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JLB

Question on the Plants Watering Watcher circuit

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Here is what you said:




The author recently told me that he may update his circuit with my improvements when he has time in a couple of months.



But it would seem in your last post that he only agreed to try the circuit with your changes in a couple of months. This is a little different than stating that he will update the circuit with your improvements.

I had no doubt that you corresponded. Just the accuracy of the statement as retold by you. And as I see...

About the filter: Yes, I meant to type low pass and typed high. No big deal since I wanted to point out that you were creating a filter here. But the rest of my statement is not changed. You have added a lowpass filter which changes the function. ANY filtering here would cause me concern. Also, I understand digital very well, thank you, as I work and design in the digital world every day. I do not have a misunderstanding of how NAND gates work. You can keep your snyde remarks to yourself in areas that I moderate or see a quick lack of posts from yourself in them. I hope this is very clear to you since you have had a language barrier with me in the past. I have told you before that if you have an argument with me, you can do it in private message. This is your last warning about this. The forum is no place for your arguments or insult and this will not be tolerated.

MP

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Here is what you said:



The author recently told me that he may update his circuit with my improvements when he has time in a couple of months.



But it would seem in your last post that he only agreed to try the circuit with your changes in a couple of months. This is a little different than stating that he will update the circuit with your improvements.

ANY filtering here would cause me concern.



MP,
I didn't say that the author WILL update the circuit with my improvements, I said that he MAY do so, which is what he says.

What is your concern about a filter in a Schmitt-trigger (with hysteresis) circuit? It makes a perfect time-delay, since the output responds exactly when the rising input voltage reaches its threshold.

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To All,
I was going to build a bunch of these cheap circuits (with my improvements), but now I realise that LED brightening (dimming in reverse) is a lousy way to indicate soil dryness, as i explained earlier (and now I have an even better idea):


When looking at the LED today, can you determine whether its brightness is more than it was yesterday (I noticed that the LED was very dim yesterday), without having a standard LED to compare it to?
Therefore another improvement would be for it to have a numeric display! (or row of sequental LEDs)


I am certainly not going to use a complicated display circuit for a dumb plant!

My new idea is to have a flashing LED indicate soil dryness. When the soil is wet then the LED is off. As the soil dries, the LED begins flashing slowly. As the soil dries even more then the flashing-rate increases. When the soil is very dry then the flashing-rate will be so fast that the LED will appear to be on steadily.
The circuit will be simple and cheap (1 CMOS IC, a couple of resistors and capacitors). The flashing will keep the average current drain low, for long battery (2-3V) life. The probes will have AC across them, preventing plating-action and corrosion. It will not cause arguments about how it works. The circuit WILL work. I could use a bunch of these.
Any comments?

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Audioguru,

As surveillance for a plant (dumb or not) it sounds like very good circuit. I have to agree on your thoughts about the form of message the first circuit gave. Your suggestion will of curse give a much clearer message. Even JLB:s application measuring salt concentration can perhaps benefit from the new circuit.
I say: fine by me!

Ante ::)

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To All,
Didn't anyone notice? The oscillator's frequency CANNOT increase when the soil's resistance also increases, as my idea requires:


My new idea is to have a flashing LED indicate soil dryness. When the soil is wet then the LED is off. As the soil dries, the LED begins flashing slowly. As the soil dries even more then the flashing-rate increases. When the soil is very dry then the flashing-rate will be so fast that the LED will appear to be on steadily.


So I'll try the original circuit with an added delay capacitor and a new 74HC132 to replace the old 4093. I'll report my findings.

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To All,
I built my slightly modified project and it works very well. ;D ;D ;D
The LED is very dim when the plant's soil is wet, gets brighter as the soil dries and shines brightly when the soil is dry.
I built the original circuit and it didn't work unless the supply voltage was 3.5V or more, because I couldn't find a Motorola 4093 chip (I used a TI CD4093 instead). The LED didn't dim, but was simply switched on or off when the soil's resistance was below or above 55K Ohms. With a supply voltage of 3.5V, the LED was very dim, when on. :'( :'( :'(
The slightly modified circuit works with a supply voltage of down to 1.8V or less (the LED's forward voltage). The CD74HC132 chip has so much drive current with a supply voltage of 3V, that I had to add a current-limiting resistor in series with the LED. With the trim-pot set to maximum (100K), the LED is off when the soil's resistance is less than 430 Ohms, it is very dim at 470 Ohms and brightens gradually as the soil's resistance increases to 56K Ohms where it draws an average current of 8mA. Above 56K to open-circuit, the LED stays bright. With wet soil between the probes (or a short-circuit), the circuit draws a supply current of only 0.2mA. The alternating voltage across the probes is exactly symmetrical, to avoid a plating action or corrosion of them. See my pics:

post-1706-14279141630113_thumb.jpg

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Ante,
Thanks.
Trial and error? Yes, I was surprised that the LED drive current was too high and I had to try first 10 ohms, then 22 ohms to find a suitable current-limiting resistor.
Other than that, it is all theory, I redesigned the circuit in my head and it works exactly as I said:



JLB,
I see your problem. I think that a capacitor is missing, ..... from the right-side probe to ground, ....... Can you try it with about 1nF? Please let us know.



