Electronic Circuits · EEE Project · 251

Smart Gardening
BJT Control System

A solar-powered automatic plant watering circuit using a Bipolar Junction Transistor to detect soil moisture and control a water pump — no microcontroller needed.

BJT (QbreakN) D1N4148 Diode Relay Switch Solar 3.7V LTspice Verified
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Interactive 3D Model

Circuit Components

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3D CIRCUIT MODEL
Smart Gardening — Full circuit layout
Solar Panel
Battery
BJT
Relay
Motor
Diode
Resistor

Live Simulation

System Dashboard

Toggle soil moisture to watch the BJT circuit respond in real time. All values match LTspice simulation results.

Solar Input
3.70V
// charging battery
Battery
87%
// 3.7V nominal
Collector Vₓ
0.09V
// BJT conducting

Soil Moisture Sensor

WET — MOTOR OFF
72% moisture R ≈ 0.28 MΩ
Toggle: Wet soil — affects BJT base voltage

Signal Flow

SOLAR → BATTERY → SOIL → BJT BASE → RELAY → MOTOR

// Wet soil → low resistance → base gets voltage → BJT ON → relay energised → motor OFF

Simulation Controls + Results

0.28 MΩ
StateVᴮVᴄBJTMotor
Wet soil0.82 V0.09 VOn — conductingOFF ✓
Dry soil0.65 V3.9 VOff — openON — watering
Current state: Wet soil → high Vᴮ (0.82 V) → BJT conducts → relay energised → motor OFF

Voltage Profile — Dry → Wet

Bill of Materials

#01
BJT QbreakN
#02
Relay switch
#03
D1N4148 Diode
#04
Breadboard
#05
3.7V Battery
#06
DC Motor
#07
Solar Panel
#08
R1 = 2000 Ω

Project Background

Why Smart Gardening?

Problem: Dhaka city residents face severe time constraints, making regular plant care difficult. Plants require consistent moisture, yet manual watering is often missed.

Solution: A BJT-based soil moisture sensor circuit powered by a solar panel automatically controls a water pump. When soil is dry, soil resistance → ∞, base voltage → 0 V, BJT cuts off, relay de-energises, motor turns ON and waters the plant. When soil is wet, resistance drops, base gets voltage, BJT conducts, relay energises, motor turns OFF.

Real-Life Implication: Grid electricity is unreliable in Bangladesh, making the solar panel critical. This system ensures plants are watered autonomously even during power outages — protecting both gardens and the environment. There is no Planet B.