I am working with the 9V sound recording module from Radio Shack. It comes with an electret mic, but I am trying to figure out how to connect headphone output or computer line out output to the mic wires (if possible)

Link to device on Radio Shack's Web site

When in recording mode, the voltage to the mic seems to be about 3.3 volts.

Is there a circuit to make this connection and have sound files 'played' to the sound module from an mp3 player, for example?


1 Answer 1


Manual here such as it is.

Disconnect microphone wires AFTER noting which wire goes where.
A photo that is detailed enough and sharp enough to allow you to tell is a good idea.
Putting a copy of the photo on disk somewhere with a file name that tells you where it is "is wise" Emailing a copy to yourself using a persistent online email account (eg gmail and many more) with key words in the message and subject line is overkill and latterly often welcome.

Connect 2 wires to where mic went with a 100 uF electrolytic cap in one wire.

Try connecting phone or line audio source to the above input - start with volume turned down, and turn up progressively while listening to result.

If level is far too high add two resistors to form an attenuator. Value shown below are a start - abut 11:1 attenuation.

Attentuation = R2/(R1+r2) if capacitor impedance is small compared to resistor values.
Making values too low (eg 10 Ohm, 1 Ohm) loads input excessively and capacotor impedance may be too high. .
Making values too high eg 1 megOhm/100k causes impedance matching problems at recorder.
R1 and R2 could be a single eg 10k potentiometer with output at wiper.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thank you. In the circuit provided, is the mp3 player on the left side, and the sound recorder on the right side? \$\endgroup\$
    – gec
    Jan 26, 2014 at 2:05
  • \$\begingroup\$ okay, I just put this together and it works pretty well! I don't understand enough audio to understand exactly why it works, and it seemed like pushing 3V back into my player was a bad idea. \$\endgroup\$
    – gec
    Jan 26, 2014 at 3:21
  • \$\begingroup\$ @gec, the capacitor blocks DC and only allows the AC component to pass so the above is safe. You should be able to check with your multimeter that the 3.3V 'disappears'. \$\endgroup\$
    – PeterJ
    Jan 26, 2014 at 5:06
  • \$\begingroup\$ +1 - Hi Russell, an excellent answer as is to be expected. \$\endgroup\$
    – Oli Glaser
    Jan 26, 2014 at 5:39
  • \$\begingroup\$ Link to the manual now is dead \$\endgroup\$
    – Qqwy
    Dec 4, 2017 at 11:24

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