Non contact detection of mains high voltage from a low voltage circuit - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: April 07, 2023, 02:42:35 pm
content: Can anyone suggest the best way to detect whether a mains wire is live or not from a low voltage circuit. Part of me had thought of using a hall effect sensor beside the wire, but that would only work when a significant enough current was flowing, whereas I'd be hoping to detect the presence of the 50Hz mains AC voltage whether it is powering something or going to an open circuited switch. I know that checking an "antenna" for 50Hz signals with lengths at large distances would have similar effects to short sections nearby). I'm thinking of a circuit I can put millimetres away from a typical UK style mains cable (the cable between the plug and a device, not the wires behind the wall, I'm not sure if they usually contain an actual shielding conductor or if earth stays as a separate strand, twisted round but not shielding the live/neutral pair) just to know if that cable is powered or not. I need to know reliably if the mains wire is powered or not, but don't need any analogue type measurements of actual voltage or current.Thanks

author: tszaboo
author_rank: Super Contributor
timestamp: April 07, 2023, 03:01:56 pm
content: If you put a small length of wire into a NOT gate, that can quite reliably pick up 50Hz noise. And from that you can do many things. Sensitivity can be adjusted with ~Mohm range resistors. Play a bit with the values. Or just buy a contactless meter, they do exist.

author: Ian.M
author_rank: Super Contributor
timestamp: April 07, 2023, 03:05:34 pm
content: Mains flex in the UK is usually round with two or three cores depending on whether on not the cable has a ground. The cores have a slight twist round each other, but the direction and rate of twist is not well controlled. Some two core cable is flat twin.Only one wire in the cable has significant voltage with respect to ground, so you'd need to surround the cable with at least three (and preferably at least six) high impedance capacitive pickups and check if any of them are picking up 50Hz. Put a grounded overall screen round the sensors and cable so external stray E fields don't trigger it.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 07, 2023, 04:01:40 pm
content: Google is such a wonderful resource:https://www.instructables.com/AC-Detector-Circuit-With-Awesome-Application/

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 07, 2023, 05:48:52 pm
content: Another simple circuit here.MOD: You could bypass the buzzer and change the 220 ohm resistor to a 1K if you wanted just the LED indicator.

author: Infraviolet
author_rank: Super Contributor
timestamp: April 08, 2023, 05:02:13 pm
content: Thanks, I guess it really is just a matter of using an antenna wire and some transistors to amplify the signal from it. I'll perhaps try a bandpass filter after it just to ignore any signals far away from 50Hz, adjusting the sensitivity and gain should be simple enough to feed the outcome of this detection to a microcontroller project.