555 timer ic

Automatic Plant Irrigation System

This project, known as the Automatic Plant Irrigation System, offers an automated solution for watering plants, eliminating the need for human intervention. Often, when people go on vacation or become forgetful, their gardens go unwatered, putting their plants at risk of damage. This project serves as an excellent remedy for such situations.


  • Block Diagram of Automatic Plant Irrigation System:
    • Main Components in Automatic Plant Irrigation System:
  • Circuit Diagram of Automatic Plant Irrigation System:

Block Diagram of Automatic Plant Irrigation System:

Block Diagram of Electronic Plant Watering System


  • The circuit is relatively straightforward in its design. It capitalizes on the principle that soil exhibits high resistance when dry and low resistance when wet. We harness this concept to create a functional system.

  • Two probes are carefully inserted into the soil, designed to conduct electricity when the soil is wet and remain non-conductive when it’s dry. Consequently, if the probes don’t conduct, the system automatically detects this state using a HEX inverter, which switches to a high state when the input is low.

  • The HEX inverter then triggers the NE555 Timer, which subsequently activates another NE555 connected to the output of the first NE555. This second NE555 is configured as an astable multivibrator and plays a role in enabling the Electric valve, permitting water to flow into the soil.

  • When water saturates the soil, the probes conduct again, causing the output of the 7404 to drop, turning the first NE555 and the rest of the circuit to a low state. Consequently, the valve is automatically shut off.

Main Components in Automatic Plant Irrigation System:

Hex Inverter 7404: The primary function of this inverter is to generate an output that is the inverse of its input, meaning it produces a signal that is the opposite polarity of the input signal. For instance, when the input to the inverter is low, it produces a high output, and vice versa. This behavior is similar to that of a standard inverter, which yields a high output when the input is low and a low output when the input is high. The 7404 IC contains six individual inverters and operates within a supply voltage range of 4.75V to 5.5V, with a typical supply voltage of 5V. These inverters find applications in various circuits, including inverting buffers, drivers, and hex inverters, among others. The 7404 IC is available in multiple packaging options, such as DIP (dual inline package), QFP (quad flat package), and others.

Inverter Pin Configuration

Circuit Diagram of Automatic Plant Irrigation System:

Automatic Plant Irrigation System

Circuit Explanation:

  • We all understand that plants can perish if the soil lacks adequate moisture. The soil’s electrical properties offer a simple yet effective method to water plants and operate the circuit.

  • To achieve this, we insert two probes into the soil, connecting them to the circuit. These probes only conduct electricity when the soil is wet (exhibiting low resistance) and remain non-conductive when the soil is dry (indicating high resistance). The battery connected to the circuit supplies the voltage required for the probes to conduct.

  • When the soil is dry, it results in a significant voltage drop due to its high resistance. This condition is detected by the 7404 hex inverter, triggering the first NE555 timer. This timer is configured as a monostable multivibrator, using an electrical signal to achieve this.

  • Upon activation of the first NE555 timer at pin 2, it generates an output at pin 3, which is then fed into the input of the second NE555 timer. The second 555 timer operates as an astable multivibrator and is triggered by the first 555 timer. It produces an output signal that controls a relay connected to an electrically operated valve via the SK100 transistor. If the SK100 transistor generates excessive heat, a heat sink can be employed.

  • The output of the second NE555 timer activates the SK100 transistor, subsequently controlling the relay. This relay facilitates the delivery of water to the plant pots through a pipe, connected to both the input and output of the valve.

  • When the transistor activates the relay, the valve opens, allowing water to flow into the plant pot. As the soil’s moisture level increases, its resistance decreases, allowing the probes to conduct. This, in turn, signals the 7404 inverter to stop triggering the first 555 timer. Eventually, the electrical valve linked to the relay is shut off. If there’s a need to adjust probe conduction, we can modify the valve using a variable resistor (R5) and a capacitor (C1).

  • The 0.01uf capacitor C5 is employed to ground the CV pin of the second NE555 timer. C3 serves to filter out AC noise, allowing only DC to pass through the remaining circuit. C4 and R3 work together to configure the NE555 as an astable multivibrator.

Values of the Components in the Circuit:

  • Capacitor (C4) = 10u 16V.
  • Capacitor (C5) = 0.01u.
  • Resistor (R3) = 27K
  • Resistor (R4) = 27K
  • Diode (D1 and D2) = IN4148
  • Relay = 6V, 150 ohms


  • Battery should be continuously monitored from power outage or simply you can use 9V DC supply adaptor.
  • Probes must be inserted into the soil. They should not be kept on the soil.
  • Electric valve should be used for best result.



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