I am trying to come up with a proximity sensing design that triggers an external interrupt on a MCU in a battery powered system (no need to sense distance). If a hand waves over the device (~<5cm distance), it will trigger a wakeup on the MCU. So I am thinking of a darlington transistor based design. However, the tricky part is that the PCB will be enclosed in an aluminum "wristband", which the user will wear. This might cause some interference - but as long as the waving hand overcomes a set threshold to trigger the interrupt, it's fine.
Any ideas/suggestions on how I can go about designing the circuit?
Thanks in advance!