alt Hacker NewsHave you considered taping a sensor to the light, or measuring the electrical current flowing through the power cord? Both should be a bit more reliable. The idea of messing with mains power is scary at first but with basic precautions it's fine, and I think you can buy current meters with various interfaces if you aren't comfortable.
Why would I buy power meters and mess around with indirect signals that don't measure what I want (how much time is left on the cycle) and instead just tell me whether it's running?
I already have an old webcam and raspi I'm not using, and they measure exactly what I care about.