Current Flowing Indicator.

[Nick]

The standard way of doing this is to insert an anti-parallel diode or two and to stick the LED over that - this leads to a small voltage drop but is VERY sensitive - I designed a master/slave power strip around this a while back - here is a schematic of two options in an LTspice IV simulation... I built the version on the left into an IEC 320 power strip and it works great - automatically turns off my Quad 405-2, which doesn't have a mains switch when the pre-amp is turned off.

Note the "multiple diode" solution is asymmetric in the diode drops it inserts (either 1 or 3) whereas the "multiplier" solution is one drop in either direction - note that the "multiplier" solution won't work with DC.

Note that in my physical implementation, the Dled was a led inside a solid-state relay (I used a Sharp S202S02) that was used to zero-crossing switch the slave on or off depending on whether there was current flowing in the master line.

These solid-state relays are very cheap indeed (mine came from Farnell) and are excellent smart switches...

EDIT: Are you charging SLAs? If so, I designed a smart charger for them a while back which might be helpful (does trickle, overcharge warning etc.)... never wrote it up properly, but here's the schematic http://www.desmith.net/NMdS/Data/SLA%20Charger.pdf