High-Accuracy and Low-Cost Sensor Modulefor Hydroponic Culture System
Tomohiro Nishimura†, Yuji Okuyama‡, and Akashi Satoh††Department of Communication Engineering and Informatics
The University of Electro-Communications, Chofu, Tokyo, Japan‡TROCHE Co., Ltd, Yokohama, Kanagawa, Japan
Abstract— A high accuracy sensor module for hydroponicculture system was developed. The module measures water level,temperature, and nutrient concentration values by using a single
device equipped with a low-cost ribbon cable and electrodes.
The module has two oscillators and the water level and theconcentration value are converted into frequencies of theoscillator signals. After the conversion mechanism andadjustment scheme are explained, the high accuracy of thesensor module is demonstrated.
Keywords—smart agriculture, hydroponic culture, Arduino,sensor module

I. INTRODUCTION
IoT (Internet of Things) technology is expanding rapidly,
and agriculture is one of promising applications where sensor
and wireless communication devices are used to monitor andcontrol cultivation environment [1][2]. However, the high costof the devices and cultivation systems only allows thistechnology to be used in a large-scale plant factory managed
by a big company or conducted as a government project[3][4][5]. However, a variety of microcomputer platforms,
such as Arduino [6] and RaspberryPi [7], and sensor devices
that can be applied to plant cultivation, are being introduced
to a consumer market recently.
We have developed a hydroponic culture system poweredby photovoltaic generation, and cultivate tomatoes as shownin Fig. 1 [8]. The system is equipped with a prototype wirelesssensor module using Arduino, supporting remote monitoringof temperature, humidity, illuminance, and water level data. Inorder to put the system into production, we developed a newsensor device that supports monitoring of EC (ElectroConductivity) for nutrient solution concentration, watertemperature, and water level using a single module. While themodule is very simple and low cost, it achieved high accuracy
measurement. In this paper, the structure and principle of thesensor module are described, and its characteristics aredemonstrated.

II. NEW SENSOR MODULE
Fig. 2 shows the sensor module stacked on Arduino, andFig. 3 is its circuit diagram. A low-cost ribbon cable is used tomeasure the water level, and electrodes for EC and atemperature sensor TI TMP20 coated by resin are connected
at the end of the cable. Capacitance of the cable changes
according to the water level in a tank, and electrical resistance
between the electrodes changes with the nutrient solution
concentration. The changes of capacitance and resistance aredetected as frequencies of oscillators connected to the ribbon
cable. Then the frequencies are converted into the water level
and the EC value. Details are described in following sections.
In the measurement of the frequency, a 16-bittimer/counter of the microprocessor ATMega328P built in
Arduino is used. However, the processor has only one
timer/counter, and thus its input is alternately used to measure
the EC value and the water level. AC current is used tomeasure the EC value because electrodes are electrolyzed if
DC current flows continuously in one direction.
III. EC SENSOR
Fig. 4 shows a three-inverter oscillator used formeasurements of the EC, where r is a resistance between theelectrodes, which changes according to the concentration ofFig. 2. Developed sensor moduleaFig. 1. Hydroponic culture system with wireless sensor module

Fig. 3. Circuit diagram of sensor modulethe nutrient solution. The oscillating frequency 𝑓𝐸𝐶 of the EC
sensor is defined as Equation (1).
𝑓𝐸𝐶 =

1
[Hz]
2.2𝐶2 (𝑟 + 𝑅2 )

C2
rR2

1
1− 𝑅2
2.2𝐶2 𝑓𝐸𝐶

[𝑆/𝑐𝑚](2)(1)

Then EC value is obtained by Equation (2) as an inverse of theresistance.

R1

𝐸𝐶 =Fig. 4. Three-inverter oscillator

It is said that typical EC value for hydroponic culture is
between 1.0 – 2.5 mS/cm. In order to minimize variation of error
between the EC values from Equation (2) and the sensor module
around this range, the circuit parameters are determined as
C2=3.25F and R2=735. Then, operating frequency of thesensor was measured by changing EC of water by mixing salt.
The EC value of the salt water was measured by the EC metershown in Fig. 5 and Table I. The measured values accord wellwith the calculated values as shown in Fig. 6 and Table II.
However, the error rates around 1.5 – 2.0 mS/cm is slightly
higher than the others in Table II. In the measurements, theelectrodes are dangled in a water container about 3-cm abovefrom the bottom. The gap between the electrodes is 10 mm and2 – 3 cm area around the electrodes affects the EC values asshown in Fig. 7. Therefore, small displacement of the electrodes
causes the increase of the error rate.
IV. WATER LEVEL SENSOR

Fig. 5. EC meter MD-TDS-EC
TABLE I.

