Noise or Sound Sensor Alarm Circuit using IC 555

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Source : EFY

Sensor circuits are widely available and are of several types each possessing significant uses. The above sound sensor alarm circuit is one of that kind where it was used to detect the sound signals in the environment. Here a microphone was used as a transducer to convert sound signal to electrical signal and in addition a 555 IC was used to ring alarm when a sound or noise in the environment is detected.


The working of the above circuit starts with turning on the switch S1 and there a LED was added to indicate that the circuit is ready to work. Initially in the absence of sound there will be no electrical signal obtained from the condenser microphone. Whenever the Microphone detects a sound in the environment , it gives the electrical signal which was fed into the Transistor T1 for the purpose of amplification. 

The amplified signals obtained from the collector of the transistor was then fed into the base of the Transistor T2 through capacitor, diode and resistor. The Transistor T2 was wired to work as a switch in the above circuit ,so whenever a signal reaches the base of T2 current flow will takes place from the collector to emitter of the Transistor T2.

In this circuit NE 555 was wired as a monostable multivibrator in which a negative trigger to the pin 2 of the IC gives out high signal and positive trigger gives low signal in the output, read detailed description working of 555 Monostable mutivibrator . So when the transistor T2 is switched on by the incoming signal, it permits current flow from C to E thereby giving a negative trigger to the NE 555. The negative trigger in turn changes the output of IC1 to high state powering the Piezo buzzer to produce sound for fixed duration. The duration was decided by means of R and C values connected to the IC 555. The time duration was given by the formula 

T=1.1 x R x C

Connecting relays at the pin 3 of the IC will enable you to drive high loads and this could allow the above circuit to be used as sound or noise sensor. 


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Frank Donald

Junior Network Engineer at Tata Communications
Frank Donald is an Electronics and Communication Engineer who loves building stuff in his free time. Video games, blogging and programming are the things he loves most

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6 Comments on "Noise or Sound Sensor Alarm Circuit using IC 555"

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Hi this is Jigar.
I want to use this with radio. But alarm circuit should activate when there is no sound come from radio. Can we use this circuit?

Please suggest.


sir,i want a circuit which sense cry of a baby and gives output as a song.pls help this is ma ID

muhammad amir

they not work what is problem


sir i need circuit of 555 to produce noise can u help me my mail id is