limitationsThere are a few limitations I wanted to point out quickly. Sadly, the ID-2 / 12 /20 , as with most readers, can only read one tag at a time. In fact, having 2 or more tags in the readers range will cause it to not read any tags at all. So, if you need to detect the presence of many tags, you will need to figure out another way, get a different sensor, or buy a bunch of these. But… more than one reader right next to each other can cause none of them to work due to interference. You can wrap the bottom and sides with foil to help cancel the interference (be careful not to short the reader with the foil). Lastly, the range, even on the ID-20 is less than 4in real world. That means you can't easily use these for detecting finish times in a race or anything like that.