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Topic: 9 output pins used for 2 digits 7 segment LED - power consumption (Read 1 time) previous topic - next topic

anton1o

Hi
I suppose to make 2 digit 7 segment  LED display, but don't have multiplexor or shift register, so I suppuse use Arduino Sensor shield to this project.
I use for power source 2 LiPo batteryes, so it's voltage is 7.2v
My Idea is to connect all LED in parallel lines, by 2 or any (till 6, as many as resolve breadboard) in line with 10 Kohm resistors one for each line to signal pin.
And the positive pin of all 7 segment LEDs connect not to +5v, but to difference Output arduino's pin.
And use positive strobe for power only one digit at a time.

When I make positive 1s output pin, will work 1st digit only, and if positive strobe will be on 2nd output pin, will works 2nd digit, not depends on signal pins state.

But i have to doubts here:
1.Does it Arduino sensor shields V.5.0 output pins has enough power to pass such a current or blow out? I can't count it to precizeance, so any idea, please.

2.Which pin i need use for strobing power - digital or analog? And why? (I think myself, that no difference, but doubt - may be difference exist?)
possible some has more current, or can switch quickly or something else?

I just newby, as you understand, but i'm afraid to blow out my pet it if i don't see something under watwer.





  Arduino sensor shield V.5.0 

Paul__B

Firstly, I can see no particular purpose in using a "sensor shield".  All it does, is to give you male pins to connect to instead of the female pin headers on the Arduino proper.

The problem with multiplexing is that even if you limit the segment current to say 10 mA (330 ohm resistors on each segment) so that its total is only 70 mA maximum, you cannot supply that 70 mA through an Arduino pin.  You need a driver transistor, if you are driving common anodes to each digit, then you need a PNP transistor with its emitter to the 5V supply and you must drive its base low via a 1k resistor to enable it.

Analog pins are digital pins.  It just happens you can if you need to, read analog voltages on them with the appropriate instruction.

anton1o

Thanks, i more understand now. But I have some new doubts about currents.
Can you explain me some more things?

Firstly, I can see no particular purpose in using a "sensor shield".  All it does, is to give you male pins to connect to instead of the female pin headers on the Arduino proper.
just because of it was already included in my robot vehicle set (there are female connectors wires set, and servo and BT and Ultrasonic has male connectors), otherwise I will need to create connection wires by myself.
Quote
The problem with multiplexing is that even if you limit the segment current to say 10 mA (330 ohm resistors on each segment)
I already tryed 0,2 Kohm, 1 KOhm, 12KOhm resistors - current over all of them forced LED glowing, as separate LED (like bulb) as digit LED - glowing point and low segment (4&5 pins on SMA42056 matrix). Of coarce, with 0,2K it more bright, but even 12K it bright enough.

I'm in doubt: is it true or I make mistake in calculations or in my measurements or my voltmeter are lying?
Because I calculate, that if 7.2 V over 12KOhm resistor current will be 0,6 mA
And my Voltmeter show 0,72 mA in real curcuit (but it very old and it's precision class are over 4%)
I must be make some error somewhere, and will be glad that somebody point me, where I make mistake.

Because if it's my measurement are true, does it mean, that all 7 segments will take 4.9 mA and if i will connect common catode to pin 0 and other to pin 1 of arduino uno, and take signals from pins 2-8, it will work?

Or i will blow my small pet? 

Quote
Analog pins are digital pins.  It just happens you can if you need to, read analog voltages on them with the appropriate instruction.

Does i right understand you, that it can be USED

1.same as digital inputs and digital outputs, generate and acept pulces
2. and low and high 2signals, and
3. has same 40mA current capacity, not more?

And does i right understand, that in addition it analog pins (A0-A5)
4.can accept analog value, so can be used as Voltmeter or audio analyser?

Thanks

Update: I had measured direct current from my power source: 3 segments eat 2 mA, 5 segments eat 3mA, 7 segments don't know, I blowed up 2 segments, but guess that 4.5MA?

Paul__B

Yes, as I said - "analog" pins are digital pins.

Now, if you are using 1k resistors (or greater) and drawing only 3 mA or so, then seven of them will be only 21 mA total (or 24 mA if you use the decimal point - and why not?) so you can drive the anode directly from the Arduino pin in that circumstance.

You will not "blow" up segments with 3 mA - except perhaps for some very early model miniature LED displays with lenses moulded into the casing.  If you have damaged them, it would be by connecting them directly across the supply voltage with no resistor.


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