So I built this device to spec -- AA battery, phototransistor recovered from an old mouse, and a mono jack. I'm trying it on my PC now using Audacity software, and it just barely responds to light. To make sure I assembled it properly, I connected my voltmeter up to each contact on the mono plug, and shined a light at it. The meter pegged. Is there some way to boost the signal? Another design I saw was the same except that it added a 4.7k resistor. Will that help?