The passive buzzer depends on pulses to make noise. A slow pace switching of voltage makes clicking noises, whereas fast enough voltage switching makes a steady audible tone.

The 555 timer wired in astable mode with a light dependent timing resistor (LDR), is an easy way to get a steady on/off square wave.

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Diagram

While the astable 555 timer capacitor is charging towards 2/3 supply voltage, the output is high.

When capacitor hits 2/3 supply voltage, then the output goes low and the capacitor discharges through pin 7 (discharge pin) until it drops to 1/3 supply voltage.

Output goes back high again once the capacitor drops to 1/3 of the supply voltage and the capacitor starts charging again.

The primary factor setting the timing for any given capacitor in this circuit is how much light is falling on the light dependent resistor. Light level sets it’s resistance.

The faster the timing goes (brighter LDR) the higher pitch or faster clicking of the buzzer.

Slower timing, due to a darkened LDR will result in slower clicks or a lower pitch buzz.

Large capacitors are more likely to click while smaller capacitors are more likely to buzz.





Power supply I use in my videos.

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Video

Quick passive buzzer circuit using 555 timer astable multivibrator controlled by photoresistor



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