There’s a door here in Harpa that has a doorbell, one of those velcro wireless things. At least, that’s what it looks like. I think it’d be fun to move it down the hall a ways, and without their plethora of cameras, they’d probably never know.

There is no way a wireless transmitter like that can know that its base is being moved, or goes out of reach. In a satellite system like that, the base just sits in the other room, waiting for incoming calls.

To bring this image to the web, it’s kind of like web monitoring; you can have a server send out pings to some receiver. But it’s only useful if the receiver keeps track of how long has passed since the last ping came in. It works either way; the receiver can call the server every so often, and complain if it’s down. But the crucial detail is the state: how long it has been since the last call went out. And there has to be a periodicity of calls.

I’ve often wanted a way to leave some possessions out in the open, unattended, for short periods of time. I think the basic setup would have to be this:

  1. I’d leave a key with the stuff, which would be a small little transponder that would call to the other part, that I have with me. If my part stops hearing incoming calls, it will tell me.
  2. Likewise, my piece would call to the key, to tell it, in a way, that it was still within reach. If the key stops getting calls, it sets off an alarm.

Most importantly, both pieces of the conversation must have a way to listen and to send.

Back to Harpa: the doorbell cannot hear its receiver. But if it could, it could listen for pings which would basically be saying “can you hear me? if you can, you’re still within 50 feet of me.” But the doorbell would also have to realize that silence itself was signficant. That’s hard for a little piece of electronics. And it takes up constant battery power. Not feasible, with batteries the way they are now (expensive and ineffective).