How can I build an AC current sensor on the cheap? It would have one
input (A/C power, 115vAC, 60hz, USA) and two outputs (the same,
unaffected A/C power as well as -5vdc for indication if there were
current draw over a certain threshold, say 200mA).

I've seen commercial (Crestron, AMX) sensors like this but they're
$200, which is probably something like a 5000% markup. Are there plans
online for this sort of thing? Thanks!

---

Clarification of Question by ostranenie-ga on 15 May 2006 08:09 PDT

Actually it looks like what I want is a Hall Effect sensor and IC in a
clamp form factor, like this - any ideas?

http://upload.wikimedia.org/wikipedia/en/thumb/9/95/HallEffCurrentSense.jpg/250px-HallEffCurrentSense.jpg

Does it light up or beep when it detects power?

Is it pluged into the circuit or is it detecting my magnetic flux or hall effect?

Is it in an all weather case?  or a cigar box?

1. It could do either, that's what the 5vDC is for - bleeding off the
house current to power a light, or buzzer, or pin on an rs232 port.

2. It's plugged into the circuit. Imagine a box with two three-prong
AC plugs ("in" and "out") and two terminals for DC output, normally
low. When there is current above a certain threshold the DC output
goes high.

3. Doesn't need to be weatherproof, just a regular enclosure is fine.

Actually imagine a box with a male AC plug (for input), a female AC
plug (for output) and two terminals for DC.. that makes a lot more
sense

Warning, you could get electrically shocked in trying to build this project.
You should have an experienced person with you.

I saw an article in an electronics magazine about how to 
measure AC current of small appliances.  Basically, they
used a small transformer with a primary winding (few turns 
of heavy wire) and a secondary windings (many turns of smaller
wire).  An AC male plug has two prongs that plug into the 
AC wall outlet.  One prong connects to the common (ground)
side of the outlet and the other connects to the hot side.
The male plug prongs connect to the two wires of the AC cord
that leads back to a small plastic box where the female AC socket
is mounted.  The common (ground) wire in the cord connects to 
the common (ground) side of the female AC socket.  The hot wire
connects to one primary lead of the transformer.  The other 
primary lead of the transformer connects to the hot side of
the female AC socket.  When the male plug is connected to the
wall socket and a "turned-on" 60 watt lamp is plugged into the 
female AC socket on the small box, the transformer secondary wires 
will produce a voltage that is proportional to the amount of AC
current that is flowing in the primary of the transformer.
You must have a transformer with very few primary turns and a
secondary with the correct number of turns to product the 
desired AC voltage over a certain load (wattage) range.
The LED indicator could be powered by the secondary voltage.
That could be as simple as a LED and a 1,000 Ohm series resistor.
Since very little power is being drawn from the transformer's
secondary wires, it is almost as if the transformer is not 
in the circuit.
Hope this helps.   :)