I was reading Hack Into a Timer AC Socket and when I got to this paragraph:
The design firstly have a RC buck? (not sure how to call correctly in English) circuit to step down the voltage from 220V AC to 5V AC, a 0.33uf safety capacitor (yellow one) used here, accompanying with a 1M discharge resistor. And after these two there is another current limited big resistor to prevent pulse and shock.
I was curious - I didn't even realize you could change AC voltage levels without a transformer (or at least not cheaper than with a transformer).
I understand that a resistor will lower voltage for a given current, but I would have expected AC mains to be too much power for a cheap resistor. And I don't get what the capacitor does - even if it is helpful here to get smoother DC power, wouldn't it interfere with anything else on the same AC line?
WARNING - ALL PARTS OF THIS CIRCUIT SHOULD BE CONSIDERED TO BE AT AC MAINS POTENTIAL AT ALL TIMES.