How to Build a Personal Alarm with a Reed Switch and Magnet

schema May 16, 2014

How to Build a Personal Alarm with a Reed Switch and Magnet

Looking for a easy but effective way to shield your non-public property or upload safety to your property? This DIY non-public alarm is compact, loud, and surprisingly effective. Whether it’s in a bag, pocket, or on a window, it reacts the instant something suspicious takes place.

Let’s break down how this alarm works and how you can make one yourself using basic components.

How It Works

This personal alarm fits inner a small plastic box. It uses a reed switch—a tiny issue that reacts to the presence of a magnet. When the magnet is near, the switch remains closed and the alarm is silent. But when the magnet is pulled away (like when a thief grabs your bag), the transfer opens and triggers a loud sound via a speaker.

You can attach the magnet to your wrist or clothing using a thin cord. If someone yanks the bag away, the magnet disconnects, and the alarm goes off. You can also do the reverse: place the alarm in your pocket and attach the magnet inside the bag.

This system also works great for doors and windows. Just place the alarm box on the frame and the magnet on the movable part. When the door or window opens, the magnet moves away and sets off the alarm.

Parts You’ll Need

Here are the electronic components required:

Component	Specification
R1	330K Ohm Resistor (¼ W)
R2	100 Ohm Resistor (¼ W)
C1	10nF 63V Ceramic or Polyester Capacitor
C2	100μF 25V Electrolytic Capacitor
Q1	BC547 NPN Transistor (45V, 100mA)
Q2	BC327 PNP Transistor (45V, 800mA)
SW1	Reed Switch
Magnet	Small magnet to trigger the reed switch
SPKR	8 Ohm Speaker
B1	3V Battery (2 x AA or AAA cells in series)

How the Circuit Works

When the magnet is close, the reed switch is closed and the circuit stays off. But when the magnet is pulled away—say, when your bag is stolen—the reed switch opens. This triggers the oscillator circuit built with transistors Q1 and Q2, making the speaker emit a sharp, loud alarm sound.

It’s a basic design, but it’s surprisingly effective and very loud for its size!

Extra Tips

Test your circuit on a breadboard before soldering.
Use a small plastic case with holes for the speaker.
Add double-sided tape or velcro if you’re attaching it to windows or doors.
Want more volume? Try using a piezo buzzer instead of a regular speaker.

Why You’ll Love It

✅ Low-cost: Very cheap to build using common components.
✅ Portable: Fits in any bag or pocket.
✅ Versatile: Use it for bags, windows, doors—anywhere!
✅ Beginner-Friendly: Great project for anyone getting into electronics.

Final Thoughts

If you’ve ever involved about robbery or break-ins, this little DIY personal alarm can give you peace of thoughts. It’s simple, loud, and smooth to carry or install anywhere you want it. Plus, constructing it is fun and worthwhile.

So clutch a few additives, a soldering iron, and start building your very own alarm device these days. You'll be amazed at how something so small can % such a punch!