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Datasheets : L293D driver - Driver Unit

What does it do?

The L293D driver subsystem is particularly useful for use with d.c. motors because it can control two motors and can drive them forwards and backwards.

How does it operate?

L293D driver circuit

Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.

The L293D IC has four input signals, labeled ‘a’, ‘b’, ‘c’ and ‘d’.

The driver boosts the current from these signals. The voltage signal at each output (‘a’, ‘b’, ‘c’ and ‘d’) is high or low when the corresponding input signal is high or low. But the current available from the output pin is much higher.

So the L293D is able to drive output devices requiring quite high currents of up to 600mA from each output pin.

The main advantage of the L293D is that, unlike a transistor, Darlington driver or MOSFET, it can drive a d.c. motor in forward or reverse.

It is particularly useful for work with PICs (hotlink to data sheet).

As well as controlling d.c. motors it is useful for work with stepper motors.


If the L293D is connected to two d.c. motors as shown on the left, and input signal ‘a’ is high and input signal ‘b’ is low, then current will flow out of output pin ‘a’, through the upper motor and into output pin ‘b’.

If input signal ‘c’ is low and input signal ‘d’ is high, then current will flow out of output pin ‘d’, through the lower motor and into output pin ‘c’. So the current in the lower motor will flow in the opposite direction to the current in the upper motor, and the motors will rotate in opposite directions.

Current can only flow in one direction (the direction of the arrow on the circuit symbol) through a transistor, Darlington driver or n-channel MOSFET, – which is why they cannot be used to reverse a motor.

When current is flowing out of the L293D this is called ‘sourcing’. When current is flowing into the L293D this is called ‘sinking’.

Possible applications

  • Driving a pair of motors in forward and reverse
  • Driving one motor in forward and reverse, and switching on up to two other output devices, such as a bulb or a buzzer
  • Controlling the movement of a stepper motor.

Making

Pins of the L293D driver

How part of the PCB might look

The blue lines show wire links on the component side.

Build and test the units that will provide the input signals before building the L293D driver.

Testing

Make sure that the signals going out (on the green PCB tracks) are the same (high or low) as the input signals (on the blue PCB tracks).

Fault finding

If there is a fault, check that the L293D is connected the right way round. Check the tracks and solder joints.

Alternatives

  • Relay – a double pole double throw relay can be used to reverse a motor, but it needs more components
  • Transistor – cheaper but cannot reverse a motor and provides less current
  • Darlington driver – cannot reverse a motor
  • MOSFET (Transducer driver) – cannot reverse a motor

Web links

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