I would like to detect a user's touch on a shop window (at a specific area, there would be a 'call to action' marking on the window, i.e. 'touch here')
Any sensor to detect this must be contained within the shop.
There is a 15cm air gap between the shop window and another internal window (behind which any potential sensor could be discreetly hidden as the internal window is tinted).
Both window panes are made of a roughly 10mm thick hardened glass. The external window area is roughly 3mx2m whereas the internal is roughly 2m x 0.3m, more like a rectangular panel.
I have been looking at using a capacitive proximity sensor such as the STM8T143, which according to the specs is capable of a range of up to 20cm but I'm not sure how the 2 panes of glass will affect this sensitivity? I know glass has a higher dielectric constant than air, but will the boundary between 10mm glass <-> 150mm air <-> 10mm glass be too much for the sensitivity of this sensor?
If a capacitive proximity sensor wouldn't be possible ,or recommended, in this scenario, could anyone recommend another way of detecting a user's touch, or very close proximity to a target area, which would fit the above use-case? The sensor would not have to be behind the interior window it could be in the air gap, but can't be outside.