Sensing AC Voltage

J.Apple

I want to build a STAMP based project that will read control voltages (24V AC) on a standard household thermostat and take certain actions based on those readings. My question is how do I reliably, and cheaply read those lines while keeping the lines electrically isolated from the STAMP? I have looked at 24V coil relays as one possibility, but they cost a bit more than I woud like.

JonnyMac

If you're just looking for the presence of AC, you could use an opto-coupler like the H11AA1. It will give you a pulse every 8.333 milliseconds (indicating the zero-cross of the aC) -- no pulses, no AC.

Mike Green

There are optoisolators made for use with AC. You can also use a standard optoisolator. You'll need an appropriate series resistor for the voltage involved and the Stamp will see pulses (for the positive or negative half cycles), but you can program for that.

If you need a steady on-off signal, you could rectify and filter the AC and use that to trigger the optoisolator or you can filter the output side of the optoisolator to smooth out the pulses.

J.Apple

Thanks Mike & JonnyMac.

As a long time mainframe/PC programmer that is new to the electronic interfacing side, I appreciate the help of being sent off in the right direction. While I do not know much about optoisolators / couplers, with the little that I have read so far, I am comfortable that I will be able to do this.

Thanks again