The Turret

The turret is a rotating cyclinder on the top of the robot, which will house a number of things. The first of these will be the camera. It will be mounted towards the front to improve the view forward. It is mounted using bearings from a bicycle headset, to provide a large diameter hole, through which I can pass USB leads and connectors, etc. To avoid excessive cable twisting the turret rotates 180° right and 180° left. The software keeps track of the absolute position and enforces the 'end stop' points. The easy solution to this is to return the turret to its 'home' position after use.

Drive

The turret is driven by a geared down stepper motor. I'm using an SM-E040 which came from an old printer or something similar. It's a 5V bipolar stepper motor with 7.5 degrees per step and in this case a 14-tooth sprocket on the output shaft. This will drives a large cog on the turret spindle (which I'm still searching for) with about 50 teeth to give a rotational resolution of about 1 or 2°.

Driver

To drive it, I'm using an SAA1027 IC as I happen to have two of these lying around. This is the data sheet (PDF). I also found a couple of old etched circuit boards from Maplin (part no. GD14Q) to match (Maplin no longer sell these). Although my motor is a 5V one, you can still drive it using a 12V supply as the IC has current limited outputs, set by a resistor (360ohms in this case).

Control

The I/O board has 5V logic and the SAA1027 uses 12V logic levels so, I've copied the transistor interface from the BBC Micro article (referenced below) to interface them.

Useful Reading

BBC Micro 'body building' article 
An informative article on driving stepper motors from an I/O port and with 5V logic levels.