Fire alarm circuit


The works is similiar from previous circuit. the LDR sending the light from lamp. When light from lamp blocked by smoke, the alarm will be activated.

Fire alarm circuit diagram

When there is no smoke the light from the bulb will be directly falling on the LDR. The LDR resistance will be low and so the voltage across it (below .6V). The transistor will be OFF and nothing happens. When there is sufficient smoke to mask the light from falling on LDR, the LDR resistance increases and so do the voltage across it. Now the transistor will switch to ON. This gives power to the IC1 and it outputs 5V. This powers the tone generator IC UM66 (IC2) to play a music. This music will be amplified by IC3 (TDA 2002) to drive the speaker.

The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V ) to UM66 .UM 66 cannot withstand more than 4V.


  • The speaker can be a 8Ω tweeter.
  • POT R4 can be used to adjust the sensitivity of the alarm.
  • POT R3 can be used for varying the volume of the alarm.
  • Any general purpose NPN transistor (like BC548,BC148,2N222) can be used for Q1.
  • The circuit can be powered from a 9V battery or a 9V DC power supply.
  • Instead of bulb you can use a bright LED with a 1K resistor series to it.