What sensor would you use to detect somone stepping on a paving stone?

[InsertCoffee]

I'm trying to create a keyless lock for my shed. Instead of a numeric keypad, I thought it would be fun to use the paving slabs in front of the sheds as 'keys', so you have to step on them in a particular sequence to unlock the door. (I already have the electronic lock and I'm happy to interface it via a microcontroller / raspberry pi / etc)

I thought a load cell would be OK, but maybe there is something simpler I'm not aware of

Sensor requirements:

• Cheap (it's only a fun project)
• Weather resistant
• Able to detect a large load (me!) but not false positives via cats / rain / etc

Any thoughts?

 

[mrinalmani]

Load cell is simple, but not really that cheap.
Perhaps a pair of infra-red emitter/ detector pair being obstructed by some mechanism each time you step on the panel. But this may be too mechanical.

 

[InsertCoffee]

Load cell is simple, but not really that cheap.
Perhaps a pair of infra-red emitter/ detector pair being obstructed by some mechanism each time you step on the panel. But this may be too mechanical.

I saw some info about DIY force sensitive resistors, using conductive foam and metal plates, but I'm not sure how long lived they would be. I don't want to be digging up paving stones very often...