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I purchased this 4 digit, 7 segment display from Adafruit:

Product: http://www.adafruit.com/products/865

Data Sheet: http://www.adafruit.com/datasheets/865datasheet.pdf

Everything I have searched for on the net either has 12 pins or 16 pins and most of those are only using 4 resistors. This one has 14 pins. I'm trying to wire it up to an Arduino but I'm not sure which pins should have resistors and which shouldn't. If I wire it up like in the product picture with 1k resistors, I get 8.8.:8.8. and as I remove resistors from 5V, I can see the different sections that are lit/not lit.

Is that the correct way of doing it? Can I now just wire those to the Arduino?

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It's just a 12 pin display, but with two extra pins for the colon.

Removing connections can help you better understand what it going on.

Resistor should be added to the anodes (pins 1,2,3,4,5,8,9,12,13) just like on the website. Pin 8 should be a different value, to match the brightness of the other (multiplexed) leds.

The other end of the resistor can be put into the Arduino. Depending on the resistor value you may need to add some transistor to drive the other pins (cathodes) on the display. Lower value resistors means a brighter display, but too much current for the Arduino to drive all 8 leds of a digit at the same time.

1k resistors, would mean about 3mA of current per segment. So if all segments are on in one digit, that would be 8*3, so 24mA. Which is approaching the maximum current an Arduino pin can source.

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  • Awesome, thanks for the reply. I'll setup the wiring just like the product image, with 1k or slightly higher resistors to be safe. What do you mean that Pin 8 should be different? Is there a specific value is should be or what is the significance of that pin? Sounds like you have used one of these before - do you recommend an Arduino library?
    – user1624184
    Commented Oct 13, 2015 at 20:01
  • pin 8 is for the colon and it has two leds attached in parallel. Since you are multiplexing the digits, that means the leds are on only 1/4th of the time. So to match the brightness you'd supply the colon leds half the current. So I'd just double the resistor value for pin 8 (unless you're not going to use the colon at all). I just looked at the datasheet. No idea what libraries are good, but I think all will work just fine on your display.
    – Gerben
    Commented Oct 14, 2015 at 15:10
  • Thanks - that makes sense about pin 8. I was able to get everything working with a library I found (minus the colon for now). I used 8, 1k2 resistors on each segment pin which should give me around 20 mA per pin and the brightness is still very good. One final question, does it matter if the resistors are on the eight segment pins or the four digit pins? I think the current would still be the same right? I have limited space so I would prefer to use 4 resistors over 8.
    – user1624184
    Commented Oct 14, 2015 at 18:22
  • No, that will not work. Having the resistor in the digit means the all the leds in digit share one resistor. So the more segments are on, the more leds have to share the current, the more dim they are. So a 1 will be much brighter than an 8.
    – Gerben
    Commented Oct 15, 2015 at 12:53

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