NOTE: Depending on where you live, operating an FM transmitter — even a very short-range one like this — may be illegal without a license. Unless you attach an antenna, it’s very unlikely that anyone will notice or complain about any transmissions you may make with this device. On the other hand, it’s very difficult to predict, before construction is complete, just where on the FM band this transmitter will broadcast. Use due caution during testing, and make sure you understand the law in your area before attaching the battery.