My nephew just gave me a lovely, slightly belated birthday present.
For a school electronics project he had to take some old device and build a modern, high tech version of it. He decided that he wanted to do something with magic and a while ago came around and had a root through my books for something that he could build. He came across an old signalling device. The original idea was that there were a number of light bulbs on a magicians table, screened from the audience's view. These were connected to switches off stage than an assistant could operate to send messages to the magician. I can't remember exactly where he found it now, in some old tome that I'd printed off the Learned Pig from the 1940s I think.
So what did he give me? A deck of cards and a little key pad of numbers. The deck of cards has got 10 little lights showing through it which can be switched on and off wirelessly from the keypad. The deck could happily sit on a magician's table without being noticed, you could probably even flash it at the audience and get away with it. I'm just looking forward to the next time I perform and can give it a test run