Help: mic amplifier transmitted via infrared, modulated in PWM

[cze_guevarra]

Hi. I've been working on this since our project was given and i could find help from any one else.

I need to build a circuit that can amplify a mic, modulated in PWM, transmit through infrared then receive, and demodulate.

This is so far what my knowledge knows:
**broken link removed**

And this is my schematic:
**broken link removed**

I know there are still a lot of bugs in this design. I really don't know how to design circuits myself.

Comments, suggestions, criticisms are welcome. Thanks.

PS: IR LED & photo-transistor are just labels. not wired.

 

[audioguru]

Hi Cze,
You do not show which type of microphone you will use. An electret mic is common but needs to be powered from a resistor from the positive supply.
Your opamp is not powered and is not biased. Its gain is too low at 3. Its gain should be about 100.
The 555 is oscillating at only 530Hz which is much too low for voices as modulation.
The ground pin of the 555 is not grounded.
Your photo-transistor should have some gain.
The lowpass filter is too simple and will filter out most speech frequencies.
The speaker must be driven from a power amplifier. Try an LM386 little power amplifier.

 

[cze_guevarra]

what then is the needed frequency for the 555 timer?
it's PWM, right? how can i adjust the frequency?

 

[audioguru]

Class-D audio amplifiers use PWM for high efficiency (low heat).
Their PWM frequency is hundreds of kHz.

If you want telephone-quality voice sounds then your PWM frequency should be 30kHz or higher.

The datasheet for the LM555 has the formula for the frequency and a graph showing various capacitor and resistor values making a frequency.

 

[Nigel Goodwin]

But the 555 is a really poor PWM modulator anyway, very low quality and restricted range.