The program uses two pulsewidth variables, pw1 and pw2; and two sets of routines, left1 and left2, right1 and right2; one for each motor. As you can see in the schematic, the first servo is wired as per the previous circuit. The second servo is now using B3 as it’s pulse out, and B4 and B5 for the SPDT switch.source

The interface uses a PIC16F876 microcontroller and not much else. It performs channel mixing, current limiting, and noise rejection. Push the stick forward, both motors move forward, move the stick to the left and the robot moves left. It makes the robot very driveable. You can use a wheel transmitter meant for cars to control it, in other words, one channel is throttle(both forward and reverse), the other steering. Speed control is smooth, and the H-Bridge transistors are sensed and protected cycle-by-cycle against over-current. Noise from the receiver, whether the transmitter is on or off, is ignored. Temperature sensing is also possible.source

The servo positions itself based on the width of positive pulses fed to it. The rate at which pulses are sent to the servo is relatively unimportant(40 per second seems pretty typical). A pulse width of 1.5 milliseconds positions the servo at its center, with 1 millisecond and 2 milliseconds being the accepted limits to either side.The Parallax Basic Stamp has a command called “pulsout” that is handy to control servos. This circuit can be driven from a radio receiver or a Basic Stamp.source

This circuit drives small DC motors up to about 100 watts or 5 amps or 40 volts, whichever comes first. Using bigger parts could make it more powerful. Using a real H-bridge IC makes sense for this size of motor.Operation is simple. Motor power is required, 6 to 40 volts DC. There are two logic level compatible inputs, A and B, and two outputs, A and B. If input A is brought high, output A goes high and output B goes low. The motor goes in one direction. If input B is driven, the opposite happens and the motor runs in the opposite direction. If both inputs are low, the motor is not driven and can freely “coast”, and the circuit consumes no power. If both inputs are brought high, the motor is shorted and braking occurs. This is a special feature not common to most discrete H-bridge designs, drive both inputs in most H-bridges and they self-destruct. About 0.05 amp is consumed in this state.source

A relay-based on/off controller on the other hand can be very efficient. A typical relay has a contact resistance of 3 milliOhm. At 20A, this translates to a voltage drop of 0.06 volts, with a power loss of 1.2W. The relay itself typically consumes around 0.4W. And, by using a double throw relay, one gets a brake for free. source