Door Intruder Alarm
Explorer · Ages 7–9
- 1Read the conceptLearn the big idea behind this project.
- 2Check the partsSee which parts the robot needs.
- 3Follow the build stepsBuild it step by step, then test it.
- 4Answer the quizTap one answer for each of the 10 questions to earn XP.
💡 The concept
A door alarm knows when a door opens. A magnetic reed switch is held closed while the door is shut. When the door opens, the switch releases, the microcontroller notices the change, and the buzzer sounds. Sensing a change and reacting to it is the heart of every alarm system.
Parts you need
- Buzzer
- Magnetic reed switch (or button)
- Microcontroller
- Battery
- Jumper wires
🔎 Parts specification
Exactly what each part is, which models to buy, and what it does.
Microcontroller▸
Suggested models: BBC micro:bit V2 or Arduino Uno R3
Features: Needs just 2 pins: one digital input for the switch and one digital output for the buzzer. It checks the switch hundreds of times every second.
Buzzer▸
Suggested models: Active piezo buzzer (3–5V)
Features: Makes a sound all by itself when you give it power — no tunes needed. It has + and − legs, so polarity matters. A longer beep pattern makes a better siren.
Switch▸
Suggested models: Magnetic reed switch MC-38, or a mini push button
Features: A reed switch is two contacts in a sealed tube that touch when a magnet is near. Stick the magnet on the door and the switch on the frame — opening the door separates them and the circuit 'sees' it.
Battery / power▸
Suggested models: 2×AA pack or 9V with clip
Features: 3V–9V depending on your board. Keep it off while wiring.
💻 Program the microcontroller
📲 App to use: Microsoft MakeCode for micro:bit, or Arduino IDE for Arduino Uno
MakeCode version: forever → if P0 pressed (button released) → play tone high pitch, else → rest.
// Arduino IDE (C++) — Door Alarm
const int SWITCH = 2; // reed switch
const int BUZZ = 8; // active buzzer
void setup() {
pinMode(SWITCH, INPUT_PULLUP);
pinMode(BUZZ, OUTPUT);
}
void loop() {
if (digitalRead(SWITCH) == HIGH) {
tone(BUZZ, 1000); // door open: alarm!
} else {
noTone(BUZZ); // door closed: quiet
}
}🧠 How it works: INPUT_PULLUP makes the pin read HIGH when the switch is loose (door open) and LOW when it's held down (door closed by the magnet). The loop checks the switch constantly — the moment it goes HIGH, tone() beeps the buzzer at 1000 Hz. Turn the beep on and off in a pattern to make a wee-woo siren.
🛠️ Build it step by step
Press Play to watch every part go into place and every wire connect. Ask a grown-up to help with real wiring!
Step 1 of 6
🔋 Power the board
Connect the battery to the microcontroller. Battery OFF until everything is wired and checked.