I'm trying to find the easiest and intrusive way to detect current on a doorbell line, so I picked up a couple of these Allegro Microsystems ACS756KCA-PFF-T linear current sensors. Their web page for this is at http://www.allegromicro.com/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/ACS756.aspx and the datasheet is here:
I'm a total n00b at electronics. I chose this particular part because it seemed from the datasheet that it would be fairly easy to connect it to an AVR microcontroller to get a good read on whether or not the doorbell line had current or not. Alas, I get minor fluctuations in the signal, but nothing to write home about.
I would put a schematic of what I did up here, but I exactly mimicked the one in the datasheet, so you'll see it there. I connected the output of the sample circuit to an ADC pin on my AVR MCU according to the way the sample schematic is shown in the datasheet for doing so. I connected a 24VAC Ikea lighting transformer with a little halogen bulb to the AC pins on the IC. When the light was off, the ADC reported a value that hovered around 500-ish. When I turned on the bulb, it might have climbed to 520, but usually didn't do much of anything. I tried a couple of the ICs, but got identical results. If I put a neodymium magnet next to it, I could get a change, so it seems the Hall effect part of it was functioning to some degree.
I was starting to think that perhaps the current that the little halogen bulb was drawing wasn't impressive enough for the seemingly super-beefy IC. To be sure it wasn't just that I was not giving it enough juice to read, I connected a 110VAC lightbulb through the sensing leads. Didn't see much difference there, either.
Could anyone with greater brain power than I have take a glance at the datasheet and tell me if they see anything that stands out that would make my setup not work? I know I'm missing something, but I'm afraid I've just been staring at it for too long or this is WAY over my head, even though the datasheet makes it look like it should be simple.