# How to identify the cathode

...In a common cathode 7 segments display?
Can someone please explain me the process?

Thank you.

I would try to read the datasheet.

The 7 segment display is just 7 leds (8 if you have a decimal point) that have either the anode or cathode connected in common. The LED specifications and pin numbers used for each segment varies from manufacturer to manufacturer so you really need the datasheet to determine pin numbers and current limiting resistor sizes.

EDIT: To add image that went missing.

Can also use 5V source with current limit resistor (470, 510, 1K, whatever) to "buzz out" the display and see what is connected to what if the data sheet is not available.
Many are pretty common - look at these to get an idea of what you're looking for re: 1 (and/or 2) common pin and 7 or 8 unique pins:

Common Anode:

Common Cathode:

Thank you all.
This display has 10 pins, 8 for segments including DP and the other 2 are both for cathodes?
I don't really understand the diagram

yes there are 2 cathodes in that case, just incase you would prefer to have the cathode coming out the left or right side of the display (I'm assuming the 3 and 8 pins are on the opposite sides of the unit)to make building a circuit board for it a little simpler.

Rookie2:
Thank you all.
This display has 10 pins, 8 for segments including DP and the other 2 are both for cathodes?
I don't really understand the diagram

Pins 3 & 8 are the cathodes, pins 10,9,7,5,4,2,1 & 6 are anode. The letters A to G are the segment names and DP is the decimal point. If the segments are small/low power then connect the anodes (through resistors) to arduino pins configured as output and connect 3,8 to arduino GND. When you set relevant arduino pin to HIGH the segment should turn on, LOW will turn it off.

Yes. 8 unique Anodes, 2 common cathode pins connected internally.
Drive the anodes high thru a current limit resistor, connect the cathodes (both) to Gnd, or an NPN transistor's collecter, with its emitter to Gnd.
Use both cathodes because you could have up to 160mA of current flowing with all anodes high, that gets shared across the 2 cathodes so you don't burn 1 out.

IF you take care to only drive 1 anode high at a time, then you could also use an arduino to sink the current from the cathode. That would be usefull for multiplexing, where you have more than 1 display with all anode As connected together, all Bs, etc. and you rapidly switch between driving the anode and then the cathode for 1 digit at a time, going thru all segments fast enough to trick the eye into thinking all are on.
More commonly, a transistor (or a buffer chip such as ULN2803) is used on the cathode and all segments for a digit are driven at the same time. This lets you cycle thru the Digits at the same rate, but looks brighter because the segments can be left on for longer.
i.e. 3 digits, 21 segments, each is on 1/21 of the refresh time - but with common cathode drive, each digit can be on 1/3 of the time. So if you left each on for 1mS, it takes 21mS to turn each one one for 1mS. Vs turning on a whole digit for 7mS, then the next for 7mS, then the last for 7mS. Will be noticeable difference in brightness.

Indeed I'm planning to multiplex a few of them.

I already have a 4 digits display based on this diagram

But a friend of mine helped me with that...pretty much like he did all the job

Now i need another one, by this time i'm by my own.