Measuring a Constant Pressure

Yesterday I decided to make a board that senses when a phone is sitting on top of it.

I was thinking along the lines of load cells and piezoelectric sensors, since I haven't used the latter before. However, I realized piezos don't measure a constant pressure. The voltage generated by a static pressure eventually fades to 0 as the charge dissipates. Good thing to know!
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A piezo would only detect me initially placing the phone down, but wouldn't know if it was still sitting there.

I started to consider load cells, but didn't want to relive the strain gauge nightmare. They're bulky and overkill too...

Wait, I can just think bigger.
My goal: Detect presence of a phone.
This does not mean I have to stick to measuring pressure!

When I place a phone down, not only does it exert pressure, but it also blocks the light underneath! Yesterday, I reminded myself of the existence of the humble photocell.

Y- axis is charge. Look at that decay!

Useful Resources: https://www.pcb.com/resources/technical-information/introduction-to-force-sensors