# Correct connections: 7-segment, 4-digit display with common anode

I see this connections for 4-digit display.

All used pins set up on output. But, really, is it correct connections?

Why it used A* input pins, and, digital pins set up on output, it is right?

Maybe all pins need used by digital pins with NPN transistors? Collector is 5V, Base - Arduino digital pin, emitter - resistor with display anode, next, collector is display segment pin, Base - Arduino digital pin, emitter - Ground.

What resistance is needed for the resistor? I think, more than 220 Ohm?

Sorry, but I'm really a beginner in electronics.

• Where is the datasheet for the display? Mar 8, 2017 at 15:24
• imgur.com/a/SIB0s
– Dave
Mar 8, 2017 at 15:40
• Where is the rest of the datasheet for the display?
– Majenko
Mar 8, 2017 at 16:02
• `(5-1.8)/220=0.014`amps is within the UNO's allowed consumption, so you're fine there. that said, i would invest in a `max7219` to free up all those pins and make coding simpler with existing libraries. Mar 8, 2017 at 16:40

That circuit looks OK to me. It will allow you to light (reliably) a single segment at a time. The A* pins will be used in digital output mode.

The A* pins are connected to the four anodes of the digits - each digit has common anodes for all the segments. They should be driven HIGH to 'select' a digit.

The D* pints are connected to the cathodes of each segment. They should be driven LOW to light that segment.

For example, to light the 'A' segment of the first digit, you would set:

• A1 HIGH, to drive the anode of the first segment
• A2, A3, A3 LOW, so that the other three segments are not driven
• D5 LOW, so that current flows from A1 to D5, lighting the digit
• D6-D12 HIGH so that no current flows from A1 through any of the other segments.

The expectation is that you will cycle around (very fast), lighting each required segment on each required digit in turn.

Without a (proper) data sheet we can't tell you what the correct resistor value is, but the 220R shown is probably OK.

• Thank you. I was looking for a datasheet on the marking, but I can not find the full description.
– Dave
Mar 8, 2017 at 16:17