Compact Anti-Theft Alarm for Bikes (Easy DIY Circuit)

I built a tiny, low-power bike alarm that detects tampering using a vibration switch and triggers a buzzer via an IRLB4132 MOSFET driver. The circuit uses a PC817 optocoupler for isolation and a Hall-effect sensor as a secure reset — the alarm can only be turned off by placing a magnet over the Hall sensor.

This compact, DIY-friendly device is perfect for anyone who wants a discreet, easy-to-make anti-theft alarm for bikes and scooters. Includes wiring close-ups and a test demo for guidance.

1. Vibration Switch (tilt/shock sensor)
2. PC817 Optocoupler
3. IRLB4132 MOSFET
4. 100Ω Resistor
5. 3.7V Li-ion Battery
6. Hall Effect Sensor
7. Buzzer (3–5V)
8. ON/OFF Switch
9. Small PCB / Perfboard
10. Wires and Connectors

I designed the circuit PCB using EasyEDA and ordered the final board from PCBWay for a professional finish.

👉 pcbway.com/?from=Promaker1

After receiving the PCB, I placed and soldered the IRLB4132 MOSFET and the PC817 optocoupler in their respective positions as shown in the attached circuit diagram/photo.

This ensures a compact, reliable, and durable build compared to breadboard wiring.

Next, I connected the ON/OFF switch, buzzer, Hall-effect sensor, and vibration switch to the PCB:

1. ON/OFF Switch: Controls power to the circuit.
2. Buzzer: Connect to the MOSFET output so it sounds when the vibration switch is triggered.
3. Vibration Switch: Detects any shake or tampering of the bike and triggers the alarm.
4. Hall-effect Sensor: Acts as a secure reset — the alarm can only be turned off by placing a magnet near the sensor.

All components are carefully soldered to the PCB, keeping wires short for a compact, tidy setup.

📷 (Insert photo showing all components fixed to PCB and wiring)

After placing all components on the PCB:

1. Trim excess legs of all components with side cutters.
2. Solder all connections properly, ensuring no cold joints.
3. Clean the PCB using liquid flux cleaner or isopropyl alcohol to remove solder residue and flux for a neat finish.

1. I used a small 3.7V Li-ion battery
2. Fixed the battery in place using a small amount of gum or double-sided tape to secure it.
3. Soldered the +ve and -ve terminals to the PCB carefully, ensuring correct polarity.

1. Measure the length and width of your PCB carefully.
2. Cut a 3mm acrylic sheet to size — I used a laser cutter for precise edges.
3. Assemble the acrylic pieces into a box shape to house the PCB and components.
4. Use gum or strong adhesive to fix the sides together securely.

This creates a compact and protective enclosure for your bike alarm.

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1. Insert the PCB into the acrylic box and secure it using gum/adhesive.
2. Connect the TP4056 charging module:
3. Solder wires to B+ and B- terminals of the module (red = +, black = -).
4. At the other end, solder a 2-pin female header for easy connection.
5. Insulate all exposed wires and solder joints using heat shrink tubing to prevent shorts.

The TP4056 module is now ready and can be used to charge your 3.7V battery safely.

1. Simply connect the female 2-pin header from the TP4056 to the male header on the PCB.
2. This allows easy plug-in charging without opening the enclosure.

1. Turn ON the switch and mount the completed alarm box securely on your bike.
2. Test the alarm: shake or tap the bike — the vibration switch triggers the buzzer.
3. Reset the alarm: place a magnet near the Hall-effect sensor to turn it off.
4. Once everything works perfectly, close the acrylic box to protect the circuit.

The DIY bike alarm is now fully functional:

1. The vibration switch detects any shake or tampering.
2. The buzzer sounds instantly when the bike is disturbed.
3. The Hall-effect sensor ensures the alarm can only be turned off using a magnet, keeping your bike secure.

📹 Watch the full working demo on YouTube: https://youtu.be/uCkH_dk6y90?si=77p7BOPlh4xo_rIQ