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Light-Activated LED & Buzzer Alarm
A 3 V light-sensing alarm that lights an LED and sounds a buzzer, with adjustable sensitivity.
DETAILS
A 3 V light-activated alarm that detects changes in ambient light and responds with an LED and a buzzer. A 5516 photoresistor and 10 kΩ potentiometer set the bias on an 8050 NPN transistor; when light crosses the chosen threshold, Q1 pulls its collector low, lighting the LED and switching an 8550 PNP high-side transistor to sound the buzzer. Members went from a printed schematic and parts kit to a soldered, working PCB.
CONCEPTS
- Light-dependent resistance
- Adjustable bias networks
- NPN & PNP transistor switching
- Cutoff & saturation
- Datasheet reading
- Systematic troubleshooting
| SUPPLY | 3 V battery pack |
|---|---|
| SENSOR | 5516 LDR + 10 kΩ potentiometer |
| SWITCHING | 8050 NPN (Q1), 8550 PNP (Q2) |
| OUTPUTS | LED (100 Ω limit) + buzzer |
- Step 1: Match each component to its PCB reference designator and verify values and orientation
- Step 2: Solder resistors and low-profile parts first
- Step 3: Install the LDR, potentiometer, LED, transistors, buzzer, and battery leads
- Step 4: Inspect for cold joints, bridges, and reversed polarized parts, then trim leads
- Step 5: Final visual inspection before connecting the 3 V supply
- LED dimmer than expected: continuity-checked the whole PCB with a Fluke 17B
- Several beginner solder joints reflowed and verified
- Circuit worked after repairs; remaining dim LED identified as a likely defective part and kept as a teaching example
- Project Management
- Daniel Bustos · William McGinley
- Schematic
- Lerry Lara · Antonio Magana · Noah Cromwell
- Soldering & Assembly
- Lerry Lara · Kaeden Wellman · William Hester · Antonio Magana · Riley Snyder · John Johnson
- Technical Documentation
- Lerry Lara · Kaeden Wellman · William Hester
SENSOR CIRCUITBEGINNER
- PHOTORESISTOR
- NPN / PNP BJT
- 3 V
- SOLDERING
