# 7-segment LED display

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#### ormo

##### New Member
Hi,

I am going to be making my own common cathode 7 segment display using 14 LEDs arranged as so:

Code:
 XX
X  X
X  X
XX
X  X
X  X
XX

where each 'X' represents an LED. Each pair of LEDs will be connected in series and will have the appropriate resistor attached. The display will be driven by a CMOS 4511 or similar (the CMOS 4511 is a standard BCD to 7-segment display driver).

What is the minimum voltage to be able to power all 14 LEDs (such as in the number 8)? Assuming each LED has a forward voltage of 2.1V:

2.1 * 14 = 29.4 volts

I know this isn't right, can someone please show me where I have slipped up?

Thanks,

#### ormo

##### New Member
Also, if you use this to figure out the resistor value:

R = (supply voltage + forward voltage) / forward current

If I have 2 LEDs in series do I just need to add up the forward voltages?

#### ericgibbs

##### Well-Known Member
I know this isn't right, can someone please show me where I have slipped up?

Thanks,

hi,

You are right, its is wrong.

Only two LEDs in series per segment.
So its 2 * 2.1 = 4.2V
If you have 10mA/segment thats a [5-4.2]/0.01 = 82R resistor per pair. OK.?

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#### ormo

##### New Member
If the LEDs have a forward current of 20mA then it would be:

(10 - 4.2)/0.02 = 290R

per pair on a 10v supply?

#### ericgibbs

##### Well-Known Member
If the LEDs have a forward current of 20mA then it would be:

(10 - 4.2)/0.02 = 290R

per pair on a 10v supply?

Sounds OK to me, have you checked the 4511 can source 20mA at 10V.

EDIT:
the datasheet says Imax of 25mA

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#### saqlain

##### New Member
I have made a counter which displays outputs from 1-15 on led's..but now i want to see it on the 7-segment display.
so can anybody suggest as to which decoder should i use and how to connect two 7-segments together..cauz i searched for 74LS47(BCD to 7-segment decoder) but that has option for only connecting one 7-segment.
plz help!!

#### saturn1bguy

##### New Member
Can anybody suggest as to which decoder should i use and how to connect two 7-segments together..cauz i searched for 74LS47(BCD to 7-segment decoder) but that has option for only connecting one 7-segment.

You'll need one counter/decoder/display per digit. The counters then get wired to carry from ones, to tens, to hundreds, etc. The counters do all the work, the decoders and displays just make the counting visible.

Or are you asking about the specific counts from 10-15, and how to display those? Ten through fifteen are invalid BCD counts. Perhap's you're using binary counters (0-15) and not BCD counters (0-9)...

#### spitfire

##### New Member
I have made a counter which displays outputs from 1-15 on led's..but now i want to see it on the 7-segment display.
so can anybody suggest as to which decoder should i use and how to connect two 7-segments together..cauz i searched for 74LS47(BCD to 7-segment decoder) but that has option for only connecting one 7-segment.
plz help!!

As Saturn1bguy said,

You will need to use 2 BCD to 7-segment decoders (1 per display) and use 'CLKOUT/CO' from the first stage counter IC to increase the count in the second stage counter IC.

What are you using as a counter at the moment?

If a CMOS counter, What type and how many counter ICs are you using? How are you incrementing the numbers/count? If (for example) the CP input on the 'MC1451' is used then it should automaticly reset its internal count to 0 once it reaches 9. However if you are loading in data via the Parallel inputs (4-bit data from 0 to 15) then it will reach a maximum count of 15. This would explain how you are counting up to 15 by using one chip.

But if you use the counter like you are now and connect it to a 'BCD 7-segment driver', any number above 9 will then display the letters A,B,C,D,E,F on the 7-segment display....something to consider.

What is the maximum number you would like to see on the displays?

I built a 24 hour digital clock using only logic ICs and made my own custom 4 digit 7-segement LED display using individual LEDs just like like you are planning to.

Pics of the Display module I made:

https://www.electro-tech-online.com/attachments/display-jpg.14304/

https://www.electro-tech-online.com/attachments/normal-jpg.14306/

https://www.electro-tech-online.com/attachments/tracing-paper-jpg.14310/

One thing to be aware of is that LEDs tend to have a terrible viewing angle so I had to roughen up the LEDs and place transparency paper infront of them as a filter...also the blue LEDs are blinding hence why I cannot sleep with the thing on.

Hope this was of some use to you.

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#### saturn1bguy

##### New Member
...and connect it to a 'BCD 7-segment driver', any number above 9 will then display the letters A,B,C,D,E,F on the 7-segment display

Careful there!

I know the DM9368 does what you say (first image).

The DM9374, CA3161, and NE587 decode counts 10-15 with '-EHLP ' (second image).

But not the traditional ones: the CD4511 blanks the display on any count above 9, and the 7446, '47, '48, '49, '246, '247, and '249 all show the same symbols for counts between 10-15 as indicated below (third image).

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