I'm working on a theater piece where about a dozen people in the audience will be holding phones which send them timed messages in synch with the story on stage. We use about a dozen phones.
We've built the very simple app that displays the messages in time, but right now the only way we have to start the sequence is to begin with all the phones backstage and have all of the apps open with a big start button. Then we frantically try to push all the start buttons within a couple seconds. It's nerve wracking and far from ideal.
Is there any way to send all of the phones a signal at the same moment to start their sequence? We don't have reliable wifi in the spaces we're performing this, and the phones don't have actual cell service. Is there a way to use bluetooth or an external device to give them all the signal?