Vinyl cut tags and their readers

Vinyl cut tags are basically an inductor linked to a capacitor. The tag then has its own resonant frequency. When an antenna is being powered by a square wave that has the same frequency, some of the power will be wirelessly transferred to the tag. The presence of the tag can be measured by means of this voltage increase.

The resonant frequency is determined by the amount of henries in the inductor and the amount of farads. The resonant frequency is equal to the square root of 1 over L times C, or the amount of henries times the amount of farads.

Because it is unclear exactly what the inductance of our inductor is, we used the network analyzer to determine the resonant frequency of the tags. You can see a drop at the resonant frequency when you hold the tag paralell to the antenna.

Eventually we would not want to have to use a machine that costs hundreds of thousands of dollars to sense the tags. So instead, we first replaced the network analyzer with a function generator that produced a square wave. When the square waves is at the same frequency as the resonant frequency of the tag, the antenna will draw more power because part of the power is transferred to the tag.

By scanning through the different frequencies with a microcontroller generating the square wave, we can detect when a tag is being read by measure the difference in voltage over the inductor. We however need to measure the difference as fast as the square wave is being generated. To do this, we are taking an approach similar to Neil's Hello Step board, where the A2D is being fired once every so many periods to be able to generate one graph.