Arduino project · Motion and security · Easy
Door Open Alarm with Reed Switch
A magnet on the door keeps the reed switch closed. Open the door and the alarm sounds.
What you will learn
- How a reed switch and magnet work
- How to detect that a door has opened
- How to invert a condition (alarm when NOT closed)
How it works
When the door is shut the magnet is close to the reed switch and the pin reads LOW (magnet near). When the door opens the magnet moves away, the condition "no magnet" becomes true, and the buzzer and red LED turn on.
Try this next
- Make the alarm stay on after the door closes again (needs a variable).
- Add a reset button.
- Use the same idea for a window and a door together.
The simulator program and explanation on this page are original. The idea follows a classic Arduino project that students have built for years.
Parts: sensors and actuators (tap to show or hide)
Try the inputs (virtual sensors)
Pins (you can change them)
| Part | Arduino pin (change it here) | How to wire it |
|---|
Every wire, one by one
Red rail = 5 V from the Arduino 5V pin. Black rail = GND. The coloured wires carry signals. Match the wire colours on the drawing to this list.
| # | Colour | From (part and leg) | To |
|---|
Steps run from top to bottom, then start again. While the simulation runs, the active step is highlighted in green.
Quantities come from the parts you added. The store links open a search for that item on Robu.in, Amazon.in and ThinkRobotics, so you always see what is in stock today. Check the model, voltage and price before you order.
| # | Item | Qty | Search term | Buy / compare |
|---|
The PDF has the circuit sketch, pin table, every wire, sequence, full code, the parts list with store links, do’s and don’ts for each part, and the safety notes. The project file lets you come back and keep editing.
In the real world
A door with a magnetic alarm
Take the magnet away in the simulator (reed switch off). The door swings open in the real room and the alarm rings.
Tip: use the gold tab at the bottom of the circuit drawing to pull the real-world view up over the Arduino board. Your parts stay visible above it, so you can press a button and watch the real scene react.
- Reed switch + magnet = the door closed or open
- Buzzer = the alarm bell
- Red LED = the warning beacon