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This PCB implements a low-battery alert system for a 48 V DC battery. When the battery voltage drops below 25%, the board activates a 5 V buzzer and indicator LED (optional).
The design fulfills all assignment requirements:
Here is a partial list of the files in the project:
low_battery_alarm/
├── 3d/
│ └── (3D STEP models for custom footprints)
│
├── BOM/
│ └── BOM.csv (bill of materials file)
│
├── footprint/
│ └── (custom .kicad_mod footprint files)
│
├── gerbers/
│ └── (manufacturing gerber files generated by KiCad)
│
├── library/
│ └── (custom symbols for this project)
│
├── low_battery_alarm.kicad_pcb (PCB drawing file)
├── low_battery_alarm.kicad_sch (schematic file)
├── low_battery_alarm.kicad_pro (project file)
└── README.md
The board is divided into four functional blocks:
The figure below describes the signal flow:
48V_IN
│
├── Fuse ─ L1 ──► 48V_FILTERED ──► DC/DC module → 5V → Buzzer
│ └──► Resistor Divider → Transistor Switch
│ └──► Cbulk, Cfast, TVS clamp
└── GND
The board begins with a two-pin battery connector (BT1). This input uses thick traces and a recommended minimum clearance for 48 V (≥1 mm).
A 500 mA resettable fuse (F1) protects the circuit from accidental short-circuits, wiring mistakes, and motor-induced surges.
The 48 V rail is subject to high-frequency noise, dips, and spikes due to:
To create a cleaner and safer rail (48V_FILTERED), the following components are used:
Reduces high-frequency noise and creates a low-pass filter together with the capacitors.
Stabilizes slow voltage changes and absorbs load steps.
Filters fast spikes in the MHz range that the bulk capacitor cannot absorb.
Clamps high-voltage spikes above ~60 V and protects the entire system from transients.
48V_FILTERED ↔ GND
To detect low-battery, a high-value resistor divider is connected between 48V_FILTERED and GND.
| Resistor | Value | Purpose |
|---|---|---|
| R1 | 220 kΩ | High-side resistor, reduces current |
| R2 | 3.3 kΩ | Creates ~0.7 V threshold for transistor base |
The divider produces a voltage proportional to the battery voltage:
[ V_{\text{div}} = V_{\text{bat}} \cdot \frac{R2}{R1 + R2} ]
At 25% battery, the divider output equals approximately 0.7 V, which is the turn-on voltage of a BJT transistor.
This provides a simple, analog threshold detector with no microcontroller needed.
A BC547 NPN transistor is used as a low-side switch for the buzzer:
When the battery falls below the 25% threshold:
This achieves the assignment requirement:
“Activate a buzzer once the charge level drops below 25%.”
R3 (75 Ω) limits buzzer current and stabilizes switching.
The buzzer requires a stable 5 V supply, so the board includes a dedicated DC-DC converter:
The converter is powered from 48V_FILTERED, through the protection network.
The buzzer (BZ1):
The result is:
This matches the assignment logic.
Footprint: Fuse_2920_7451Metric
Chosen for mechanical robustness and compatibility with Digi-Key part F9958TR-ND.
Footprint: L_6.3x6.3_H3
Chosen for inductors in the 6.0×6.0 mm SMD class.
Footprint: D_SMB
Matches the SMBJ package used by SMBJ58A parts.
CP_Radial_D10.0mm_P5.00mmTHT, 5 mm pitch. Chosen for beginner-friendly soldering and high-voltage spacing.
THT footprint provided by KiCad library for IA48xxD.
Standard TO-92.
| Ref | Qty | Description | Digi-Key Part Number |
|---|---|---|---|
| C1 | 1 | 100 µF 63 V electrolytic | B41827A8107M-ND |
| C2 | 1 | 0.1 µF ceramic 0603 | 2679-C0603X7R101-104KNE-CT-ND |
| D1 | 1 | SMBJ58A TVS diode | SMBJ58ALFTR-ND |
| F1 | 1 | Polyfuse 500 mA | F9958TR-ND |
| L1 | 1 | 47 µH SMD power inductor | SRN6045-470MCT-ND |
| PS1 | 1 | 48→5 V DC-DC module | 1470-1378-5-ND |
| Q1 | 1 | BC547 NPN transistor | BC547-ND |
| R1 | 1 | 220 kΩ | CF18JT220KCT-ND |
| R2 | 1 | 3.3 kΩ | CF18JT3K30CT-ND |
| R3 | 1 | 75 Ω | CF18JT75R0CT-ND |
| BZ1 | 1 | Buzzer | 668-1652-ND |
The BOM list can also be found in the file BOM/BOM.csv.
This PCB meets all assignment requirements using:
The design is electrically safe, easy to assemble, and demonstrates correct use of filtering, protection, and sensing techniques in mixed-voltage systems.
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