Choice of sensor for detecting touch (on/off)

Question

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.

Answer 1

I would be thinking in terms of running a grid of some very fine wires stretched tightly across the inside of the outer glass — single strands of AWG40 spaced every 3-4 inches, horizontally and vertically, would be nearly invisible.

Scan the wires individually for increased capacitance; if the wires that cross in your "target area" show increased capacitance at the same time, trigger your event. Such a system would allow the creation of multiple "hot spots" of various sizes, each triggering its own event.

Answer 2

You might be interested in IR or ultrasonic proximity sensors, much easier to install I think. They sell them in ready to go modules at places like sparkfun or adafruit as linked above.

They have no problem sensing movement up to 80cm for IR and 25ft for ultrasonic. You could put this on the inside of your shop facing the "touch here" location and when an outside user placed is hand in front of it you'd get a huge signal change that you could use for whatever your application desires.

It might take some signal processing to make sure the change was the right magnitude to prevent people just walking by from setting it off but lots of these sensors advertise no dead-zones which would make this a simple task.