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13. Buzzer: an alarm mode

Bench build · Fundamentals · Beginner · about 30 minutes

Step 13 of the automatic light: Q4, driven from the same 555 output, switches BZ1 through SW2 (alarm on/off). With the alarm on, whoever trips the PIR at night gets light and a beep for the whole 11 s. Component 13 of 14: the buzzer. An active buzzer has its own oscillator inside: put DC across it and it beeps (about 2.3 kHz). A passive buzzer or a speaker needs an AC signal instead. It draws 20–60 mA, so a transistor drives it, never a chip pin directly.

13. Buzzer: an alarm mode: the finished build running on the simulator's bench

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Parts list

Test procedure

Each test has an expected value and a pass range, like a factory test sheet:

Alarm on, movement at night: buzzer sounds
Alarm off: silent, lamp still on

How it works

The buzzer. An active buzzer has its own oscillator inside: put DC across it and it beeps (about 2.3 kHz). A passive buzzer or a speaker needs an AC signal instead. It draws 20–60 mA, so a transistor drives it, never a chip pin directly.

Reading it. The + leg is longer and marked +; rated voltage on the label (3–12 V here). An active buzzer usually has a sealed back; a passive one shows the circuit board.

Testing it. Put it across a 9 V battery the right way round: it beeps. Ω reads tens to a few hundred ohms (the coil).

In the build. Q4, driven from the same 555 output, switches BZ1 through SW2 (alarm on/off). With the alarm on, whoever trips the PIR at night gets light and a beep for the whole 11 s.

Builds on lesson 12: everything from before is still there.

Next: lesson 14, solar panel, charge controller and battery.

Things to try

Toggle SW2 and trigger the PIR.
Meter the buzzer current: about 50 mA, through Q4.

Background

One circuit, built up one part at a time: by lesson 14 it is a solar-powered, dusk-sensing, motion-triggered timed lamp with an alarm.