After some experimentation with tiny N20 5V DC motors, the same type found in MG905 servos – I realise that they run just too fast. For the DC motor to work I have used a small 2N7000 / 2N7002 / NDS7002A — N-Channel Mosfet.
I will use a few of these DC motors in the forthcoming installation for the final project, however, because the RPM is too fast, I will be converting some of the servos I have into continuous and much slower, rotating servos.
This is achieved by cutting the physical obstruction on the main gear and disconnecting the potentiometer inside, replacing with 2 x 2.2K ohm resistors as voltage divider. I have also tested the more reliable MG906 metal gear servo with the PCA9685 16 channel controller board with an Arduino Uno. I drive the board with a 10A 5V fanless power supply with my own printed enclosure holding a dual pole 30A 240V switch.
I will now be looking at converting as many of the plastic geared servos as possible using surface mount 2.2K ohm resistors (+/- 1% accurate), on order from RS Components ; 100 for 70p!
Heres a short clip of 3 motors in action: