This is a project of a smoke detector circuit. The circuit can detect the smoke produced in the result of burning of anything. We can also call it optical smoke detector because it is using an LED and LDR to sense the smoke.
The circuit can be divided into three stages.
The first stage is built with a dark detector circuit using two transistors. The second stage is a timer circuit built with a 555 timer IC and the third stage is an alarm circuit built with UM3561 IC.

Working of the circuit is simple, the LED light will continuously fall on the LDR from a small distance of few inches and when any type of smoke enters in between the LED and the LDR then the resistance of the LDR increases due to which the Q1 become in off state while Q2 become switch on and triggers the 555 timer IC. When the 555 timer IC triggered its output goes high for a preset time period and activate the UM3561 alarm IC and hence a loud audible alarm is produced. When the preset time period of the 555 timer IC is completed the IC automatically gets off hence the alarm will also gets deactivated.

The sensitivity of the circuit can be adjusted with the help of the 100K and 1K variable resistor. The 100K variable resistor is used to control the brightness of the LED and the 1K variable resistor is used to adjust the sensitivity of the LDR.

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Smoke Detector Circuit Diagram