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Built a smart Li-ion charger with 10-LED fuel gauge. Features BQ24075 for charging & LM3914 voltage monitoring. Full build guide available!


Tired of guessing your battery's charge level? In this Instructable, I'll show you how to build a professional-grade lithium battery charger with a visual 10-segment LED indicator. This project combines the reliable BQ24075 charging IC with the classic LM3914 display driver to create a compact, feature-packed module perfect for powering your DIY projects. Whether you're building a portable speaker, robot, or any battery-powered device, this charger will keep your power in check!

What You'll Learn:
  1. How to design a circuit with charging and monitoring capabilities
  2. Proper LM3914 configuration for accurate voltage reading
  3. Professional PCB assembly techniques
  4. Troubleshooting common circuit issues
Supplies

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Components List:
1 BQ24075 IC Li-Ion battery charger
1 LM3914 IC LED driver
10 LEDs (5mm) Mix colors for visual effect
1 USB-C connector Power input
1 JST 2-pin connector Battery connection
Various Resistors Values per schematic
1 PCB Custom designed
Complete BOM list [here]
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Tools & Materials:
  1. PCB Fabrication:JLCPCB (Green solder mask!)
  2. Soldering:
  3. Soldering iron or hot air station
  4. Solder paste
  5. PCB stencil
  6. Tweezers
  7. Debugging:
  8. Multimeter
  9. Microscope
  10. Design Software: Altium Develop
Circuit Design Explanation

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The core of this project uses two main ICs working together. The BQ24075 handles the battery charging process safely, while the LM3914 drives the 10-LED display to show battery level. I designed the voltage dividers to scale the battery voltage properly for the LM3914's input range.
The Two Main ICs:
  1. BQ24075- Handles battery charging safely
  2. LM3914- Drives the 10-LED display

Key Design Points:
  1. USB-C input for universal compatibility
  2. Voltage dividers scale battery voltage for the LM3914
  3. MODE pin selection (dot vs. bar display)
  4. Proper decoupling capacitors for stable operation
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Pro Tip: Run a Design Rule Check (DRC) before ordering PCBs!

 Design Files:

  1. Full Schematic PDF
  2. Altium develop project Online access
Design Tips:

  1. Place power components on top side
  2. Keep analog and digital sections separate
  3. Use thick traces for high-current paths
  4. Add test points for debugging
 Manufacturing:
  1. Export Gerber files from your EDA software
  2. Upload to JLCPCB (or similar service)
  3. Choose your preferred color and finish
  4. Order a stencil for easy assembly
Assembly Process

Needed Data files:

GERBER Files [Download here]
BOM File [Download here]
Pick&Place File [Download here]

Assembly started with the bottom side using solder paste and stencil.
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After component placement, I used a hot plate for reflow. For the top side, I used a 3D-printed jig to hold the PCB while applying paste and components.

The Jig STL files are available through [this link]
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Bottom Side Assembly:
  1. Align stencil and apply solder paste
  2. Place components using tweezers
  3. Reflow using hot plate (show temperature profile)
  4. Inspect under microscope
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Top Side Assembly:
  1. Use 3D-printed jig to hold PCB
  2. Repeat paste application
  3. Place remaining components
  4. Use hot air gun for reflow
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Troubleshooting


Initially, all LEDs stayed on regardless of battery voltage. The issue was incorrect voltage scaling for the LM3914. I had to rework the voltage divider resistors to properly scale the battery voltage to the chip's 1.25V reference range.
The Problem:
"After first assembly, all LEDs lit up regardless of battery voltage. This is a common LM3914 mistake!"
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The Solution:
  1. The LM3914 needs scaled voltage inputs
  2. RHI must connect to the 1.25V reference
  3. Battery voltage must be divided down
  4. Use proper resistor values for 2.7V-4.2V range
Testing & Final Result

After fixing the circuit, I tested with various battery states. The LEDs now accurately show charge level from empty to full. The system properly charges batteries while giving clear visual feedback through the LED bar graph.
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The completed charger reliably charges Li-ion batteries and provides accurate charge level indication. The compact design combines both charging and monitoring in one professional-looking board ready for use in various projects.

Initial Tests:
  1. Visual inspection for solder bridges
  2. Check power consumption with no battery
  3. Verify LED blinking pattern (no battery detected)
  4. Test with fully charged battery (all LEDs on)

Charging Test:
  1. Connect partially discharged battery
  2. Monitor LED progression over time
  3. Measure actual battery voltage vs. LED indication
  4. Validate full charge detection
The finished charger reliably charges Li-ion batteries while providing accurate charge level indication. It's a compact, professional solution that combines both functions in one board. Perfect for powering your DIY projects with clear battery status visibility.

This follows your style of short, technical explanations with just the essential details needed to understand and build the project.