Mar 312015
 

Darkness detector circuit with audio output using 555

This darkness detector circuit beeps when the light drops below a certain level. The main elements of this darkness detector circuit are the 555 integrated circuit, that is configured to operate as an astable multivibrator and an LDR which is used as darkness sensor.

Darkness detector circuit operation

The pin 4 of the timer 555 is the enabling pin of the integrated circuit. The 555 IC will begin to work only when the voltage level on this pin is on high level

Darkness detector circuit with audio output using 555

The LDR and resistor R3 form a voltage divider. When the LDR does not get enough light, its resistance increases and the voltage across its terminals rises to almost the voltage level of the power source. When this happens, the 555 is enabled and starts to work.

When the 555 is active, it deliver on its output (pin 3) a square wave.
This square wave causes the MOSFET transistor passes continuously from the cutting region to the salturacíon region, sounding the speaker.

If the light level increases, the voltage level on pin 4 decreases, the 555 timer is disabled and the audio signal is interrupted.

To better control the level of light or darkness, which causes the circuit starts its operation, the resistor R3 is replaced by a 1K resistor in series with a 100K potentiometer. By varying the potentiometer, you can make the adjustments.

Darkness detector circuit components

  • 1  555 timer (IC1)
  • 1  LDR (photoresistor)
  • 1  47K resistor (R1)
  • 1  1K resistor (R2)
  • 1  10K resistor (R3)
  • 1  0.047uF electrolytic capacitor (C1)
  • 1  0.1uF capacitor (C2)
  • 1  IRFZ44 MOSFET Transistor (Q)
  • 1  8 or 4 ohms speaker
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