Hello,
Brand new to arduino and love it so far. I was curious to see all the sketches using leds, have a resistor. I just got a 7 segment display from maplins. Looking at all the led schematics online they use a resister per led segment.
I have a common cathode (or common anode - cant remember) Can I just put in 1 (pull down) resister between the gnd and common rather then at the other end which requires 7 resistors?
thanks in advance
nz-cam
Have to see a drawing to answer your question.
The term of 'pull-up & pull-down' resistor normally only applies to digital inputs, not digital outputs. Current limiting resistors to drive LEDs are wired in series between the I/O pins and the LED elements. How to wire up LED displays can get a little confusing as first one has to define if one is going to use a multiplexing drive circuit (uses less I/O pins) or direct drive.
Again a schematic drawing is worth 1K words. 
Lefty
Hi Lefty thanks for your speedy reply and clearing up the pullup pull down stuff 
There are 16 resistors in this pic. Why cant you directly connect the pins without resistors . But put 1 resistor between gnd and common cathode?
(i'm not trying to be cheap ...just have 2 resistors )
If the LED work on the same voltage as the controller then the don`t even have to have one resistor.
The resistor is for current limiting .
So if need a different value of resistor to power 1 LED than 2 or 3 etc
If you know every LED letter had the same number of leds lighting you could use a fixed vaule resistor .
So you need to ouput at the same voltage as the LED need or use a resistor for each LED pin.
Arhh yes i see!
So as long as I'm prepared to accept that letters like "I" will be brighter than "8" then I should be good to go!
Thank you very much for the info.
nz-cam
If the LED work on the same voltage as the controller then the don`t even have to have one resistor.
Come on you know that is rubbish. Don't go confusing a beginner.
You need one resistor for each segment. You can't use on resistor for several segments because then the current will be divided up as well and the LED brightness will change depending on how many are on.
That may not work. Each segment is a different LED and unless they all have exactly the same characteristics, some segments may not light up. Its worth a try but you will be better off with resistors for each segments.
Sorry why ?
If you are using a 3.3 volt led and a 3.3 volt controller you shouldn`t have to use resistors.
But this is not the case , so I agree that each segment should have a resistor .
Righto,
back to maplins tomorrow for a bag of resistors!
Sorry why ?
If you are using a 3.3 volt led and a 3.3 volt controller you shouldn`t have to use resistors.
But this is not the case , so I agree that each segment should have a resistor .
3.3 volts is not the working voltage, its the forward voltage or the voltage required to overcome the bias of the diode. Below 3.3 volts no current will pass, above 3.3 volts current is only limited by the resistance of the circuit or the LED acting as a fuse and burning out. They are not the same as flashlight bulbs. They need a voltage above the forward voltage and a resistor to suit. Grumpy mike is right.
Oops, I guess you learn something every day.
To me the led should self regulate, If supply voltage only equal to the forward bias voltage, but I most be wrong.
The resistor calculators don`t help and just say 1 ohm what ever current led you use , But I think the 1 ohm is just a minimum resistor you can get.
But most hi brightness led torches just use a number of leds in series and don't use any form of resistor.
The resistor calculators don`t help and just say 1 ohm what ever current led you use , But I think the 1 ohm is just a minimum resistor you can get.
But most hi brightness led torches just use a number of leds in series and don't use any form of resistor.
More rubbish. I know this is the Internet and everyone has a voice, but one should know their subject better before giving advice that can lead to needless component damage.
Also high power LED torches (flashlights here!) use either a series limiting resistor (not efficient) or better yet a constant current regulator to drive the LED string. So there is current limiting being done either way. An LED must have current controlled or limited externally to itself.
As far as those multi-segment displays, I pretty sure that most are designed such that all the segments give the same brightness level given the same current, otherwise their spec sheets would give information on the specific current requirements for each segment. What a design hassle that would be.
Lefty
To me the led should self regulate,
Sadly physics gets in the way of what you want.
As they say in parliament, I refer my right honourable friend to the answer I gave some time ago. :-
Oops, bullet-proof vest fitted, check.
It was after midnight, I should have fitted an “it's late I want to go to bed” filter on my browser.
Yes, thinking about it in the morning light, It's a diode so self regulate may not be a part of it's make up.
But in the end the main answer will still be the same, yes you should use a resistor for each segment.
Can I please remove the bullet-proof vest now ?
@ Mike, I've read your post and fully understood it, great explanation. - Thanks
Can I please remove the bullet-proof vest now ?
Yes
There are some people who persist in this myth and I think it should be jumped on so as not to miss inform beginners.
My fault, I though you were one of them but you are really one of us.