How The 'Sound Trigger' Works

The Sound Trigger circuit consist of six distinct stages which most of the stages are coupled to the following stages and most stages are coupled to the following stage through a capacitor.

STAGE 1 : Microphone

An electrets microphone is used in this stage, MIC1, which depends for its action on the changes in capacitance that occur between a fixed plate and a plate that is being vibrated by sound. This type of microphone has a permanent charge across the capacitor, produce by heating the dielectric during manufacture while maintaining a strong electrical field between the plates. After a while, the microphone will be cooled and the electric charge remains.

This microphone also includes f.e.t.amplifier and requires a current to power it. After the sound is detected, there will be a voltage signal generated across the microphone and the signal will passes across the capacitor, which will bring us to the second stage, Inverting Amplifier.

STAGE 2 : Inverting Amplifier

The amplifier has an f.e.t. inputs and it is used in inverting mode with its specified gain set by the ratio of resistors. In this stage, the trimmer potentiometer, VR1 will adjust the voltage at the non-inverting (+) input to make it equal to the steady inverting (-) input after the sound is absence. Then, the output voltage of the amplifier (pin 6) will passes across capacitor (C2) to the next stage.

STAGE 3 & 4 : Diode Pump and Trigger Switch

The diode pump will rectify the signal and to produce a cumulative effect. The diode pump is used to avoid the cancelling triggering when the voltage swings to negative. Only when positive pulse generates, it can switches on the MOSFET TR1 (trigger switch) via capacitor C4.

           

STAGE 5 & 6 : Timer, Lamp Switch and Buzzer

When the TR1 is switched on, the timer which is IC2 555 will automatically trigger. This is wired as a monostable multivibrator which then produces high output pulse from pin3. The length of the pulse from IC2 (timer) depends on the value of resistor in this stages, R6 and C6. The value of the resistor is already set for timer: 50second to 60 second, which the time for the lamp and buzzer will come on. The changes of the length of the pulse can be made by choosing an appropriate values for R6 or C6. 

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