There are 2 types of proximity sensors [switches]:
Inductive: These build a magnetic field around the sensor. Inductive proximity switches will only trigger when in the presense of a magnetic object (iron, cobalt, or nickel). Hint: This would be handy for detecting when you are over the steel mesh.
Inductive Proximity Sensors
Capacitive: These use an electric field to detect metallic and non-metallic objects. Every element has it's own dielectric constant (k). The sensor uses a reference material and detects a difference in capacitance. These can be used to detect other robots, containers, etc.
Capacitive Proximity Sensors
Remember, you can only use the sensors from
Digikey or
Future-Active. If they carry that manufacturer and model of sensor, then use it if you have them lying around.
If they are non-powered, they don't count toward the $200 electronics limit.
If they are powered, you have to count them.