Giorgos Lazaridis writes:
An AC Hum touch sensor is a very special technique, rarely used for switching applications, because it has a great disadvantage. In order to operate normally, an active AC power line has to be near by. More info about this type of touch sensor, along with other types (including the capacitance method), can be found in the corresponding theory page, how the touch buttons work..
DIY AC Hum Touch Button – [Link]