Working with Motors

There are 3 basic types of motors: 

Step motors, or Stepper motors

Servo motors, and

DC motors - with a brushed DC motor, speed is proportional to voltage, and torque is proportional to current.

DC Motors

"TT Motor Gearbox" is a complete DC motor with integrated 1:48 gearbox and 5.5mm flattened shaft which gives a convenient way to connect to wheels or rotary encoders.  It is rated for 3V (120RPM no load shaft speed) to 6V DC (250RPM), with max voltage of 8V.  With load and using gearbox, the angular speed is 26 to 54 RPM, current draw is 150mA.  At 6VDC, stall torque is 0.8kg.cmm, and stall current 1-1.5A.

A typical DC motor without any gears will have a high shaft speed but low torque, so will not be able to drive a load.  Additionally, it can be complicated to design the motor bracket and mechanical connection to an axle driving wheels.

Powering the TT Motor Gearbox. 

Evaluating battery options:

A standard 9VDC battery (PP3 size - historically PowerPack by EverReady, introduced in 1956) has zinc-carbon or alkaline chemistry and is designed for low-current applications (much lower than 150mA).  It is not suitable for driving a motor for any length of time because it typically provides 0.5 Ah of current life (500mAh) and a good rule of thumb is Ah/20, so in this case approx 25mA typical steady-state draw.  If it were good enough, it would require a buck converter such as the LM7805 to step down the voltage to between 3-6V.  Source

CR123A batteries (lithium-ion chemistry) have a max discharge rate of 1.5A.  They can safely provide up to 300mA continuous current draw for 6 hours (or 1800mAh), or a pulse discharge of 1.8A (3 sec on, 7 sec off) for 1 hour.  We can run this directly at 3V or use a boost converter to raise to 5V, or 2x 3V batteries in series.  Without a voltage regulator, you'll have voltage related sag as you drain the battery or pull a high amount of current through.

Buck (step-down) converters

LM7805 is a fixed 5VDC output regulator taking 7-35V input (7-13p ea).

LM2596 is an adjustable buck converter taking 3-40V input and producing 1.5-35V output (£1.30 ea)