
We've cracked it.
I've just tested a set with Davidson's help.
The buttons are all different designs, with two important things in common: They are all metal buttons with a shank style of fastener and a metal backing that fits onto the front.
I've put a notch in the metal backing on each one: When the backing is lined up with the 'T' for the top of the button, the button is set to send a message. When it's lined up with the 'B' on the bottom, it receives.
Unlike Hermione's, one doesn't trace the letters to appear round the edge of the coin. Instead, one taps the button in a pattern. Davidson and I adapted it from Morse Code so that it's not quite the same but close enough that anyone who knows Morse could adjust fairly quickly. There are sure to be quite a number of Muggles who know it.
The best part is that it doesn't require a wand. It's simply on, all the time. We'll instruct everyone to keep their buttons set in receiver mode unless they need to send a message, and to switch it back as soon as they've finished.
Davidson, Moony and I were discussing it and think that the best way to use them is sewn on to a shirt or jacket button, at the wrist, or perhaps a collar or somewhere else it'll be in contact with skin, so it'll be felt when it pulses. We can charm them so that once sewn in place, they shan't fall off (but they can be removed and sewn onto another garment if desired).
I've set the spell so that each message will be preceded by a single long, five-second pulse. It will feel like a vibration against the skin. Then the message will start. Anyone receiving it can pinch their button for another five seconds to request a repeat; it can be repeated up to three times before it expires.
Now that we know it works, I'm going to make up sets of twelve each. Each set will go to a camp, with a 'master' button that Davidson and his group will hold. They can send messages to any camp that way, but the messages in the camps will go to a limited number of people and can spread from there. Davidson also mentioned the tactical advantage of a trusted agent outside each camp and I agree, completely.
Alice, I can make a set for the Moddey Dhoo kids' parents and give you the master button for someone at the sanctuary, if you like. It's probably going to be a fair amount of work monitoring the communications; that's why Davidson volunteered one of his people with experience in Morse (or 'Maurice' as we're calling our variation). Then again, if a large number of the recipients in the camps are our kids' parents, then perhaps we don't need a separate set - only I didn't think everyone in their camp would appreciate knowing details about the children, necessarily.