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For all my platforms, I use a direct friction drive. This type of drive system is simple and allows for easy and fast resetting of the platform.

The drive system powered by an RB35 1:600 geared DC motor (Conrad electronics) and a 1:20 worm drive and a steel drive shaft driving the platform by friction. Due to weight of the Dobson and the vertical force transmission, the friction is high enough to prevent slipping. The relatively thin drive shaft (5mm) causes in the long term some wear on the segment surface. It is therefore better to use a somewhat thicker shaft. 

The motor speed is controlled by the motor voltage, which can be achieved by an adjustable resistor (most simple solution, as for my 14"er) or by a voltage control unit. Both solutions give satisfying results, but the voltage control unit has the advantage that the motor voltage does not need to be re-adjusted as the batteries get weaker. 

The  RB35 1:600 geared DC motor can be run between 2 and 12 V and consumes 50 mA. A battery pack easily lasts for 20 to 40 hours of operation. 

The table shows the approximate dependence of motor speed on motor voltage for RB35 1:600 motor. 

The voltage control unit employs an LT 1086 low-drop voltage controller (Conrad electronics). The input voltage should be at least 0.8 V above the required output voltage (max 30 V). 

A small adjustable resistor R3 of 500 Ohms allows to adjust the output voltage into the desired range, which can then be fine tuned by a potentiometer R2 of 100 Ohms. A fast ceramic and a a tantal capacitor (obey +/- !) bridge the output. The capacitor at the input is not necessary if you use batteries. 

A further alternative for the control electronics are pulse-width-modulating modules, which are commonly used to control the speed of DC motors. They can be ordered as completed modules or as kits from common electronic surplus companies.

Another very simple possibility to drive your platform was realized by Nick Hill, UK. He uses a commercially available drive motor as it is offered by Synta/Skywatcher/Celestron for about 30 Euros. The housing comprises the motor, electronics and a 9V cell, which, according to the manufacturer, should last for 40 hours.

The sense of rotation can be switched and the drive speed regulated over a very wide range. The motor can be attached directly to the drive shaft using a flexible adaptor as shown in the picture.