Easy

Magnetic Reed-Switch Door Alarm

A reed switch closes in the presence of a nearby magnet — glue the magnet to a door and the switch to the frame, and opening the door breaks the contact and triggers the alarm.

Circuit Diagram

+9V Reed switch 28x3mm magnet pull-down R (base → GND) Q BC547 C → buzzer E → GND BUZZER Magnet near reed → contact closed, Q off, silent. Door opens → buzzer sounds

How to Build It

  1. Connect the reed relay in series between the positive rail and a BC547 transistor's base, with a pull-down resistor to GND on the base.
  2. Connect the transistor's collector to a buzzer, the buzzer's other terminal to the positive rail.
  3. Connect the emitter to GND.
  4. Bring a magnet near the reed switch — the circuit should stay silent (contact closed, transistor OFF).
  5. Pull the magnet away (simulating the door opening) — the buzzer should sound.

Bill of Materials

ProductCodeQty
Buzzer LD-BZPG-2312 1
Relay (12/24VDC/auto) 50 406 1
Resistor DR47K-8/9 1
Transistor (incl. IGBT) — 1
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