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soil moisture tester circuit schematic

Soil Moisture Tester Circuit

For those of us who don’t like to get their hands dirty, this simple soil moisture tester quickly checks the state of their plants and how much attention they need. Recommended for all plant owners!

Plants seem to have something in common with pets. They are usually acquired or given with the best intentions, but not everybody seems to be able to look after them properly. Of course we do not expect everybody to have green fingers, but when plants are not watered enough they simply die. In any case, too much neglect usually has fatal consequences. Cactuses seem to survive such a careless treatment the longest and we have to admit that these are the only plants that manage to survive at our offices.

So what can we do about it? It’s simple really. All it needs is for regular checks to feel if the soil in the pot has become too dry. But what is ‘too dry’? Some people just don’t seem to have the right fingers for this task. A little electronics can be used to rid us of this problem forever.

The circuit described here might be very simple, but it’s a very useful soil moisture tester. Two electrodes are stuck in the soil and the moisture level is shown on an LED display. The LEDs have been arranged into three colours: green LEDs indicate that the soil is damp, yellow LEDs that it’s getting a bit dry and red LEDs warn that immediate action is required!

Soil moisture tester circuit diagram

soil moisture tester circuit schematic

A quick look at Figure 2 is enough to ascertain that the full circuit is barely more complex than the block diagram. It’s only really the supply that is extra. Even this is very simple, consisting of only a small mains transformer rated at 6 V/200 mA, a single rectifier and smoothing capacitor (D1/C1) and a voltage regulator which provides a stable +5 V. The AC supply fed to the electrodes is obtained in a very simple manner: by taking it from the supply just before the rectifier.

The preset used to set the sensitivity can be found as P1. D2, R1 and C3 rectify the moisture dependent AC signal, which is then fed to pin 5 of IC2, the heart of the circuit. This IC used here is an old favourite, the LM3914 bargraph display driver. This 18-pin IC converts an analogue input to drive a 10-LED (linear) display. The IC contains 10 comparators, which each are connected to a reference voltage via a precision resistor network. The inverting inputs of the comparators are connected to the analogue input via a buffer stage. The LEDs are driven directly by the comparator outputs.

Pin 9 is used to set the display to bar-mode or dot-mode. In the first case JP1 should be shorted, in the second it can be left open circuit. Obviously the dot-mode gives the least current consumption of the IC. Potential divider R2/R3 sets the reference voltage. The total value of both resistors also determines the brightness of the LEDs.

Moisture tester construction

Due to the small number of components used, it’s unlikely that the construction of the tester will give you sleepless nights. Start the construction with the lowest components (resistors); that tends to be easiest. You should preferably use a socket for IC2. Take care that you get the polarity right for the diodes, electrolytic capacitors (C1, C2 and C3) and the LEDs (short leg = cathode).

The small mains transformer (Tr1) is mounted onto the PCB last. Make sure that you use a sound and well isolated cable (with a strain-relief) between the mains and the primary of Tr1. Carefully check the finished PCB before applying mains power and never work on the circuit when it’s plugged into the mains!

Moister measure PCB layout

The circuit should be mounted in a safe plastic case, with a label stuck on the bottom, stating the mains voltage and the value of the fuse. A pair of sockets for banana plugs is mounted on the case for the connection to the electrodes.

The electrodes are made from two lengths of stiff, isolated copper wire, about 10 cm long and 1 mm thick. 4 cm of insulation is removed from the ends, which are then tinned. This is to prevent the copper wire from oxidising. The connection between the electrodes and the circuit could be made with two lengths of flexible stranded cable.

Moisture tester calibration and usage

Once the supply has been switched on and the electrodes have been connected, the tester is as good as ready for use. But first preset P1 needs to be adjusted. All you need for this is a glass of tap water. The electrodes are inserted into the glass of water and should be kept between 1 and 2 cm apart. This corresponds to the maximum moisture level, so we have to adjust P1 until the top green LED (D3) just lights up and D4 just extinguishes. When the electrodes are removed from the water, you should see one of the red LEDs (D10, D11, D12) light up.

Since this absolute maximum level will not occur very often, the tester could be calibrated more practically. A pot plant should be watered liberally, after which the electrodes are inserted into the soil, again keeping them between one and two cm apart. P1 is then adjusted until one of the green LEDs lights up. You will probably find many opportunities to check that one of the three red LEDs lights up when testing a plant that hasn’t been watered for three weeks. And that’s it!

After the previous description it should be clear how the tester should be used. The electrodes should always be kept the same distance apart (between one and two cm), perhaps using a spacer, and the tinned ends should always be completely inserted into the soil.

