An very simple smoke detector circuit can be constructed using a 555 timer circuit and a photo interrupter . The photo interrupter module is used as the smoke detector, while timer 555 is wired in astable configuration as an AF oscillator for sounding alarm via a loudspeaker.

In the absence of any smoke, the gap of photo interrupter module is clear and the light from LED falls on the phototransistor through the slot. As a result, the collector of phototransistor is pulled towards ground. This causes reset pin 4 of IC 555 to go low. Accordingly, the timer is reset and hence the alarm does not sound.

When smoke is present in the gap of the photo interrupter module, the light beam from LED to the phototransistor is obstructed. As a result, the phototransistor stops conducting and pin4 (reset) of IC 555 goes high to activate the alarm. The photo transistor and LED must be housed inside an enclosure with holes to allow entry of smoke. This smoke detector circuit project require a power supply between 9 and 12 volts .

Photo electric Smoke detector using 555 timer

ASTABLEMULTIVIBRATOR

output frequency The circuit is arranged as shown in the figure and the 555 timer is made to work in astable mode and output is connected to the led and if the 555 timer is correct the led blinks continues to produce the continues square waves. The circuit diagram of an Astable Multivibrator using IC 555 Timer is shown in figure. There is no stable in the output (Astable Multivibrator) and its output changes continuously between the high and low states. The output in an Astable Multivibrator oscillates between the two states at a constant rate. As a result, square wave signal is given at its output. For this reason the Astable Multivibrator circuit is also called rectangular wave oscillator. An Astable Multivibrator in digital system is often called a Clock Pulse Oscillator. An Astable Multivibrator provides a very stable As shown in figure. Here, the input trigger pulse at Pin No2 has not been provided. Pin No 2 is connected with the Pin No 6 from where a capacitor C1 is connected to the ground. The discharge Pin, pin No 7 is connected to +Vcc through a resistance R1.The threshold Pin No.6 is connected to the Pin No.7 through a resistance R2. When this circuit is switched ON then the capacitor is discharged and SSSSSSS the trigger input is low. Hence, the output is high. Now the capacitor C1 charges to wards +Vcc through the resistance R1 & R2.When the voltage across the capacitor C1 reaches two-third of +Vcc then the output becomes low and the capacitor C1 begins to discharge. But when the voltage drops to one-third of +Vcc then the output again becomes high. This complete cycle is repeated again and again. The time when the output is high, is called charging time. This is given by: T1=0.693(R1+R2)*C1 The time when the output is low (Discharging time) T2=0.693 R2*C1 Thus, the total time for one cycle T=T1+T2 =0.693(R1+2R2)*C1 Frequency the oscillator Duty cycle of the output wave form can be adjust by selecting the

correct values of R1 and R2. Ratio of the high state time and time taken to complete one cycle is known as the Duty cycle. Duty cycle=T1/T2 =0.693(R1+R2)C1/0.693(R1+2R2)C1 =R1+R2/R1+2R2 *100%. The output of the astable multivibrator can be connected to the buzzer for the indication of alarm indication, connected for relay circuit for alternate switching, and also as a square wave pulse generator.