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
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.
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
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.
Then, put it down on cardboard (desktop calendar), and then cut to the desired size, as Figure 4.
We test them it works so well we are very happy.
As Figure 10 and you can watch the video we test them below.