Circuit diagram and description
1. Microphone takes any voice signal or sound over a certain region.
2. Voice signal is a high frequency signal. So HPF filter is incorporated in this circuit.
3. HPF is made by 2 passive elements – 0.1 micro farad capacitor and 100 kilo ohm resistor.
4. Input signal is fed to the HPF for noise reduction. HPF passes only high frequencies and attenuates the low frequencies.
5. Next stage is amplifier. It is made by an Op-Amp LM324N with a resistive network. The amplifier configuration of LM324N consists of a 100 kilo ohm feedback resistor and a 1 kilo ohm resistance.
6. Filter output act as an input of the amplifier. The input is applied to the non inverting terminal of the Op-Amp. The inverting terminal is dedicated for resistive network.
7. The amplification factor is 100.So the strength of input voice signal increased 100 times after amplification.
8. The amplified input experienced a peak detector circuit, made by a PN junction diode 1N4148 with a parallel combination of 10 micro farad capacitor and 100 kilo ohm resistor.
9. Peak detector circuit holds the signal for a few mille seconds. The holding time is depends on the value of capacitor and resistor.
10. Last stage is comparator. It compares the amplified signal with a fixed reference voltage.
11. Comparator circuit is designed using a LM324N Op-Amp with a voltage divider circuit made by a 100 kilo ohm and a 10 kilo ohm resistor. The voltage divider circuit is incorporated here to make the reference voltage 0.45 volt.
12. The amplified signal applied to the non inverting terminal of the comparator. The Inverting terminal is responsible for the reference voltage.
13. When amplified voice signal (input) is higher than the reference voltage, the comparator output goes high i.e. +5 volt. Otherwise the comparator output is low i.e. ground.
14. LED with a 100 ohm resistor used at output end for indication purpose. This 100 ohm resistor is used to save the LED.
15. When comparator output goes high, LED glows. When comparator output is low LED doesn’t glow.
16. The activation of LED indicates the presence of the voice signal.
17. The comparator output is set to low initially.