Beginner
Simple Alarm Circuit
Build a battery alarm that uses a reed switch or sensor to trigger a buzzer through a transistor.
Safety first, always.
ElectroLab AI teaches theory, low-voltage electronics, and planning concepts. Mains voltage, switchboards, fixed wiring, high-current systems, and legal electrical work must only be performed by licensed electricians where required.
Open Safety CenterBuild target
Use a transistor as a switch and build a practical sensor circuit.
Build steps
1.Place the transistor and identify base, collector, and emitter.
2.Wire the buzzer in the collector path with correct polarity.
3.Add a base resistor from the reed switch or trigger node.
4.Connect the battery and test the trigger action.
5.Mount the reed switch and magnet so the alarm changes state reliably.
What you are learning
1.A transistor can let a small trigger current control a larger buzzer current.
2.The base resistor protects the transistor input.
3.Reed switches open or close when a magnet is nearby.
Bench tests
1.Test the buzzer directly from the battery briefly.
2.Measure base voltage when triggered.
3.Move the magnet slowly and note the switching distance.
Fault finding
1.Buzzer always on: reed switch logic may be reversed.
2.Buzzer never sounds: check transistor pinout and buzzer polarity.
3.Weak sound: battery may be low or buzzer current too high.
Upgrades
1.Add an LED indicator.
2.Add a latch so the alarm stays on.
3.Add a delay capacitor for entry timing.
Project safety
Use a small battery supply only. Do not connect alarm prototypes to fixed wiring or security systems.