• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Help: mic amplifier transmitted via infrared, modulated in PWM

Not open for further replies.


New Member
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:

And this is my schematic:

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. :)


Well-Known Member
Most Helpful Member
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.


Well-Known Member
Most Helpful Member
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.
Not open for further replies.

Latest threads

EE World Online Articles