Imported from GitHub: Matan-Vinkler/low-battery-alarm · commit 0ea567f · license MIT

Description

A simple PCB design that detects low battery levels using a high-voltage divider and triggers a buzzer via a 5 V DC-DC module. Includes input filtering, surge protection (TVS), and transistor-based threshold switching. KiCad project + BOM + gerbers.

README

Low Battery Alarm - PCB Design Documentation

48 V Low-Battery Indicator with Buzzer Alert

• Revision: vNOV 2025
• Author: Matan Vinkler
• Date: 27-11-2025

Overview

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:

• Accepts 48 V DC input using a standard connector
• Monitors battery voltage using a high-value resistor divider
• Activates a buzzer once charge level drops below 25%
• Includes a filter stage to reduce voltage spikes and noise caused by variable loads
• Generates a stable 5 V rail using a DC-DC converter
• Provides complete schematic, BOM, footprints, PCB design, and fabrication files

Project Structure

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

System Architecture

The board is divided into four functional blocks:

1. 48 V Input Stage
2. Input Protection and Filtering
3. Voltage-Sensing Divider + Transistor Threshold Switch
4. 5 V Power Stage and Buzzer Driver

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

48 V Input Stage

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.

Input Protection & Filtering

The 48 V rail is subject to high-frequency noise, dips, and spikes due to:

• Motor loads
• Cable inductance
• Sudden battery reconnection
• Transient switching

To create a cleaner and safer rail (48V_FILTERED), the following components are used:

• L1 — Series Power Inductor (47 µH)

Reduces high-frequency noise and creates a low-pass filter together with the capacitors.

• Cbulk — Bulk Capacitor (100 µF, 63 V)

Stabilizes slow voltage changes and absorbs load steps.

• Cfast — Ceramic Capacitor (0.1 µF, 100 V)

Filters fast spikes in the MHz range that the bulk capacitor cannot absorb.

• D1 — TVS Diode (SMBJ58A)

Clamps high-voltage spikes above ~60 V and protects the entire system from transients.

All components above connect between:

48V_FILTERED ↔ GND

Voltage-Sensing Divider

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.

Transistor Threshold Switch (BC547)

A BC547 NPN transistor is used as a low-side switch for the buzzer:

• Emitter → GND
• Collector → buzzer ground
• Base → divider node

When the battery falls below the 25% threshold:

• Divider voltage < 0.7 V
• Base current stops
• BJT switches OFF
• Buzzer turns ON (wired in pull-up configuration)

This achieves the assignment requirement:

“Activate a buzzer once the charge level drops below 25%.”

R3 (75 Ω) limits buzzer current and stabilizes switching.

5V Power Stage

The buzzer requires a stable 5 V supply, so the board includes a dedicated DC-DC converter:

• PS1 — IA4805D (48 V → 5 V Isolated Converter)

• Input: 43.2–52.8 V
• Output: +5 V (100 mA)
• Isolation: 1 kV
• Simple THT footprint in KiCad

The converter is powered from 48V_FILTERED, through the protection network.

Buzzer Output Stage

The buzzer (BZ1):

• Is powered from +5 V
• Connects through current-limiting resistor R3
• Is switched by transistor Q1

The result is:

• High battery voltage → buzzer OFF
• Low battery voltage → buzzer ON

This matches the assignment logic.

Component Footprints & Footprint Rationale

• Fuse (F1)

Footprint: Fuse_2920_7451Metric Chosen for mechanical robustness and compatibility with Digi-Key part F9958TR-ND.

• Inductor (L1)

Footprint: L_6.3x6.3_H3 Chosen for inductors in the 6.0×6.0 mm SMD class.

• TVS Diode (D1)

Footprint: D_SMB Matches the SMBJ package used by SMBJ58A parts.

• Capacitors

• Cbulk: Radial electrolytic, footprint CP_Radial_D10.0mm_P5.00mm
• Cfast: 0603 ceramic SMD Chosen for easy sourcing and compact layout.

• Resistors

THT, 5 mm pitch. Chosen for beginner-friendly soldering and high-voltage spacing.

• DC-DC Module

THT footprint provided by KiCad library for IA48xxD.

• Transistor (BC547)

Standard TO-92.

Design Rule Checks (DRC) & ERC Notes

• PWR_FLAG is added to both +48V_RAW and +48V_FILTERED to satisfy KiCad ERC
• High-voltage clearance rules are applied (≥1 mm)
• Tracks for 48 V supply and ground are thick (≥50 mil)
• Sensitive analog node (divider output) has short trace to transistor base

Bill of Materials (BOM)

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.

Conclusion

This PCB meets all assignment requirements using:

• A robust input filter
• High-voltage safe resistor divider
• Simple analog threshold detection
• Reliable 5 V DC-DC conversion
• A transistor-driven buzzer alert

The design is electrically safe, easy to assemble, and demonstrates correct use of filtering, protection, and sensing techniques in mixed-voltage systems.