FEATURE OF MD-TDS-EC

Measuring range
Accuracy
Operating temperature0 – 9.990 mS/cm±2.0%
0.1 – 80.0 °C

The Schmitt-trigger relaxation oscillator shown in Fig. 8
is used for the water level sensor. 𝐶 is the capacitance between
two adjacent wires of the ribbon cable, and changes according
to the substance around the cable such as air or water. The
capacitance is grounded, and thus the GND line is shared witha temperature sensor attached to the electrodes to reduceresources. The ribbon cable was soaked into saline solutions,and oscillation frequency was measured by changing waterlevel and EC value. As shown in Fig. 9, not only the waterlevel, but the EC value also changes the frequency because itaffects the capacitance C. The capacitance C of the cable isdefined as Equation (3), where 𝑓WL is the oscillating
frequency of the water level sensor.
𝐶≅1(𝐹)
𝑓WL 𝑅4(3)

Fig. 10 shows the capacitance of the cable calculated byEquation (3). The capacitance is linear with respect to thewater level for each EC value, and thus the water level can beeasily calculated from the capacitance.

Fig. 6. EC vs. oscillating frequency
ERROR RATE OF EC VALUES

Calculated(mS/cm)
0.75
1.10
1.46
1.91
2.57
3.00

R4Error rate (%)

R3

0.082
0.381
1.355
1.885
0.044
0.006
Electrodes

C34.5

Water
Fig. 7. The area which can measure EC

EC=0.17EC=1.13EC=3.00
EC=5.00
EC=10.0

4
3.5
3
2.5
2
1.5

Measured area

R5

Fig. 8. Schmitt-trigger relaxation oscillator

Frequency fWL (kHz)TABLE II.

Measured(mS/cm)
0.75
1.096
1.482
1.942
2.564
3.002

Fig. 11 shows an equivalent capacitance model of thecable. 𝐶𝑣 is the capacitance inside of coating determined by020406080

Water level (cm])
Fig. 9. Oscillation Frequency vs. water level100length L of the cable. The capacitance 𝐶𝑣 which is outside of
the coating changes according to the water level x. When 𝐶𝑣
changes to 𝐶𝑣′ , then the total capacitance of the cable 𝐶(𝑥) isdefined as Equation (4).
𝐿𝐶𝑐 𝐶𝑣′
𝐶𝑐 𝐶𝑣
𝑑𝑥 + ∫𝑑𝑥′
0 𝐶𝑐 +𝐶𝑣
𝑥 𝐶𝑐 +𝐶𝑣
𝐶𝑐 𝐶𝑣′𝐶𝑐 𝐶𝑣(𝐿 − 𝑥)=𝑥+′
𝐶𝑐 +𝐶𝑣𝐶𝑐 +𝐶𝑣′𝐶𝑐 𝐶𝑣𝐶𝑐 𝐶𝑣=(−)x′𝐶𝑐 +𝐶𝑣 𝐶𝑐 +𝐶𝑣
𝐶𝑐 𝐶𝑣+𝐿 (F) (4)𝐶𝑐 +𝐶𝑣𝑥𝐶(𝑥) = ∫Here,𝐶𝑐 𝐶𝑣
𝐶𝑐 +𝐶𝑣

𝐿 is the capacitance of the cable when the waterlevel is 0. Then, Equation (4) becomes as follows.
𝐶(𝑥) = (𝐶𝑐 𝐶𝑣′
𝐶𝑐 𝐶𝑣−) 𝑥 + 𝐶(0)′𝐶𝑐 +𝐶𝑣 𝐶𝑐 +𝐶𝑣

V. CONCLUSION
We developed a sensor module for a hydroponic culture
system, which can measure water level, EC, and watertemperature values with a single device using a low-costribbon cable. The water level and EC values are converted
from oscillation frequencies related to the capacitance andresistance of the device using the cable ribbon. The conversion
method with approximation calculation were described and its
high accuracy was demonstrated. However, it was found that
there is a need to review the structure of the electrode formeasuring EC.

