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Temperature demo 2, using 2 digit LED display CC


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A poster asked about using Common cathode display, just happens that is the type I had in hand.

And had been wanting to use the 74hc595 in driving LED's, so this is what I came up with.

 

I used of code I created for my first LED display project as a base.

 

I have drawn a schematic in Eagle for this project, which I have posted on my website in PDF format for those without Eagle.

 

The code has some interesting bits in it.... Binary to Decimal conversion, Seven segment truth table example, writing to the 74hc595, multiplexing LED's.

 

 

https://sites.google.com/site/ericstringer/home/projects/msp430launchpad/temperature-led-demo-version-2

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I have one suggestion. Since you have only 2 digits, instead of the second 595 you could use 2 NPN transistors and save a chip.

Mac suggested once a very clever way to drive 2 digits with 2 transistors and a single pin, basically the base of the second transistor is connected to the collector of the first one. This way if the first digit is on, the second is off and vice versa. You can connect the base of the first transistor directly to MCU or if you are using just 7 segments, to the 8th output of the first 595.

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I have one suggestion. Since you have only 2 digits, instead of the second 595 you could use 2 NPN transistors and save a chip.

Mac suggested once a very clever way to drive 2 digits with 2 transistors and a single pin, basically the base of the second transistor is connected to the collector of the first one. This way if the first digit is on, the second is off and vice versa. You can connect the base of the first transistor directly to MCU or if you are using just 7 segments, to the 8th output of the first 595.

 

 

The two transistor is not a bad idea, I have used in the past a simple transistor as a inverter gate to invert a signal.

Or could use two pin's from the MSP430 if you have them to spare.

 

In a final design I would add transistors to drive the digits (or some other device), just because of the current limitation of the 595. Also current limiting resistors would be on the list.

 

I have a second display I going to add, my next project will be to have time and temperature displayed.

 

I

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