Huy guys,
I want to connect 3digit 7seg to arduino
I want to ask if i need to plug resistor between the 3 digit 7seg and the arduino?
And what resistance do I need
Thanks in advance
8 resistors and 220-330ish
If you are multiplexing them then the digit line takes the current from several LEDs which is
going to overload a pin - so transistors are needed to handle that larger current. 7 red segments
each with 220 ohm will take 7 x 16mA = 0.11A on the common (digit select) line.
Anyway what are these displays? Are the common anode or common cathode?
So i need 8 resistors (220)?
If i connect three 3digit 7seg in parallel
Do i need 8 resistors or more
I use 3 pins (one for each) to enable them
So i need resistor for these output?
Hi,
The displays are they common cathode or common anode?
Tom.. 
Sorry just saw MarkT post.
Hi,
If common cathode then a circuit like this, less one display and transistor.
Tom....
I think we are talking about mutiplexing here, not connecting in parallel. The digit select lines carry more current
for upto 7 segments at a time and cannot be driven direct from an Arduino pin. Yes the segment lines should
have resistors on to limit the current, and they can be driven by Arduino pins direct as the current is within the
Arduino pin limit (for the Uno, Mega, though not necessarily for the Due and other boards with less current
per pin).
So 7 (or 8 if decimal point) segment pins via resistors to the bussed segment pins.
3 pins driving transistors then switch the digits one at a time.
For common cathode use NPN transistors to low-side switch the digits,
For common anode use PNP transistors to high-side switch the digits (and the segment pin logic will
be active-low, ie LOW means ON).
Basically like that circuit but no resistors on the collectors of the transistors, but on the segment
lines instead (otherwise '1' will be much brighter than '8' which looks awful)
Hi, try this.
Tom...
Yeah, that's what I meant. I have sometimes done it the other way, but the brightness variation between
1's and 7's and the rest is pretty obvious and annoying...
(Though the a constant current chip its not an issue of course!)
I did not understand why the transistors
TomGeorge:
Hi, try this.
Tom...
i use three component like this(each component is three digit)
i need more transistors and resistors?
or should i plug it in parallel?
Parallel all identical segments.
If you use a MAX72xx chip... you will only need the one resistor and it multiplexes the light on/off so fast you dont see it.. and they all look lit at the same time..
You can also control up to 64 leds with one chip.. and daisy chain even more chips together (if ever needed)
avibd:
I did not understand why the transistors
Because an Arduino pin cannot handle 120mA. I said that in posting #2
avibd:
I did not understand why the transistors
When you multiplex a 7seg display.
The current flows from each Arduino output through the respective segment.
Then back through the common pin of the display.
The current flowing out of the common pin is the sum of all currents of the segments you have lit up.
This current is more than the Arduino pin is rated for.
So you use a transistor to control that current, using a smaller current from the Arduino
Hope it helps.. Tom...
Hi,
How many 3digit displays do you aim to have connected to your Arduino?
With the more digits you have, a problem with multiplex flickers can occur, as the controller is doing all the switching process.
You may be better off and easier to use modules that have onboard drivers to do the multiplexing for you.
https://www.aliexpress.com/item/0-36-4-Digit-LED-Display-Module-with-Decimal-Point-TM1637-Drive-Mini-7-Segment-LED/32648785549.html?spm=2114.search0304.4.1.NePRHN
It only uses two control lines and is available from most Arduino suppliers, and libraries are available.
They will simplify your code as well.
They are 4 digit but you don't have to use one of them, just blank it off if its behind a panel.
Tom...
INTP:
Transistors are a bit much just for 3 digits if multiplexed.It's literally lighting up just 8 LEDs, well within the capabilities of an Arduino board without needing the transistors.
1 LED 15mA, 8 LEDs 120mA, Arduino pin absolute max 40mA, so NO, its not within the capabilities to multiplex
direct from Arduino pins unless your display needs a lot less current (like 4mA/segment).
Also, without the transistors, the digits that have more segments illuminated will be dimmer. It can be subtle, so some designs just shrug and accept it. But you can not exceed the 20mA (not 40mA!) safe maximum for an Arduino pin.