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Topic: 30 pin 4 digit display (Read 7925 times) previous topic - next topic


It seems to me to be a bit convoluted. Why clear the registers each time? Just have a pattern per digit, and send that pattern out. I suggest you want an array per digit, (off/on/off/off/on etc.) and for a particular digit just output the array contents.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics


standard 4  display

I understand that, which will work for a standard 4 segment schematic like above..but if im not mistaken...

this 4 digit display

..this one's input/output isn't seperated logically per segment and you really need to be continually swapping the voltage for each number..thus a costant need to refresh the screen. im new to this, but please let me know if im incorrect in this


It's the same thing isn't it? Except the anodes and cathodes are around the other way.

All I am saying is your method of setting the voltages seems unnecessarily long.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics


You can not connect the two ground pins directly to the arduino pins or the shift register pins. This is because they carry the current for a whole bunch of segments. This is too much and so needs buffering with a transistor.
What value of resistors are you using for each segment? Do not say you are not using resistors please.


I didn't connect resistors because in the alarm clock I ripped it out of , it went straight from this display to microcontroller. The display is on its own board , so I assume the resistors are there under the display?

Otherwise would I connect the led display pins to resistor to power  (arduino pins)? I'm new but I thought resistors were always in between ground and cathode of led no?

How would I buffer with the transistor?

Right now the display is working properly, it's just not as bright as I would
Like it...but maybe this is just the way it is due to delays?


in the alarm clock I ripped it out of , it went straight from this display to microcontroller.

Chuck it up, our leds-have-to-have-resistors god/godess, :)

There must be a way to simplify your code: it is too complicated. If you think about it, you are essentially driving "two digits". So there must be ways to arrange the pins so that there is some consistency between those digits.


Dec 30, 2012, 06:41 pm Last Edit: Dec 30, 2012, 06:52 pm by Grumpy_Mike Reason: 1
Please ignore dhenry he is the forums equivalent of a village idiot.

It is unlikely you have built in resistors, the original display could have had a constant current driver so not requiring resistors.
Using a transistor for a buffer is easy, see the first diagram in this link:-
It is likely that the poor brightness is due to lack of a proper driver trying to force too much current down a pin.
Try and simplify the code, have you read:-


I just double checked and there is 3 39ohm resistors (I think?) on the 2 grounds (pin 1 and 2) and the 5th pin...


Are you sure? It is a bad idea having resistors on the ground. This is because it can take the current from several segments depending on what is displayed. Therefore the current / brightness changes depending on what is being displayed.


THANKS GUYSI got it to working pretty bright! dhenry, the "2 digits" remark got me thinking.

I rewrote the way it worked, so that you basicially call a function show_num(a,b,c,d) and then it will slowly build the segments to light up on pin 1 in an array and the same for one on pin 2..then at the end it displays each with a small delay in between! less delays, so more bright. video and code below


code available here: http://codeviewer.org/view/code:2dc5

@grumpy_mike,well im not entirely sure about the resistors because im just looking at the old alarmclock circuit board.. in theirs..they have  ground to pin 1 and actually vcc to pin 2...and then 2 39ohm resistors, one between each ...the ground also has a diode on it...a little confusing for a newbie like me. below is an image

this is what im doing below..im still a bit confused on the transistor in terms of where it would go


Code: [Select]
[quote]code available here:[/quote]

the show_nm() seems to be very complicated.

I would do something like this:

void led_display(void) {
  static unsigned char current_num=0; //current number to be displayed. 0=left most, 1=right most
  turn_off_all_digits(); //by pulling the two ground pins high
  switch (current_num) {
    case 0: show_num0(); current_num=1; break; //display num0
    case 1: show_num1(); current_num=2; break; //display num1
    case 2: show_num2(); current_num=3; break; //display num2
    case 3: show_num3(); current_num=4; break; //display num3
    case 4: show_dot(); current_num=5; break; //display dot
    case 5: show_dotx2(); current_num=0; break; //display doublt dot
    default: current_num=0;

show_numx() can be implemented separately, or if needed, can be implemented here.

I would also use a display buffer for the content, and a look_up table that maps the digits into segment information, for speed and flexibility.



I happened to have a similar LCD lying around, so decided to give it a try. I am only using the third and fourth segment though, as I do not have a 74HC595, and limited pins on the Arduino Uno.

It seems to work fine, and I'm planning to use this to display sensor output (e.g. temperature) in the future.

I have some questions on the transistor part though:

  • Are the PNP transistors wired up correctly?
    In fact, in this case, the transistors seem overkill; I did some current measurements, and current out of LCD pins 1 and 2 never seemed to exceed 15mA. It seems to work fine without transistors as well. Any feedback appreciated.

  • Is it necessary to put an additional resistor between the Arduino pins (12 and 13) and the PNP transistor bases? According to http://arduino.cc/forum/index.php?topic=18703.0, I should. Again, in this case it seems overkill, as the currents I measured at the base were in the order of 1mA.

Attached a Fritzing schematic and the full code.

Any feedback would be greatly appreciated!


Replying to my own (old) question.

1. As the LCD pin current (15mA) seems to stay well below the maximal allowable pin current of an arduino pin (40mA), it seems safe to hook things up without transistors.

2. It is good practice to put a resistor between an arduino pin and a PNP transistor base, to limit the amount of current sunk into the arduino pin (or, in case of NPN transistor, drawn from the arduino pin).


You also need to consider the total current draw, not just individual pins, however in your case you seem to be within the limits.

Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

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