Automation Cool homemade utility Gadgets / circuit projects ECE engineering projects Sensor based smoke/gas sensor

Smoke detector alarm circuit | Fire safety alarm

Hello Engineers. Sometimes due to small fires or smokes sources, large disaster takes place. For this, a smoke detector alarm circuit is used to detect smokes or even some of the gases interfering through them. A smoke detector alarm circuit has a smoke/gas sensor in it as the main element. So let’s start with our topic.

Gas/Smoke sensor

There are two kinds of gas sensors/smoke alarms. One of the most common is the Ionization smoke detector and the other one is Photoelectric smoke detector.

Ionization based smoke detector:– This kind of smoke sensor are most common to use. They have a voltage divider inner circuitry. These sensors check gas concentration in the atmosphere. These contain two aluminum metallic pieces coated by tin and mainly enclosed in a steel sieve or mesh. They have a substance called americium-241 that converts gas molecules into positive and negative ions. Due to the law of attraction, negative ions stick positive plates and positive ions stick to negative plates. This ion exchange completes the circuitry.

When smoke enters the sensor, the resistance of sensing element changes and this makes it possible to detect gas.

Photoelectric smoke detector:- In this kind of smoke alarm, there is a straight light beam similar to laser running across the chamber. A photosensor is also there. Once smoke enters the chamber, it scatters the light. some of the light is also scattered towards the photosensor. This then results in triggering of the alarm.

smoke detector alarm circuit

MQ2 Gas sensor

Smoke detector alarm circuit

MQ2 is a Gas/smoke sensor that measures the gas concentration to detect gas. It is also based on the concept of ionization type smoke detector. MQ2 is a gas sensor one of amongst MQ series of gas sensors. The material of MQ2 is SnO2 which has lower conductivity coated by tin and mainly enclosed in a steel sieve or mesh. With the Gas concentration rising, the conductivity of the material also rises. The change in conductivity results in a change in output. When smoke enters the sensor, the resistance of sensing element changes and this makes it possible to detect gas. There are 6 terminals connected to the sensing element. Two leads work to heat element and the other 4 are for output. It can detect many kinds of gases like propane, butane, LPG, Hydrogen, Smoke, etc. A very commonly used smoke sensor is MQ2.


After sensing the change in gas concentration its resistance when decreases, it provides TTL output 5v. This sensor can provide both, digital as well as analog output.


  • LM7805
  • C1 330nf Capacitor
  • BC547 Transistor
  • C2 100nf
  • R1 680ohms-1k ohms
  • MQ2 Gas sensor
  • 12V Relay
  • D1 1N4007
  • BUZZER or you can use any kind of bell or alarm circuit.

Circuit for smoke detector alarm

smoke detector alarm circuit

In the circuit above, when smoke or any gas concentration sensed by the sensor, it gives a TTL(Transistor-Transistor Logic) output. The output pin used in the diagram is AO(Analog Output) pin. The sensor gives 5v analog output to the base of BC547 Transistor, the collector turns the relay on by connecting its other end to ground. Another end of the 12v relay already connected to +12v.

The relay then shifts its COM pin’s connection from NC to NO, where the Gong bell is connected. Then the Gong bell gets access to the AC supply and a loud alarm bell is rang.

I hope you guys like this tutorial. Please comment if you have any doubts. Also, subscribe to our newsletter to get the latest post update. Thank you.

Check out other security based automation projects.

Content Protection by
Aabhishek Sharma is a Computer Science Engineer and a Digital Marketing Trainer and Consultant. Aabhishek Sharma also loves working on Electronics Projects and working with microcontrollers and Designing Home automation Gadgets.

4 Replies to “Smoke detector alarm circuit | Fire safety alarm

Leave a Reply

Your email address will not be published. Required fields are marked *