Build a 3.7V 18650 Battery Level Indicator using BC547 Transistors

Posted on by

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

Build a 3.7V 18650 Battery Level Indicator using BC547 Transistors
Build a 3.7V 18650 Battery Level Indicator using BC547 Transistors
Build a 3.7V 18650 Battery Level Indicator using BC547 Transistors

Download Gerber File

Download

Special Thanks to Our Sponser – JLCPCB:

No project is complete without the right tool and materials. That’s why our sponser JLCPCB stepped into provide essential material for the project. JLCPCB is a leading provider of high quality printed circuit board and PCB assembling services.

Simply head over to JLCPCB, Upload your gerber file, select specification and just place your order.

48-Hour Turnaround for 6 Layer PCBs!

The LM358 is a reliable, affordable, and versatile operational amplifier that is ideal for a wide range of analog applications. Whether you're building audio amplifiers, filters, voltage followers, or sensor signal conditioning circuits, the LM358 is a solid choice. Its ease of use, low power consumption, and dual op-amp configuration make it a go-to component for both beginners and experienced engineers. Proximity Sensor Circuit
Proximity Sensor Circuit


Discover Easy, Affordable, and Reliable PCB manufacturing with JLCPCB! Register to get $60 New customer coupons.

How To Order PCB

Order PCB Now

Circuit Diagram

Build a 3.7V 18650 Battery Level Indicator using BC547 Transistors

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

More Projects