You've probably seen them. Those coaster or puck size electronic things they give you at a restaurant to buzz you when your food is ready.
A seemingly common element of their design is that they have a pair (sometimes more) of metal posts that (apparently) run through the device. These facilitate the stacking of the devices in to a pile on a charger. I assume the current just runs up the common bus created by the posts as they are aligned to be on top of each other.
Another feature of the pager is that they occasionally strobe an LED. Mostly I think just to let folks know the things are alive and charged.
However, when combined in to a stack (both on and off of the charger), the pagers will orient themselves so that when the pagers strobe individually, they will do so in order, say, from bottom to top. Giving the entire stack an attractive, animated light display.
So, my question, is how do the pagers self orient? Given that their only connectivity is through the pair of charging posts? Now, they may have an optical sensor to tell them they're underneath something. The may have a magnetic sensor to tell them they're near each other. But I'm still not sure how they coordinate to determine not just who's on top or bottom, but, indeed, their place in the stack.
What are some mechanisms that can facilitate this kind of connectivity and ordering?
Addenda responding to comments:
The animation behavior happens on and off the charger. It's very possible the LED blink could be a PING of some kind from the central server ("not only is the puck alive, but the server can sense it as well"). It's possible that the pucks can talk back to the server. How would the puck know the ID of the device is was placed upon to relay to the server (or was placed upon it). If the server is driving the ping, then it's transmitting the ping packets in order, with the animation mind. If you have several stacks of the pucks, you will have several animations (though I can't say if they happen simultaneously, this is all casual observation in a restaurant). It's very possible the devices might have other optical or proximity sensors, but seems excessive to do that just for this feature, which suggests they manifest it with the hardware they already have.
No, I'm not familiar with the 433Mhz protocol.