Neat little ISD1820 voice recorder. How do I make an Aux line in?

Thread Starter


Joined May 8, 2011
See pictures below. I bought a bunch of these on Ebay they work pretty good. But the microphone is low quality. What I want to do is ditch the microphone and use the headphone jack on my computer as a source to record sound. I believe I can remove the microphone and R3 and attach the input directly to C4. As I understand it I might need a voltage divider to attenuate the signal from my headphone jack but I have no clue how to do it or the resistor values. Could I just use a very low volume on my computer instead? Do I need a resistor to match impedance?



Joined Nov 30, 2010
This chip wants Mic in less than 100 mv P_P.
I would guess about 50:1 would be the maximum attenuation you would need.
With a volume control in the computer, maybe 5:1 max attenuation.
Zin is 10K, so 39k in series with the computer or a 1K volume control pot after a 3.9K resistor from the computer.
Make sense?