Why Interface Circuits?

Why Interface Circuits?

        Computers don't exist in a vacuum.  They have to interface with the world in many ways.  You can sit at a keyboard and type and you're using a computer interface.  You click a mouse button and that's another interface.

        Measurement devices often need to communicate with computers also.

These kinds of needs - and many others - show why interface circuits are necessary.

        You'll need to use a comparator, sooner or later.  Here's what you'll need to learn about comparators.

What Is A Comparator?

        A comparator is the simplest circuit that moves signals between the analog and digital worlds.  What does a comparator do?

        Just to give you an idea of how a comparator works, here is a simulation of using a comparator.  Set the voltages on the control panel to adjust the voltage inputs to the comparator.


What About Real Comparators?

        Real comparators may work like the one in the simulation, but there are sometimes other considerations.  For example, a common comparator is the LM339, which come on a chip with four comparators.  The four comparators are all open collector outputs.  We need to discuss that.

        In this situation, you don't need to know a lot about transistors (although it's a good idea to learn that if you don't know it!).  What you need to know is that, in this situation, the transistor acts like a switch.  A transistor doesn't always act that way, but it does in this situation.

        The way you connect the comparator is to put your load between five volts and the collector connection on the chip - like this.

Finally, if you want to use an LM339, you'll need this pin-out.  You can click here to get the pin-out in a separate window.


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