I am trying to modify this circuit to be activated by a smartphone headphone output. Do you think it is possible?

http://www.kitsrus.com/pdf/k126.pdf

Thank you,
Tim

 

you will have to modify the input circuit, the electret micorphone does draw a little current, and that biases the + input of the amp.

 

I imagine that driving a transistor at its base to bias it in the same way as the electret mic would. Any suggestions?

 

electrets use fet's for the internal amp. adjuxgt bias accordingly.

 

If your using an headphone output, i would omit the first stage of the op amp, and input directly into C3, that should give you a dc voltage across C2 depending on the level of the headphone volume.

 

I am trying to modify this circuit to be activated by a smartphone headphone output. Do you think it is possible?

http://www.kitsrus.com/pdf/k126.pdf

Thank you,
Tim

Replace the mic with an identical resistor to R8 - that biases the op-amp input to 1/2 Vcc, you need a DC blocking capacitor say about 0.1 to 0.47uF. Its probably a good idea to include a load resistor on the phone side of the blocking cap, probably about 32 or 64R - but the DC winding resistance of a headset as measured should be close enough.