I've read some concern here about the DC motor bit not being reversible without using a hardware switch. That may one of the reasons I haven't bought one. Well, that and the lack of precision - they just spin. But servo motors are a completely different thing! They are precise, and repeatable. They (generally) don't spin, but rather they move.
So last weekend, while shopping at Fry's I came across a device called a 'Parallax continuous rotation servo motor'. Now that just seemed weird - cause "continuous" and "servo" seem incompatible. None-the-less, I bought the cheaper one, a medium sized motor, with plastic gears and a standard 3 wire cord and connector for about $17.
Now comes the cool part: The connector looked like it would fit in the same socket as the smaller littleBits motor on the servo Bit. So ... I removed the motor from the Bit, and plugged in the Parallax continuous motor. - NOTE: be sure it's plugged in correctly. Looking at the 'front' of the bit (away from the printed name) the signal wire is on the right.
In a simple circuit with a slide dimmer, and it worked! It spun around and around - not to fast, but no slowpoke either. AND when the input voltage was reduced with the slide dimmer, it slowed, stopped at about the midway point, then reversed and spun backward as the voltage was further decreased!
In short, this motor turned my servo Bit into a reversible DC motor Bit! here is a movie to demonstrate:
Ok, so one of my concerns was that this larger motor would draw way too much current, so I tested the current draw using a proto module and a milliammeter. It required about 100mA regardless of the direction or speed. It used about 20mA when at rest. In contrast the standard servo uses about 20mA at rest, and 85mA when moving, so the new motor does use about 18% more power. And this may indeed have an effect on the overall circuitry. But for now, it's my new reversible motor!