Here is a design for a full-featured motor shield that will be able to power many simple to medium-complexity projects.
4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, internal kickback protection diodes. Can run motors on 4.5VDC to 25VDC.
Up to 4 bi-directional DC motors with individual 8-bit speed selection (so, about 0.5% resolution)
Up to 2 stepper motors (unipolar or bipolar) with single coil, double coil or interleaved stepping.
Pull down resistors keep motors disabled during power-up
Big terminal block connectors to easily hook up wires (18-26AWG) and power
Arduino reset button brought up top
2-pin terminal block and jumper to connect external power, for separate logic/motor supplies
Tested compatible with Arduino Mega 1280 & 2560, Diecimila, Duemilanove, and UNO
L293D motor shield, the input voltage DC4.5-25V
600mA OUTPUT CURRENT CAPABILITY PER CHANNEL
1.2A PEAK OUTPUT CURRENT (non repetitive)PER CHANNEL
LOGICAL “0” INPUT VOLTAGE UP TO 1.5 V(HIGH NOISE IMMUNITY)
INTERNAL CLAMP DIODES
The Device is a mnolithic integrated high voltage,high current four channel driver designed to accept standard DTL or TTL logic levels and drive inductive loads (such as relays solenoides, DC
and stepping motors) and switching power transistors .To simplify use as two bridges each pair of channels is equipped with an enable input. A separate supply input is provided for the logic, allowing operation at a lower voltage and internal clamp diodes are included. This device is suitable for use in switching applications at frequencies up to 5 kHz.