Please read the description/textbox first. I show the principles of a 1 transistor (read: Field effect transistor, FET) moisture indicator where a red LED (could also be a green LED) lights up when the soil gets too dry.
Much more info in the video. The circuit, as it is, is surely a good instrument/indicator, but I want to make another video where you can align perhaps better (via a potentiometer) for different soil-moisture situations. So e.g. when the soil is moderate moist-the LED lights up. Now, with this circuit, you (the gardener) has to see/use it in practice when the LED lights up, given a certain amount of moisture in the soil. Could be very easy to find your way and thus interpreting the amount of light that the LED shows related to the moisture content of the soil, by the way, that is also the reason why I publish the circuit. You can easily get aquainted.
Other option: us a Micro Ampere meter (100 uA-500 uA) instead of the LED and read the scale to see how moist the soil is. Mount a potentiometer of 1 K or 500 Ohm parallel to that (say) 500 uA Micro Ampère meter, over the (+) and (-) and use the tap of the potmeter (and the - or + electrode of the uA meter) to align to certain moisture levels in practice, in hobby gardening or professional gardening.
Corrections to the video and/or important info:
1) On approx. 1.55 I say that "when the Gate of the FET is clamped to ground (negative) there is no current flowing in the Drain-Source circuit", but of course there is some current, though tiny (300 Micro Ampère)
2) The resistor in the Drain lead is not 190 Ohm, but half of that, thus (say) 100 Ohm, because I paralleled two resistors of 190 Ohm (!)
3) On 3.33 I say “the LED is clamped to ground, negative” but of course I mean "the Gate of the N-FET is clamped to ground, negative", as long as the soil is wet or moist.
4) The test/moisture electrodes must be made of the same material, otherwise they will give a potential difference in the wet soil, that deteriorates the functioning and can give a false reading. Copper is easy and cheap. With steel you can get unexpected potential effects due to corrosion. Iron oxide has another potential compared to pure iron in a wet/moist environment.
5) The LED must be of a normal type (watch the video). So no white LED, no “power” LED, only a normal small RED or perhaps a green LED will work. Has to do with the proper barrier voltage of a LED to make this electronic circuit work. Red and Green LEDS can work in the 3 Volt range that is used here.
Furthermore: everything is explained in the video.
The schematic is clear/OK and works. Of course: when you want to make this in a proper way for your garden: all the electronics must be mounted in a water/moisture tight box, only 2 electrodes of the same metal (copper) must stick out of that box, no rain/water may get into the electronic circuit.
If so (wet or moist), the whole electronic circuit gets useless, also because such a FET is extremely sensitive on its Gate, so the slightest resistance/voltage on the Gate (of whatever cause) can have an effect on the LED lighting up or not or can give a false reading.
Perhaps an idea to pot the circuit in Polyester resin(?). Not tested (!).
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I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 28 May 2022.