I would like to find or create a small low-power proximity detector to tell a device when another device is near. This would be coupled to a small microcontroller like the AT-tiny 45. It doesn't need much resolution as it will only influence right and left turns, so I could just have two directional detectors and the device will turn in the direction of the strongest signal. Ideal range would be 5cm-1meter, but I'd be happy with 20cm-50cm.
The idea of using an IR LED and photodiode could work, but I'd prefer an option that works better in the sun, so am looking for other options. I had initially thought of making a tiny antenna and EMF detector antenna, but the proximity resolution is really poor. I'm wondering if someone can suggest ideas relating to small solid state magnets and pickup coils.