Simple solar plant watering alarm

This is a simple Solar plant watering alarm circuit. It will sound alarms when the soil is dry. The trees have died. It the solar sell is 6V DC power supply. Thus saving convenience and does not require batteries.No noise at night.

How the circuit works
As circuit diagram in Figure 1


Figure 1 the circuit diagram of the Solar plant watering alarm.

The main principle is that when the soil is dry, with high resistance. However, when the soil is wet with minimal resistance.

Usually the wet soil will have resistance is about 6K the electricity current will flow to the probes to base of transistor Q1-2N3904 ,makes it works conducting current from the positive voltage to CE to ground.

Thus, the transistor Q2-2N2222 not has a bias current to base its, so Q2 not works no sound from a buzzer.

solar plant watering alarm circuit
Hand draw use AAx3 batteries 4.5V

But when dry soil, high resistance between probes so there aren’t the current flow to base of Q1 so it not conducting. Thus the current from the positive flow to R1-10K to base of Q2 it works conducting current to Buzzer sound.

LED1,LED is connected in series to display power on. R2 is used to limited current to LEDs

Parts will you need
Q1: 2N3904 40V 200mA NPN transistors
Q2: 2N2222 40V 500mA NPN transistors
LED1,LED2: LED 3mm as you like
R1: 10K 0.25W resistors
R2: 100 ohms 0.25W resistors
VR1: 50K Potentiometer
BZ1: 6V Buzzer
B1: 4.5V (1.5V AA x 3 battery)

How to assemble circuit
This projects we use cardboard instead PCB because it is easy and economical. First of all, draw the component and wire layout on paper thin as Figure 2 and Figure 3 the complete component layout.

draw the component and wire layout on paper thin
Figure 2 draw component layout on paper

the complete component layout
Figure 3 the complete component layout.


Then, put it down on cardboard (desktop calendar), and then cut to the desired size, as Figure 4.

cut to the desired size
Figure 4

After than, Drill the component pins by nail as Figure 5
Drill the component pins by nail
Figure 5

Next, put the parts onto the cardboard as component layout. As Figure 6
put the parts onto the cardboard

wiring the leg of all parts
Then, Wiring the leg of all parts by soldering iron as Figure 7

install the 6V solar cell on the cardboard
Next, install the 6V solar cell on the cardboard as Figure 8

we created Simple solar plant watering alarm project successfully
Finally, we created this project successfully as Figure 9

We test them it works so well we are very happy.
As Figure 10 and you can watch the video we test them below.

we test them it works very well
Figure 10

If you want to build the PCB for this project. My son design it below.
Component layout

Actual-size of Single-sided Copper PCB layout

Get Ebook: Simple Electronics Vol-03

Get Updates via Email

We always try to make Electronics Learning Easy.

10 thoughts on “Simple solar plant watering alarm”

  1. love you site,

    could you draw a circuit including your plant sensor to open a 12 volt water solenoid and turned off when the plants and sensor was wet.

    when the plants in my mini hot house need water can the sensor tell the power to open the water solenoid
    the misting sprinklers spray the plants till the sensor is damp and it stops.
    if you can do this can you email me regards Vin thanks


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.