LED Flash — Portable UPS & Emergency Light
Manimohan T. · Department of Electronic & Telecommunication Engineering, University of Moratuwa · July 2023
Overview
The LED Flash is a 12 V portable device designed to eliminate disruption caused by power failures. It provides continuous illumination for up to 12 hours, can act as a router backup (12 V DC jack), and doubles as a mobile power bank (5 V/1A USB). The system switches seamlessly between mains power (230 V AC) and battery, and automatically cuts off the battery at 10.5 V to prevent deep discharge. A battery capacity indicator displays the remaining charge level at all times.
Motivated by personal experience during Sri Lanka's power crisis — losing internet during online lectures — this product solves three problems in one: emergency light, phone charger, and router UPS, removing the need to buy three separate devices.
Specifications
| Output | LED Flash Panel + 5 V/1A USB × 2 + 12 V DC Jack |
| Input Supply | 230 V AC (mains) or 12 V rechargeable battery |
| Battery | 12 V rechargeable, ~12 h light / 7 h router backup |
| Current | Up to 1A continuous output |
| Deep-Discharge Cut-off | 10.5 V (auto-disconnect to protect battery |
| Weight | ≈ 350 g |
| Estimated Cost | 16,290 LKR (≈ US $50) |
| PCB Size | 100 mm × 60 mm (Altium Designer) |
| Enclosure Size | 150 mm × 100 mm × 40 mm (SolidWorks) |
System Architecture
The circuit has two power paths that feed a common output rail via relay switching:
- Mains path: 230 V AC → transformer (230 V→15 V) → bridge rectifier (GBU8005) → smoothing capacitors → IC 7812 / IC 7805 regulators → regulated 12 V and 5 V outputs. The NE555 timer IC monitors the mains voltage and controls the relay.
- Battery path: 12 V rechargeable battery → Schottky diode (IN5822) → load. The Zener-based circuit disconnects the battery at 10.5 V to prevent deep discharge. A second relay switches between charging mode (mains) and UPS mode (battery).
- Protection: Current limiting resistor (5 W 3.3Ω), Zener-based voltage clamping, and indicator LEDs (Red = power-on, Green = charging, Yellow = battery full, additional for line-mode).
PCB & Enclosure Design
PCB (Altium Designer)
Single-board design, 100 mm × 60 mm. Clearance 0.203 mm between copper traces, trace width 0.254 mm, through-hole size 2 mm. Schematic, footprint, and 3D model were all verified before fabrication.
Enclosure (SolidWorks)
150 mm × 100 mm × 40 mm plastic enclosure with handle. Parts: front body, base, handle, and a custom D/C jack holder (with injection-mold design). Top/bottom planes 1.5 mm thick; side walls 2.5 mm. Holes for USB port, DC jacks, AC socket, LED indicators, rocker switches, battery monitor display, and PCB mounting pillars. Three conceptual enclosure sketches were evaluated by a Design Driven Innovation (DDI) matrix — Sketch 1 was selected for portability and ergonomics.
Bill of Materials (Key Components)
| Component | Qty |
|---|---|
| Transformer 230 V → 15 V | 1 |
| Bridge Rectifier GBU8005 | 1 |
| IC NE555P (timer) | 1 |
| Relay SRU12VDC-SL-C | 2 |
| Regulator IC 7812 (12 V) | 1 |
| Regulator IC 7805 (5 V) | 1 |
| Zener BZX55C6V2 (6.2 V) | 1 |
| Zener 9.1 V + 6.2 V | 1 |
| Transistor BC547BRL1G | 1 |
| Diode IN4007 | 2 |
| Diode IN5822RLG (Schottky) | 1 |
| Capacitor 10 nF | 2 |
| Capacitor 220 µF | 1 |
| Capacitor 2200 µF | 1 |
| Capacitor 1000 µF | 1 |
| Resistor 1 kΩ | 8 |
| Variable Resistor 20 kΩ | 2 |
| USB Female Port | 1 |
| LED Panel (White) | 1 |
| LED indicators (mixed) | 4 |
| Rocker Switch | 2 |
| Rechargeable Battery 12 V | 1 |
| Battery Capacity Monitor | 1 |
Sourced from Mouser Electronics, Tronic.lk, and Duino.lk. Total BOM cost: 16,290 LKR.
Results
The assembled product successfully powered a LED panel lamp and a Wi-Fi router simultaneously from battery during a simulated power cut. Mobile charging via USB was also demonstrated. Indicator LEDs correctly reflected each operating mode (power-on, charging, battery full, and line mode). The battery capacity display showed the remaining charge percentage in real time.