COMPONENTS LIST
Resistors:
R1 = 100kΩ
R2 = 2kΩ7
R3 = 2kΩ2
P1 = 500kΩ preset H
Capacitors:
C1 = 470μF 16V radial
C2,C3 = 10μF 16V radial
C4 = 100nF
Semiconductors:
D1 = 1N4001
D2 = 1N4148
D3,D4,D5 =LED, green
D6-D9 = LED, yellow
D10,D11,D12 = LED, red
IC1 = LM7805
IC2 = LM3914-N
Miscellaneous:
Tr1 = mains transformer, secondary 6 V 200mA ( e.g., Monacor/Monarch VTR-1106)
F1 =fuse, 200 mA, with PCB mount holder
JP1 = short-circuiting jumper
K1 = 2-way PCB terminal block, lead pitch 7.5mm

34 Comments

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  • Abby

    On the white LED lihgts you’re fine. In a lot of cars, LED brake lihgts have been commonly used for the past five or so years. Now I am beginning to see LED white lihgts for the license plate area.There are some companies out there that are making replacement LED bulbs that fit the same bayonet type application of your OE style incandescent lihgts. So long as the luminosity of the bulb is matched, there shouldn’t be a problem.As far as the blue dome light is concerned this may require some clarification. In the old days, a dome light was the name given to the light fixture that is attached to the headliner of a car. If that is the light you’re referring to, then there is no problem on the color, type, etc. If by dome light you’re referring to an emergency vehicle light (AKA Federal siren / light) then that will fall under guidelines by your state. Some colors are allowed for use by civilians (contractors. first responders, etc.) while others are in violation. In the case of vintage emergency vehicles, these lihgts are allowed for authenticity, but they cannot be used unless the car is in private property, or at an event under the allowance of the local law enforcement authority.

  • estefan2020

    I personally would make a few changes, tr1 and ic1 I would change with a dc to dc up/down converter ( better battery efficiency). I would use surface mount components, leds 2ma versions, and use 3 aaa batteries. The project should fit on the back of the 3 aaa battery holder. The green leds can be eliminated, as if the plant has sufficient water, you would know by no leds and save battery power.

    But that’s my take on the project.

  • Rohit Bhavekar

    Their are 10 LED’s 3* red , 3* green, 3* yellow then which is then 10th one ??
    Upto to how many volt we can supply power to this tiny kit ??
    Is this really works???

    • estefan2020

      The schematic shows 10 led’s, color is your choice. Its designed to use mains 120/220 ac, but if tr1 is removed, 8-15v dc can be used 7805 will convert it to 5v.

  • Sweep3

    I am extremely new to electronics, but am willing to figure things out. That said, I want to build a wood moisture meter and need sharp pins as sensors that will penetrate the wood (to about 3/8″ or so) and a display in percent moisture. I can see the possibility of using leds to display moisture in percentages with 8- 10 steps being enough to cover a range that makes sense. I have seen very old schematics meant for wood, but nothing recent save a few seemingly too simple to really work examples. Would love to hear your thoughts on this.

  • estefan2020

    2 metal wires of same type. though due to the water, I would try copper better conductor of electricity for short term use. If you plan on using this for long term use, then stainless steel to slow down the rusting of the metal.

  • honey

    i have completed the whole project but please let me confirm WHAT TYPE OF ELECTRODES ARE USING AS A SENSOR ..REPLY AS SOON AS POSSIBLE .

    WAITING FOR YOUR REPLY AS SOON AS POSSIBLE ITS URGENT

  • estefan2020

    Sounds cool.
    What would be great, low number of components under 10 perhaps. that could be left in the plant, and a blinking led starts up when dry off when damp. many $5 or 6$. Would be great for any potted plant, low enough to be affordable for each pot. More of an idiot proof pot plant.

  • ajay

    hey. m also doing this type of project in my college with the help of arduino… is this a good idea or not? can we also introduce gsm module and wireless energy transfer in this project …? …. and can we also experiment with light intensity in this project??

    • Fadedrew

      Howdy from Cape Town, South Africa! I’m trying to reilaseA controller with java like the haken continuum. (www.hakenaudio.com)I’m using a wiimote aimed up through a glass sheet and a framearound it beaming IR from LED’s reflecting off a silver foil fingerglove. I’m looking to colloaborate on this since I’m new to java(bout 2 months now) and although very MIDI savvy I’d like tofind out from you how you implemented the bluetooth stackand MIDI. Please email me so I can get more details on yourMIDI implementation like the pitch bend range (maximum 36semitones??)ThanksCampbell

  • SHAH

    What does the grounding mean in this circuit?
    i mean where to connect the grounding symbol?
    New to electronics.. need help

    • estefan2020

      Ground is mostly the negative of the battery. especially in this circuit.

      When dealing with amplifiers that use +v and -v, there will be a virtual ground. simply its like connecting 2 9v batteries in series, and the common will be ground, not the negative of the battery. Though some amplifiers may use 2 identical resistors in series as a voltage divider, then the minus of the battery will be ground.

  • estefan2020

    nice, but a bit complex.

    What would be nice is a simplified design way more simplified. One that would blink one led if attention is required, off when moist. one monitor per plant, a couple of transistors resistors, caps, battery.

    Why spend electricity when no attention os required?

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