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Reply to "Design of a $10-20 DCS-TIU Port Tester Tool?"

 

gunrunnerjohn posted:

...

In looking at the HCT part, it's current outputs are pretty low, maybe a larger resistor on that LED...

Well now we're entering the eyes-glazing-over zone, but the 330 resistor is no problem in this application.  The 4 mA output current characteristic for the 74HC and 74HCT family is simply a point on a curve.

74hc output load line

For example, with a 5V chip supply voltage, the output pin driving the LED can supply 10 mA if you don't mind the output voltage dropping to 4.4V.  Or you can get 20 mA if only 3.7V is suitable.  There are considerations and equations that get messy when you push the envelope but for this application 330 ohm resistor is not asking for anything untoward.

 

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  • 74hc output load line

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