This Smoke Detector Circuit sense the smoke in the air and displays the level of smoke in PPM (parts per million) on LCD display. This circuit triggers the Buzzer when Smoke level becomes higher than 1000 ppm, this threshold value can be changed in the code according to the requirement. This circuit mainly uses MQ2 Smoke/Gas sensor and Arduino to detect and calculate the level of smoke. MQ2 gas sensor is also sensible to LPG, Alcohol, and Methane etc. In the schematic diagram, we have a comparator circuit for comparing output voltage of smoke sensor with preset voltage (output connected at pin D7). Also smoke sensor output is connected at an analog pin of Arduino (A0). Buzzer is connected at Pin D9. And LCD connections are same as Arduino LCD examples that are available in Arduino IDE (12, 11, 5, 4, 3, 2). Remaining connections are shown in the circuit diagram. For the complete DIY project and step by step guide to build this project, check out the [project page on circuit digest][1]. [1]: http://circuitdigest.com/microcontroller-projects/arduino-smoke-detector-on-pcb-using-mq2-gas-sensor