Plant humidity monitor

This little circuit monitor the humidity of plant soil and should be a welcome help for forgetful readers who occasionally find their plants on the window sill suffering from acute drought.

The plant humidity sensor consists of a handful of inexpensive parts and gives a reliable Indication when the soil is too dry. Circuit IC1 arranged an oscillator that supplies two complementary switching signals. Q and Q\, at a frequency of about 58 Hz. Theoretically, the Q and Q\ signals are never active at the same time and thus form a perfect alternating current source that prevents electrolysis n electrodes A and B.

The potential at the wiper of P1 is a function the electrode resistance (RA-B), and thus, spends on the soil humidity. By comparing this potential with a reference. IC2 indicates whether the soil humidity falls below a certain level when the plant needs watering. The reference is derived from the symmetrical electrode current. and is taken from junction R2-R3, Which is at a constant potential of 2.5 V with respect to ground.

The operation of the comparator is straightforward in spite of the alternating electrode current. A simple d.c. voltage comparison is, therefore, out of the question. Assuming that Q=0 and Q/=+5 V, the wiper of P1 is at a potential U1 with respect to ground.

Consequently. the wiper will be at 5-U1 when the electrode current is reversed (Q=+5 V; Q\=0 V). If in the first situation (Q=0 V and Q \=+5 V), the wiper potential is, say, higher than the reference, it follows that it is lower than the reference in the other situation. To make sure that the comparator output is actuated in both cases, the input signals are swapped by the Q and Q\ outputs of IC1. This is achieved with the aid of the four electronic switches contained in IC3.

The red LED, D2, lights when the soil is too dry. This happens when U1 is greater than 2.5 V. Depending on the setting of preset P1, this corresponds to a soil (inter-electrode) resistance of 0-1.82 kΩ. The further P1 is turned towards the electrode connection, the higher the soil resistance (i.e., the drier the soil) required for the green LED to go out and the red LED to light.

Interestingly, the capacitance of the soil may cause the red and the green LED to light at the same time, which gives a useful indication of ‘between wet and dry.

Plant humidity monitor Schematic diagram

Plant humidity monitor Schematic diagram

The electrodes are best made from the carbon rods salvaged from used batteries. This is an inexpensive solution, which also prevents corrosion. The electrodes are pushed into the plant soil about 4 cm apart. The exact adjustment of P1 will depend on the type of plant to be monitored and has to be established empirically. In most cases, however, good results will be obtained with the P1 set to mid-travel.

Since the sensor requires a stable 5 V supply. power is derived from an inexpensive mains adaptor whose direct output voltage is ‘cleaned’ and stabilized with the aid of a 7805 regulator. As the sensor draws a current of only about 5 mA, the supply shown here may be used to power a number of sensors.

In addition to providing the supply voltage to the sensor units, the power supply also functions as a central (remote) indicator. LED D1 lights when one of the sensor units connect-ed reports ‘dry soil’. If none of the sensors reports ‘dry soil’, but at least one of them between dry and wet’, LED D 1 lights at reduced intensity, since the voltage applied to the LED control input is then between 2 V and 3 V. Clearly, this means that all sensor outputs must be connected to the LED control In. put on the supply (wired-OR configuration).

Parts list  (Monitor circuit)
R1 = 100 kΩ
R2, R3 = 15 kΩ
R4 = 820 Ω
R5, R6 = 680 Ω
R7 = 22 Q
P1 = 11kΩ preset
C1 = 39 nF
C2 = 100 nF
C3 = 10 μF, 16 V
D1 = LG3369EH . (3 mm, low current green)
D2 = LS3369EH (3 mm, low current red)
D3 = 1N4148
Integrated circuits:
IC1 = 4047
IC2 = TLC271
1C3 = 4066
Enclosure 65x50x30 mm, e.g., Bopla EG406
PCB Ref. 934031
Parts list: (Power supply circuit)
R1, R2 = 10 kΩ
R3 = 220 Ω
C1 = 100 μF, 25 V. radial
C2, C3 = 100 nF
C4 = 10 μF. 16 V. radial
D1 = 1N4001
D2 = LED. 5 mm, red
T1 = BC547B
Integrated circuits:
IC1 = 7805
Enclosure 65x50x30 rum, e.g.. Bopla EG406
PCB Ref. 934032
* Siemens (in UK: ElectroValue at 0784 442253
Phoenix Mecano at (0296) 398853
Available from Elektor Electronics


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