The voltage output directly from the GPIO pins is only 3.3 volts so, there aren’t many devices other than LEDs that you can drive with such low voltage. If you wanted to control, say, a Lego motor that runs on nine volts, you would need a nine volt voltage source yet prevent accidently sending nine volts through the Trakr’s processor and frying it.
When deciding what kind of circuit to connect to the GPIO pins, always apply the maxim, “Keep It Simple, Stupid” (KISS). If, for example, you wanted to drive a single motor in only one direction, the simplest device to keep a higher voltage source–the one used to drive a motor–from accidently passing through the Trakr’s processor is an electronic relay. This Instructable will demonstrate how to use a relay to control an external device with the Spy Video Trakr. You'll learn how an electronic relay works, what the electronic scematic diagram for a relay looks like, and what an actual electronic circuit diagram in which the relay is used looks like. You'll learn how to make conversion cables to connect the Trakr to external devices. You'll learn how to make a larger cargo deck for the Trakr and see some examples of how some devices can be mounted on the Trakr. You'll learn how to tap into the Trakr's 9 volt and five volt power supplies. Finally you'll learn how to use the electronic relay to control an foam dart missile launcher.