Smoke detector circuit using LDR and 555

When speaking of Smoke sensor circuits we often think about implementing it using Microcontrollers or using a development boards. But in fact there are possible ways you can build a simple smoke alarm without using dedicated sensors or microcontroller. In this design I have used LDR for this purpose. Also this smoke detector circuit uses a timer 555 for activation of buzzer which produces an intermittent sound as result of sensed smoke.


  • Battery 12V
  • Battery 9v
  • LDR
  • Potetiometer 50k Ohm
  • NPN Transistor 2N3904
  • Resistor 82k Ohm
  • Resistor 220 Ohm
  • Capacitor 100uF
  • Timer 555


The working of this circuits starts with LDR which is the smoke sensing element and a light bulb.  Set an LDR and a 12V bulb right next to it, we can adjust the distance between them as much as we want. But it should be close enough for the light from bulb to fall on LDR. When smoke is emitted, light from the bulb will get obstructed. This results in a variation in the light that’s received by the LDR due to smoke. As a result transistor Q1 will turn ON and collector voltage switches to high state.

Voltage in the collector terminal of transistor Q1 will cause a trigger in the timer 555, which will activate the buzzer connected to 555 output. The buzzer will continue to sound until smoke exists in the environment. Use a 50k potentiometer right at the base of the transistor to calibrate the triggering point for the smoke. By adjusting the value of resistance using the Potentiometer we can set the voltage level at which the transistor should turn ON and trigger the IC 555. This in turn will sound the buzzer acting as an alarm.


  • Bulb should remain ON in order for this smoke detector circuit to work.
  • LDR should be arranged in such a way it gets less disturbed by ambient light, you can cover the surroundings of LDR making it to interact only with the light from bulb.
  •  This circuit cannot be a replacement of Smoke sensor since the accuracy the smoke sensing will be be dependent on incident light and the ability of smoke from fire to mask the light.
  • Rather this circuit can be used for hobby projects and suited well for beginners.

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