The link provided by Randy Moe is instructive. Up to that point in the thread, I was just shaking my head in skepticism. The first part of the Lomo article shows the shortcomings of the sound method. How are you to measure a time from this kind of plot? Why were the red lines placed here rather than there?
Attachment 183725
The schematic with a light sensor (phototransistor) is better than similar ones seen elsewhere on the internet, because the current path is closed inside the schematic itself, not inside the (unknown) inside of the phone's or computer's soundcard. What is also good is the fairly low value of the resistor 4.7kΩ, ensuring the response is relatively fast.
Attachment 183726
In case you must use a microphone or line input that is not powered, you can include the power source as follows:
Attachment 183727
One drawback of soundcard inputs is they have a high-pass filtering, in practical terms, they have a series capacitor, which is why even the better Lomo schematic produces this kind of response, instead of the expected "brick" shape; nevertheless good enough for measuring times. I found that high-pass filtering sometimes more pronounced on the mic input than on the line input, presumably to prevent "proximity pop". And a Terratec external USB sound card to have more extended low-frequency response.
Attachment 183728
The ideal circuit uses a photodiode operated a zero-bias, an op-amp wired as current-voltage converter (transimpedance), and a hacked soundcard with a short-circuit across the input capacitance. Or a digital scope, if you have one.
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