Inject tone into computer

Question

I'm using the circuit at the bottom of this post to generate a triangular wave, which I'd like to inject into a computer's microphone jack.

My computer has a single audio jack. When I plug a TRRS splitter into the jack and run a male-to-male (TRS) stereo line into the microphone side of the splitter I observe that there's 3V between the tip and the sheath as well as the ring and the sheath, while there's 0V between the tip and the ring. This suggests to me that this is a mono mic jack.

And herein lies the problem. The circuit I have looks like it requires 3 lines: ground, power, and signal. But the microphone jack looks like it only provides a power and a ground.

How can I connect adapt the circuit to drive the mic input?

Answer 1

The mic input has a signal and a ground. The signal line also outputs a 3 V bias voltage supply, to supply power to some types of microphones needing this power. This is a very weak supply, provided through something like a 2.2k resistor.

For you to feed a signal into the microphone input, you can simply connect the output of your circuit straight into the mic input (as long as it's output voltage is not too high - I haven't calculated the output voltage of your circuit). The op amp has enough drive capability to sink the 3 V bias voltage.

For good measure, you could add a coupling capacitor in series with the signal. Then your op amp won't need to sink the bias voltage supply.

You obviously need to supply the 9 V power for your circuit from elsewhere.

Answer 2

Microphones need bias supply (the power) and the same signal is used for audio. Just isolate op-amp output with a capacitor so there is no DC current flow and it should work.

Just note that microphone signals have very small amplitude, much less than 1Vpp, so most likely you have to attenuate the triangle wave output before connecting to PC microphone input.