AC Power Sensing
Warning! Any voltage above 30 is considered unsafe. While the circuits presented here work and are safe, they are not guaranteed to be. Exercise your own caution when using these designs as I will not be held liable for any injury.
When there is 120vRMS AC power present, the voltage at R3 will be near to 5 volts. This makes this circuit compatible with a 5v microcontroller's IO. Very useful as it can directly interface with minimal components. Useful for detecting whether a switch is turned on, or whether there's a power outtage.