Brief Passive Buzzer Controlled by 555 Timer Astable Multivibrator set by Light Dependent Resistor LDR Circuit

One way Passive buzzers can be switched on and off is with an NPN Bipolar Junction Transistor (BJT). The NPN BJT can be controlled by a 555 timer output. An Astable mode 555 timer’s high and low output speed can be set by a Light Dependent Resistor (LDR).

Basic electronics circuit fragments and simple circuits 22 Light Dependent Resistor LDR 555 timer controlled 2N2222 NPN BJT passive buzzer load diagram by electronzap electronzapdotcom
Basic electronics circuit fragments and simple circuits 22 Light Dependent Resistor LDR 555 timer controlled 2N2222 NPN BJT passive buzzer load diagram by electronzap electronzapdotcom
  • Passive buzzer makes clicking noises while it is turned on/off slowly.
  • Speeding up the clicks enough turns them into a buzz. The buzz gets higher pitched as you keep speeding them up (at a higher frequency).
  • Brighter light on the Light Dependent Resistor (LDR) charges and discharges the capacitor faster than lower light.
  • Capacitor discharges through pin 7 (discharge pin) to ground while output is low.
  • 555 output is set low after the capacitor rises above 2/3 of the supply voltage.
  • There is a high output after the capacitor drops below 1/3 of the supply voltage.
  • NPN Bipolar Junction Transistor (BJT) switch turns on when the 555 output is high, and off while the output is low.
  • Current flows through the buzzer when NPN BJT is on. No current flows through it when the NPN BJT is off.

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