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Topic: Can I drive 16 x 7-segment (or 8 with DP) displays, with no shift register? (Read 3509 times) previous topic - next topic

fatcharlie

The board checks out fine - phew! Just goes to show a little knowledge is dangerous!


PaulRB

Good news. And the display?

While you're waiting for those max chips, here's how you should wire an led display directly up to an Arduino.

Notice there is one resistor per segment. One per digit would be incorrect, because the brightness would not be consistent between say a "1" and an "8". This is because of Ohm's law.

Also the digit lines each have a transistor to take the strain off the Arduino pins. For the segment pins, you can get away without transistors because each Arduino pin only has to supply current to one segment. But for the digit pins, the Arduino might have to supply enough current for 7 or 8 segments, which would go over the maximum (40mA). You could use higher series resistors so that this current stays under 40mA, but that would make the display quite dim. But the transistor amplifies the current, so that the Arduino pin only has to deal with a few mA, but the digit gets as much as it needs, e.g. 160mA.

fatcharlie

OK, so the good news is the Max7219s have arrived - but unfortunately the capacitors didn't.

What do they do in the circuit? Are they completely necessary? If so, I can see two options, either wait anothe couple of days to get some (probably the postage would be more than they cost!) or hack some from the numerous old circuit boards I have lying around. Any advice on the latter?

Thanks,

Roger.

fatcharlie

Capacitors,


Before, I just ordered some off eBay but i think that was a mistake. Now I look into it more throughly, I find a bewildering array of types - some pictures show them with voltages written on them as well as the capacitance.

I need a 10uf and a 0.1uf capacitor for each 7219. But should they be ceramic, electrolytic, or other types? should they be rated for 5V or is that part irrelevant?

Thanks,

Roger.

Edit: and, for that matter, where can I get them - a bit confused, here...

Grumpy_Mike

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But should they be ceramic,
The 0.1uF should be. The 10uF can be electrolytic. The voltage can be anything over the voltage you are using.

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What do they do in the circuit?
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html


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Are they completely necessary?
Yes. Even if you can not immediately see any difference they are not optional.

 

fatcharlie

Thanks :-)

Just read the link - I understand now. Cheers.

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