EC=0.17EC=1.13EC=3.00
EC=5.00
EC=10.0

Capacitance (pF)

654
32020

We set the parameters 𝛼 = 9.81, 𝛽 = 0.0981, 𝛾 = 33.6
to minimize errors between measured and calculated
capacitances. As shown in Fig. 12 and Table III, the measured
capacitances are in agreement with theoretical values, andsmall error rates less than 1 % were achieved.

(5)

87𝐶𝑣′ changes with EC, but when EC is constant, 𝐶(𝑥) is linear tox. 𝐶(0) is not affected by EC because it is the value of thecable in the air. Therefore, the water level can be calculated
from the capacitance of the cable. When EC is increased, thecable capacitance is saturated as shown in Fig. 10. Therefore,
the slope of 𝐶(𝑥) can be calculated from two points on
𝐶(𝑥), for example, 𝐶(90) and 𝐶(0) . Here, we assumed
relation between EC and the capacitance as 𝐶(𝑒), where e isthe EC value, 𝐶𝑠𝑎𝑡 is the saturated capacitance and 𝛼, 𝛽, 𝛾 areparameters.
𝛼𝐶(𝑒) = (1 −) 𝐶 [𝐹](6)𝛽 ∙ 𝑒 + 𝛾 𝑠𝑎𝑡

40

60

80

100

Water level (cm)Fig. 10. Capacitance vs. water level

Fig. 11. Equivalent circuit of ribbon cable

The sensor module is being tested in the new hydroponic
culture system of Fig. 13 that is 8-times larger than previous
one of Fig. 1 for actual application and commercialization. Inaddition to tomato we cultivate various plants such as melon,zucchini, pumpkin, eggplant and herbs, all at the same time.
We are also developing a raising seedling system as shown inFig. 14, where the sensor module is used. Not only the three
sensor values, but more parameters of cultivation environment
and wireless communication will be supported by the module
to introduce the systems into various locations in city area.
Our target is not to raise production efficiency of a single
vegetable in a large plant factory, but to provide fun ofcultivation with a variety of plants to users as a service
business. In addition to the hardware development, we aregoing to develop remote management software on mobile
terminal and communication tools for the user to expand new
agriculture for fun.

Fig. 12. Capacitance vs. EC
TABLE III.

EC (mS/cm)
0.17
1.13
3.00
5.00
10.00

ERROR RATE OF WATER LEVE

Calculated
value (pF)
5.73
6.63
6.90
6.98
7.05

Measuredvalue (pF)
5.73
6.63
6.89
7.03
7.12

Error rate (%)
1.6310-5
7.7310-5
0.115
0.691
0.960

Fig. 13. New hydroponic cultureFig. 14. Raising seedling systems

REFERENCES
[1]

[2]
[3]

[4]

[5]

[6]
[7]
[8]

W. S. Lee, "Sensing technologies for precision specialty crop
production," Computers and Electronics in Agriculture, Elsevier, vol.
4, No.1, pp. 2–33, 2010.
T. Fukatsu, et al., “Field Monitoring Using Sensor-Nodes with a Web
Server, J. Robotics and Mechatronics, vol. 17 No.2, pp.164-172, 2004.
“ICT Case Study in Agriculture” (in Japanese)
http://www.soumu.go.jp/johotsusintokei/whitepaper/ja/h26/html/nc142320.html, 2014.
“About the AI agriculture initiatives” (in Japanese)
http://www.maff.go.jp/j/shokusan/sosyutu/sosyutu/aisystem/pdf/ai_tor
ikumi.pdf, 2014.
“Where the ICT agriculture is and where it is going” (in Japanese)
http://www.maff.go.jp/j/shokusan/sosyutu/sosyutu/aisystem/pdf/ict_ai.pdf, 2015.
“Arduino - Home,” https://www.arduino.cc/“Raspberry Pi - Teach, Learn, and Make with Raspberry Pi,”
https://www.raspberrypi.org/T. Nishimura, et. al, “Development of hydroponic cultivation systemusing a microcomputer control”, Proc. DICOMO 2015, pp.230-242,2015. (in Japanese)