555 timer – Buzzer output – Astable multivibrator mode – Light dependent resistor LDR controlled circuit

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Buzzers are nice for beginning electronics because you can hear when steady current goes through active buzzers, or when current is changing rapidly through passive buzzers. 555 timer works well to keep circuits on/off over a period of time, or to rapidly turn something on/off. Light Dependent resistor make a good variable resistor for changing the output frequency of a 555 timer.

555 timer astable buzzer light dependent resistor LDR controlled circuit schematic diagram by electronzap electronzapdotcom
555 timer astable buzzer light dependent resistor LDR controlled circuit schematic diagram by electronzap electronzapdotcom

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555 timer pin layout illustrative diagram by electronzap electronzapdotcom
555 timer pin layout illustrative diagram by electronzap electronzapdotcom
  • When power is applied, the capacitor starts charging from the 5V supply through the fixed resistor and the light dependent resistor. At that time the output is also high (as close to the 5V supply that the output can provide. A 1,000µF capacitor is a good value to help keep the active buzzer buzzing for a decent amount of time. A passive buzzer will just make a single click right when the output goes high. So you’d probably want no more than 100µF capacitor for the passive buzzer to speed up the clicks.
  • When the capacitor charges to 2/3 of the supply voltage, then the output goes low (a direct connection to 0V ground), and the active buzzer turns off whereas the passive buzzer makes a single click.
  • As soon as the capacitor discharges to 1/3 of the supply voltage, then the output goes high and the capacitor charges again. The process repeats as long as power is applied.
  • Threshold pin (6) senses when the capacitor is above 2/3 supply voltage.
  • Trigger pin (2) senses when the capacitor is below 1/3 supply voltage.
  • Discharge pin (7) connects to 0V ground when the output is low. The capacitor discharges through it while it does so, and the current from the supply (5V in this circuit) also goes right to ground, so it doesn’t affect the discharge time. When the output is high, pin 7 basically turns off and no current flows through it. So it doesn’t affect the charging time of the capacitor.
  •  Light dependent resistor (LDR) and fixed resistor control how much current flows into the capacitor from 5V. The value of the capacitor determines how much current is needed to raise it’s voltage. The capacitor discharges through the LDR alone, so it will discharge anywhere from a little bit to a lot faster than it took to charge. LDRs usually have a lot more resistance than 220Ω, but they can drop well below 220Ω if it is really bright.

Video

Astable 555 timer buzzer active passive using light dependent resistor LDR circuit electronics

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Transistors will probably be the most challenging components to learn. Understanding them will help you understand all of electronics much better, and help you the most in being creative while designing your own circuits.

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