Future Kit Light Activated Alarm FK245
by not_ryoga10 in Circuits > Audio
842 Views, 5 Favorites, 0 Comments
Future Kit Light Activated Alarm FK245

For this project, I made a light-activated alarm circuit using the Future kit FK245. The circuit uses a photo-transistor as a light sensor, which triggers an audible alarm through an 8 Ω 0.25 W speaker when exposed to light.
This project demonstrates the use of electronics being controlled by light, and shows how light can be turned into an electronic signal to give an output. It is a simple example of sensors, signal processing, and sound generation. The circuit can also be adapted for basic security systems, such as detecting when a drawer or box is opened.
Supplies


FK245 from the future kit website
FB15 from the future kit website
Power Source:
9V battery holder
PCB Board:
Future Kit FK245
Speaker:
8 Ω, 0.25 W
Photo-resistor (light sensor)
LED (red)
Resistors:
120 Ω
1 MΩ
1k2Ω
10 kΩ (×2)
50 kΩ
150 kΩ
Electrolytic Capacitors:
22 μF (×2)
4.7 μF
0.1 μF
Ceramic Capacitor
Diodes: IN4148 (×2)
Transistors:
C1815 (×2)
C9012 (×2)
Trimmer Potentiometer:
104
Contact pins:
x4
Wires
Side cutters or diagonal cutters (Both work!)
Hand Drill + 5mm and 6mm drill bits
Wire Stripper
2 Heat Shrink pieces
9V Battery Clip
Hot Glue Gun
2 Male/Female Jumper
Lighter
2 Heat Shrink
Annotating and Preparing All Components



- Lay out all the parts on a white piece of paper
- Label resistors, check color codes, and the value of each resistor.
- Mark capacitor +/– polarity and diode orientation.
- Keep all the components organised before soldering. (All Images from Future Kit)
Safety Setup

- Wear safety goggles.
- Clean soldering iron tip.
- Keep a damp sponge nearby.
Soldering the PCB (LOWEST TO HIGHEST COMPONENTS)

- Start with the lowest, smallest components then move on to the larger components gradually.
- Insert resistors, bend legs slightly to hold them in place, and trim leads after soldering each resistor.
- Insert diodes (check polarity), and solder.
- Insert small capacitors (ceramic + electrolytic), check polarity, solder.
- Insert transistors (C1815, C9012), solder.
- Insert photo-resistor + LED (long leg = +), solder.
- Insert trimmer potentiometer (104), solder pins.
- Solder wires for speaker and 9V battery holder.
Testing the Circuit

- Inspect PCB back for cold joints/bridges.
- Connect working 9V battery.
- No light = silent, light = alarm.
- If it's not working, check the polarity, short circuits, battery, or misplaced parts.
Preparing the Box for My Project

- Find a box that fits PCB, speaker, and battery.
- Use a hand drill and drill a 5 mm hole for photo-resistor, 6 mm hole for switch.
Inserting Components Inside the Box


- Insert photo-resistor + switch into drilled holes.
- Hot-glue speaker to box wall.
- Fix PCB + battery holder with double-sided tape.
- Extend LDR with male/female jumpers if short.
- Add heat shrink to battery/switch wires.
Final Assembly and Testing


- Close box carefully.
- Reconnect battery.
- Cover LDR = silent, light on LDR = alarm.
- Replace oversized battery holder with smaller clip if it doesn't fit.
- Optional: seal box with hot glue if you want to shut it at all times.
Troubleshooting and Warnings
Troubleshooting:
Alarm silent: check polarity, solder joints, battery.
Alarm always on: check LDR orientation, solder bridges.
Weak sound: check speaker wiring and any flaws.
Inconsistent: verify component values.
Warnings:
Soldering iron around 450 °C, wear goggles, avoid burns.
Hot glue gun around 120–215 °C.
Do not force components into PCB.
Don’t use damaged batteries.