3 digit 7 segment display help


Joined Mar 24, 2008
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Joined Mar 14, 2008
If you have an open collector type driver (output logic active-low) then you use the common anode version of the display (anodes go to V+) as shown in the bottom schematic. If you have an output logic active-high driver then you use the common cathode version (cathodes go to ground) as shown in the top schematic.

In either case, unless the driver is a constant-current type, you need a resistor in series with each segment driver to limit the LED current to the proper value.


Joined Mar 14, 2008
Here is part of the schematic I'm trying to copy. I'm using BC 547 instead of the BC 557.
You can't use a BC 547 in that circuit since it is an NPN which requires the opposite signal polarity from the BC 547 which is a PNP. You need a PNP. Just about any small signal PNP will work.

Thread Starter


Joined Sep 28, 2011
Oh yes, of course. Thanks for pointing that out. I should of checked. I forgot to order them so used the 547 which i had laying around.
I'm confused on the led display. It has 5 pins one one side and 6 on the other. I don't inderstand the led schematic.


Joined Mar 14, 2008
The numbers on the schematic correspond to the pin numbers. Normally pin 1 is on the top right, looking at the bottom, with the numbering going clockwise from there.

For your circuit you need the display shown in the bottom schematic.


Joined Dec 8, 2009
According to the drawing, pin 1 is labeled
on the lower left when viewed from the front.

The pins should be in order 1-12 going counter

Oddly, from what I gather from the drawing, there is
no pin 6. If the bottom row has 5 pins the numbers
would be 1-5 (starting from the left) on the bottom row,
and the top row would be 7-12 (starting from the right).
Without seeing it that would be my guess.

Do you have the other specs (Vf, If, Reverse voltage, etc.)?
If you wish to verify pin numbers by applying
power, through the appropriate resistor of course,
be careful not to exceed the maximum reverse voltage
in case you accidentally apply power the wrong way.
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