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Robot Building for Beginners, Third Edition by David Cook

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CHAPTER 16

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Transistor Switches

When a chip pin provides current like the positive terminal of a battery, it is said to “source” current. When a chip pin receives current like the negative terminal of a battery, it is said to “sink” current. These terms are reminiscent of the water analogy, where water comes out of the source and then goes into the sink.

Many chips have the same limitation as the LM393 comparator as far as their inability to source or sink significant amounts of current. Not only is maximum current an issue, but also older chips can’t source as much current as they can sink. So, designers connected LEDs and other circuits “backwards” ...

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