Individual transistors can be used to drive higher power output components such as
motors and lamps. Some times it is necessary to drive a number of outputs at a time.
Instead of using many transistors, integrated circuits are available that contain
a number of transistors. These are called darlingtron arrays and two such components
are the ULN2803 and ULN2003.
The advantage of transistor arrays is that it allows much smaller circuit board to
be designed since a single chip takes up less space on a PCB than a number of separate
ULN2803 Darlington Array
The PIC chip needs a voltage of 4.5V to work but the use of the ULN2803 and a separate
24V battery allows the PIC to control high power 24V lamps.
The circuit diagram doesn’t show it but the Gnd pin must be connected to the 0V rail
of the power supplies (the negative terminals of both batteries).
ULN2003 Darlington Array
The ULN2003 practically identical to the ULN2803 but it has one transistor.
L293D Half H-Bridge Driver (not GCSE)
This chip allows motors to be switched on and off from a low power circuit such as
a PIC chip. What is most important is that it also allows the direction of the motor
to be controlled. It is therefore a great chip for building robots and other moving
The L293 has two enable inputs which can be used to deactivate the outputs of the
drivers when required. These are digital inputs and can therefore be either a logic
1 or a logic 0 (high or low).
• The enable 1 input enables drivers 1 and 2
• The enable 2 input enables drivers 3 and 4
The L293D has the bonus that the outputs can supply voltages of up to 36V (a PIC
can only manage 4.5V) and each output can supply a current of up to 600mA which is
much greater than a PIC or similar circuit can cope with.
The Vss pin is the low voltage power input which should be connected to the same
power supply as the PIC chip or other circuit which is connected to the inputs of
the L293D. This can be a maximum of 7V.
The Vs pinis the high voltage power input and can be between 4.5V and 36V, this
is what allows the L293D to drive much higher voltages and currents.
The Gnd pins are the 0V connections and should be connected to the 0V rail that is
shared with the input circuit. There are four because the pins are used to help dissipate
the excess heat generated by the chip.
The L293D heats up when it is in use which can cause damage if it becomes too hot.
This can be prevented by connecting a heatsink to it. The cheapest way is to include
a large area of copper on your PCB connected to the Gnd pins to spread the heat away
from the chip. Another method is to attach a metal heatsink to the top of the chip.
Metal heatsink fixed to top of an L293D chip.
Heatsink made by including a large area of copper on the underside of the PCB soldered
onto the Gnd pins of the L293D chip.
Controlling Motors With an L293D
Copper area on bottom of circuit PCB.
Switch 1 (Input 1)
Switch 2 (Input 2)
The circuit below shows how an L293D can be used to control a DC motor. The truth
table shows how closing the switches controls the direction of the motor. The L293
is capable of controlling two motors but the diagram only shows 1.
The truth-table to the right shows how changing the logic levels on the inputs effect
the motion of the motor.
You can see, the motors can run off a higher voltage power supply. The L293D can
also be connected to a PIC chip.