Build a 3.7V 18650 Battery Level Indicator using BC547 Transistors

Lithium-ion 18650 batteries are widely used in DIY projects, power banks, and portable electronics. But to ensure safety and efficiency, it’s important to monitor their voltage levels. In this blog, we’ll walk you through building a simple battery level indicator using BC547 transistors — no microcontroller required!

Components Required:

• BC547 Transistor
• 1k ohm
• 220 ohm
• 1N4007 Diode
• 18650 Battery
• Connector
• Red and Green led
• Battery Holder

Gerber file

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Circuit Diagram

All About the BC547 Transistor

The BC547 is one of the most popular and versatile NPN bipolar junction transistors (BJT) available to electronics hobbyists and professionals alike. Whether you’re building amplifiers, switches, or basic electronic projects, the BC547 is often a go-to choice due to its availability and performance.

Key Specifications of BC547

• Type: NPN Bipolar Junction Transistor
• Collector-Emitter Voltage (Vce max): 45V
• Collector-Base Voltage (Vcb max): 50V
• Emitter-Base Voltage (Veb max): 6V
• Collector Current (Ic max): 100mA
• DC Current Gain (hFE): 110-800
• Transition Frequency (ft): 150 MHz
• Package Type: TO-92

Pin Configuration

• Pin 1 (Collector): Connects to the load
• Pin 2 (Base): Input control signal
• Pin 3 (Emitter): Connects to ground or negative terminal

How Does BC547 Work?

The BC547 is an NPN transistor, which means it turns on (conducts current) when a small current flows into its base pin. When the base-emitter voltage (Vbe) exceeds approximately 0.7V, the transistor allows a much larger current to flow from the collector to the emitter.

Modes of Operation

1. Cutoff Region:
   • Vbe < 0.7V
   • Transistor is OFF, no current flows from collector to emitter.
2. Active Region:
   • Vbe > 0.7V but not saturated
   • Transistor amplifies current.
3. Saturation Region:
   • Vbe > 0.7V and Vce is low
   • Transistor is fully ON, acts as a closed switch.

Applications of BC547

• Switching circuits: Like LED drivers and battery level indicators
• Amplifier circuits: Small signal amplification
• Oscillator circuits: For signal generation
• Pulse-width modulation (PWM) control: In motor drivers

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