Smoke detector circuit

 A smoke detector circuit is one which is used to detect un-burned fuel smoke created due to burning process. Such circuits are useful for building safety and house hold safety requirements. Such circuits are compulsory in some countries to be installed in the buildings if you are going to build new building or house. Without such alarm circuit installation, the authorities would not pass your building and will now allow residents to live in such building due to fire hazards and lake of fire safety.

There are many kind of fire alarm circuit available in the market. Some uses special sensors and detectors to perform the job. Whereas some uses very basic sensors for such kind of job. The below circuit is also using a very basic concept and very basic circuit design for the purpose. It uses an LED and an LDR to detect smoke passing between both of them. This a must for this circuit to fix it in the path of smoke to pass the smoke between the led and ldr. Otherwise if smoke is not passing between led and lrd the circuit wouldn't detect the smoke and would not alarm in case of fire or smoke. Such circuit have a beauty that it is very simple and very low price and also is consuming very low power from the power supply or from the battery. There are some other kind of sensors available in the market which are a bit expensive and also they are consuming too much power form the power source. Such circuits details article would also be available here on this blog.

Smoke detector circuit using basic sensors and components

In the above circuit when smoke is passed between led and ldr, the ldr resistance changes due to change in the light intensity passes through the smoke. The ldr resistance decreases with more smoke. The would decrease the base current of the transistor. The transistor is connected here in this circuit in common collector configuration. Then the output of this transistor is fed to the cob IC which is a pre amplifier for the small signal received from the transistor. The first VR which is 100K in value under the ldr is used to calibrate the circuit. The output of the cob ic is fet to the ic2 which is a power amplifier and is TDA2002 though another VR which is also of 100K value. The output of the power amplifire IC TDA2002 is fed to the loud speaker which is of 8R impedance through a capacitor which is of 1000u and 25v value. The rest of the components resistance and capacitors are making filter network for the output of the audio power output IC TDA2002. The circuit is powered up from a 9V battery or a 9V PP3 battery. Power supply is off course better because battery would drain up with the passage of time. The other way is to use a rechargeable battery with appropriate charging arrangement to make the circuit battery powered and also free of battery replacement tension.