Question on the Plants Watering Watcher circuit

[MP]

This is what I suggested you do in reply #8. We look forward to your circuit post.

MP

[Ante]

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
   

[audioguru]

Thanks, Guys.
Another improvement:
A bi-colour LED that flashes Green when the soil is not too dry, and flashes quickly Red when the soil is very dry.

[Ante]

audioguru,

Why not! And it

[audioguru]

Ante,
Ha, Ha, and S-O-S to you too.

[audioguru]

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.

[audioguru]

To All,
I built my slightly modified project and it works very well.  
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:    

[audioguru]

Wet soil, but not soaked:

[audioguru]

Dry soil, this plant needs a drink, NOW:

[audioguru]

My latest schematic:

[audioguru]

Now all I have to do is change R4 into a regulated current source, so that the LED's brightness doesn't reduce when the battery's voltage runs down.

[Ante]

audioguru,

You have been busy; I

[audioguru]

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?

[Ante]

audioguru,

Yes the boards look good, are they as strong as the blue once? And is the copper