The LTV-8141 that GRJ uses in his circuit should work. 65 cents at DigiKey...plus shipping.
http://www.digikey.com/product...-8141&vendor=160
You still need the resistor as GRJ shows. There are 4-pins on the LTV-8141 (GRJ's diagram shows 6-pins but that is incorrect per the DigiKey datasheet link). 2-pins are inputs going to the back-to-back LEDs. 2-pins are outputs which would replace the photoresistor on the eBay unit. The hookup would look something like this:
The LTV-8141 is a so-called AC optocoupler - it has 2 internal LEDs to handle either polarity of input. You can also buy DC optocouplers which have only 1 internal LED but they require the correct DC polarity input to work.
If you go this route, the detector side of the optocoupler is implemented with a photo-transistor which is a semiconductor and hence has polarity. It must be hooked up with the proper polarity so inquire again. The eBay module as supplied uses a photo-resistor which does not have polarity and can be hooked up either way.
