Mining Plant DIY ANY Sensor OR INSTRUMENTATION Concepts/IDEAS

I need a set of original, build-from-scratch ANY SIMPLE SENSOR OR INSTRUMENTATION-system concepts that can realistically be deployed in a mining process plant for ongoing monitoring duties. Rather than... (Budget: $30 - $250 USD, Jobs: Arduino, Electrical Engineering, Electronics, Instrumentation, PCB Layout, PLC, Product Design, Raspberry Pi)