But he never replied again, so I tried it.

Do you like my Chinese-copied epoxy-fiberglass Veroboard?

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Hi there Audioguru:

I was a little bit shocked by the level of controversy that my simple question started.

As a matter of fact I started to look into other circuits as this one
( as it was) had some additional problems like the 10% mark to space
ratio that polarized the probe, and the fact that I need a linear output
as the media conductivity changes.

But with the changes you suggest, including the use of the CD74BC132, I will give it a second try.

Your help has been great !!!!

Don't be surprised if I request your help once again in order to finish my project ( sorry ! )

Best regards, JLB.

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JLB,
Sure, we are glad to help you.
Thanks for pointing-out the errors in our original project:
1) No dimming, as was promised by the author.
2) Very dim LED.
3) Asymmetrical duty-cycle at the probes, contrary to the text.
4) No function with most original ICs at 3V or less.

Did your typing slip on the CD74HC132?

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Sorry, I meant to type CD74HC132.

By the way, do you think that wit the suggested modifications the
circuit will show a linear response respect to the media conductivity?

I plan to measure the conductivity of salts dissolved in water from 0
parts per million to 3000 parts per million.
This is equivalent ( roughly) to a conductivity range of 0 mSiemens/cm to 4 mSiemens/cm.
Will your changes provide a linear response to increased conductivity?
( Twice salt concentration = twice conductivity = twice output from
the circuit ??? ) I plan to rectify the output signal to the LED.



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JLB,
I tried my circuit with the probes in a glass of tap water, then activated-charcoal-filtered water. They both made the LED dim, like a 470 ohm to 2.2K resistor, depending on how much (1/8 inch to 1 inch) of the narrow (like a toothpick) probes was in the water. My probes are spaced 0.6 inches apart. Is my water that bad?
I have my trimpot set to maximum (100K) and it works well that way for soil resistance. When I reduce the trimpot, then the circuit becomes an on-off switch, because then the added capacitor seems to be too small.
Then I increased the capacitor to 11nF (11 times) and decreased the trimpot to 9K and the LED brightness responded to a resistance from 39 ohms to 5k (1/11 of before). The plain water dimmed like a 1.2K resistor. Then I mixed a level teaspoon of iodized table salt in 16oz of water (it didn't taste stong) and the LED became dim like an 82 ohm resistor. I don't know how many ppm that is but I think that I am on the right track. A limit will be reached where the CD74HC132 just cannot supply enough current to a very low resistance.

You do not need to rectify the pulse-width-modulated signal to the LED, just a series resistor and a filter capacitor to ground will be fine. Your DC voltage is measured across the filter capacitor. With a 3V supply, infinite resistance measures 1.5V, and a shorted resistance measures 3.0V. The voltage change certainly isn't linear with resistance change but appears to be logarithmic. It is difficult to tell since how much is 1/2 of infinity, and how much is double a short?

The symmetry of the waveform across the probes is not exactly 50-50 as I thought. It is actually about a 60-40 duty cycle since the oscillator's Schmitt threshold is less than 1/2 supply voltage.
I hope that you can still use this circuit.

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Audioguru, Thanks again.

If you mixed a level teaspoon of salt ( about 5 grams) in 19 oz ( 454 grams ) of water you had some 11,000 ppm solution.
Your probe is simmilar to the one I use, so your info is very interesting.

I will see if I can overcome the alinearity .......... or live with it.

Best regards, JLB.

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Hey Guys,
Last night, after watering my plants, I turned-off the lights and noticed that the LED on my slightly-modified project was very, very dim. I never looked at it in the dark before. I previously replaced the useless 100K trimpot with 220K so that the LED wouldn't indicate wet, low-resistance soil.
Replacing the soil with a resistor, the LED reliably indicates a smoothly changing brightness from 39 ohms to 100K ohms, a range of more than 2,500! ;D

I tried a PNP transistor with emitter resistor and voltage reference on its base as a current-source, to replace R4 which caused the LED to dim when the battery voltage ran-down to 2V. It didn't make much difference. So I tried Ante's trial-and-error method and ended-up with that PNP current-source directly driving the LED and the voltage reference being driven by the CD74HC132. It worked very well with a supply voltage from 2.3V up to 6V (the IC's limit). But since it also dimmed the LED below 2.3V, I gave-up and are now using the much simpler R4 instead, to current-limit the LED, and stay with a 3V battery. The dimming down to 2V isn't much. :-\

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Ante,
Yea, I actually had to try out different resistors (tweaking) and exercise my solder-sucker for a change.
It was because my LED current source didn't work as designed, since it was operating with only a 2V supply while powering the 1.8V LED, and didn't have much voltage remaining to control itself. The transistor wasn't saturated but refused to work properly when its collector voltage became less than its base voltage.

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