Amplifier circuit for mic input

Projects General Guidance
1

Hi

I need to amplify the pulses generated by a digital pin for connecting into a notebook mic input. I want to use 5V from the USB so no batteries.

Bipolar transistor or FET?

Any examples available?

Thanks

2

A microphone signal is typically around 10mV depending on the sensitivity of the mic, the loudness of the sound, and the distance from the mic.

You need attenuation, not amplification. I'd start with a 1/100 [u]voltage divider[/u] (2 resistors). Maybe try 10K and 100 Ohms. If the signal is still too strong you can adjust the resistor values.

3

You may also need a capacitor between to pin and the mike input so you are not making the notebook part of the DC circuit.

Paul

4

DVDdoug:
A microphone signal is typically around 10mV depending on the sensitivity of the mic, the loudness of the sound, and the distance from the mic.

You need attenuation, not amplification. I'd start with a 1/100 [u]voltage divider[/u] (2 resistors). Maybe try 10K and 100 Ohms. If the signal is still too strong you can adjust the resistor values.

Thanks it sounds even easier that I thought!

5

Paul_KD7HB:
You may also need a capacitor between to pin and the mike input so you are not making the notebook part of the DC circuit.

Paul

And which is the value? A ceramic or electrolytic cap?

6

rva1945:
And which is the value? A ceramic or electrolytic cap?

Put the capacitor on the feed to the voltage divider, instead of the output. It will allow you to use a lower value, and make it easier to achieve a good low frequency response.

7

Thanks guys, it is working nice, I have 8 PPM channels fed into the mic from a Nano, as intended. Later of course each channel will have the corresponding pulse width according to the inputs (pots, switches, etc.), so far 1500us as an example. I'm making the notebook believe I am using a RC transmitter.

I am not using any capacitor and I guess that maybe causing this behavior: if I move my hand close to the breadboard the signal is corrupted and then I get 4 to 12 channels randomly. Or perhaps that happens because I am using a breadboard with some wires and instead I should mount everything in a shielded case.

Thanks again!