Line-2-mic for notebook

Modern laptop and notebook PCs almost invariably come with a microphone input but not a line input for audio signals. A reader asked us for a simple circuit that world enable him to connect an audio line signal to the microphone input. We do like the easy ones! Because the microphone input is usually monaural, a stereo line signal requires not only attenuating but also summing to turn stereo into mono. A voltage divider comprising just three resistors (R3, R4, R5) does the job.