

 

[audioguru2]

OOps, thats right. When the soil is damp then the LED is off so the output of the gate is high. You can't increase the value of R8 from 47k or the output voltage folds and works like a mirror. The circuit will need an opamp to shift the output voltage from 0.0V to 5.0V.

 

[sattyshack]

another question the circuit seems to latch in the high state and when i abruptsl contact the probes together it goes back to the low state. My high state shorted probes is 4.65v and my low state high-z on the probes is 2.12v does this seem reasonable. I also had another question in my case i am only looking for 3 threshhold DC voltages to trigger watering, so my microcontroller will be looking for only 3 set dc voltages. Would it be easier to use the soils resistance to adjust the gain on an op amp producing a range of values based on my op amps gain. Almost like a thermister would be used for temperature. Im not doing data logging so i only need minimal accuracy say dry, medium dry, and wet. But if the output from your design is supposed to be from 2.11  - 4.65V and i van get rid of that latching effect then i can make that work.

 

[audioguru2]

Hi Satty,
I have made many of these projects and they don't "latch". Did you substitute any parts?

Maybe it does something funny with a 5V supply. I can have a 100k pot beween the probes and the LED is off when the pot is zero ohms to 47 ohms then it gradually gets brighter until the pot is 100k or is not there.

Another member wanted 3 coloured LEDs to indicate the moisture level of the soil. We designed a circuit similar to yours but with opamps as comparators to drive the LEDs:

View attachment 40206

 

[sattyshack]

i used all parts the same as specified accept on the electriltic input cap i used a 50V because i didnt have a 16v but that i dont think would matter. i also used ceramic disc caps for all other that still shouldnt matter right?

 

[audioguru2]

Your capacitors are fine.