For a long time, I have been thinking about making a set of pin badges that signal to each other using an IR pulse, and blink with a visible light. If a few of them were together, they would synchronize their lights, like fireflies.
To do that, I’d want to detect IR, and reject IR that was not from one of the badges (like light from a fire or the sun). The IR receivers that go into TVs do that, and there are super-small versions available for surface-mount construction.
Osram’s SFH5410 looks perfect, but was discontinued, this time in 2010, and without a suggested replacement.
Sharp makes the GP1US30XP series in surface mount packages about 4x4mm. Their sensing direction is normal to the PCB, which is ideal for aiming them away from the wearer. This would be the ideal part, if but it was also discontinued.
For purposes of prototyping, I can use some larger parts that I already have, but the ultimate design will be very small. Someday, perhaps I’ll have them fabricated professionaly and give them away at Firefly.