2 Smoke Alarm Circuit

This circuit  alarms you when smoke is detected. If used well, this can help you prevent fire or prevent fire from spreading and causing further damage.

smoke alarm schematic
Circuit operation
The LED will light up the light dependent resistor (LDR ) and making its resistance very low (as low as 500 ohms). In this case,  transistor Q1 and Q2 are off. When there is smoke in the area, light from LED will be blocked resulting to an increase in LDR resistance. In this case, both transistor are on, and 555 timer starts to generate sound to the speaker.

Part List:
R1-300 ohms
LED-white or any high  intensity LED
R3-100k potentiometer
R2-10 kilo-ohms
Q1-2N222,cs9013, or  similar NPN transistor
R4-1.2 kilo-ohms
R5-1.2 kilo-ohms
Q2-cs9012, any general purpose PNP transistor
c3-2.2uF electrolytic capacitor
R6-1.2 kilo-ohms
R7-5 kilo-ohms
555 IC
9V battery or DC supply
speaker (8-ohms)

Before you finalized the installation, first turn on the circuit and test it. While LED is on, adjust the value of potentiometer R3 until no sound is heard in the speaker.Then try to block the light from LED to LDR with a piece of paper to make sure if it is working, if not try adjusting R3 again.


  1. is it possible to make http://www.youtube.com/watch?v=QFCequolK84 the blinking and beeping i mean by modifying the circuitry a bit

  2. may I ask the resistance of your ldr?