I was browsing one of my electronics catalogs and I came across the Fluke 1AC-A1-II Volt-Alert AC Non-Contact Voltage Tester, so I bought one to try it out. Some of my student that work with AC a lot really liked it. However 25 bucks a student was kind of expensive, so I came up with this design for under $2.00.
Clarifications to schematic, forgot to add a power switch (doe!), Probe connects to clock pin 14
How it works:
The heart of this circuit is the CD4017 Decade Counter/Divider with 10 Decoded Outputs. Basically when a AC signal is in close proximity to the non contact probe the counter counts from output 0 to 9 in sequence one single output on at any given time then lights up the MAN74 Seven segment display in order from segment a,b,c,d,e,f then resets back to a producing a chase pattern. The decimal point on the MAN74 is a power on LED indication. The non contact probe is made from insulated 22AWG wire in a loop.
Additionally this circuit will also detect any clock or data line in a digital circuit. If you want to control the sensitivity you can connect a 100K potentiometer to the probe to ground, the closer the wiper is to ground the less sensitive it is.