7 Segment Clock

Hi Guys,

New to the forum, sorry if this post is in the wrong section. So me and a buddy are trying to build a large 7 segment clock with HH:MM:SS displayed. Each digit comes prebuilt and is about 7 inches high.

The problem we’re having is this:

The arduino runs on 5v and each segment runs on 12 V & 27mA/segment. We have tried to use a NPN transistor just to try and blink one segment but we have had no luck. Can anyone point us in the right direction of how we should be wiring the panel with the arduino? Thanks!


add a ~1k resistor between the arduino output pin and the input of the pair

and these are available in IC packages (ie TO220) if that makes life easier

Thanks for the fast reply. I'm going to try this as soon as I get home. Just a follow up question for my own knowledge...

I tried using an NPN transistor as a switch as it is described on this website:


It didn't really work, I'm just wondering why a darlington pair should work and the other example didn't.

Thanks a lot!

Grumpy Mike explained it to me one day, but I cant find that post and I am no EE, so I am having trouble finding the exact formula and terms that would describe it

but you can sort of think of it as a latched door, its much easier to open if there is not a ton of people pushing on the other side (voltage), theoretically at some point the force on the latch will make it impossible to move

(if you followed that grats lol)

Thanks a lot Osgeld! I actually "fixed" my original circuit. We weren't connecting the ground of the arduino to the 0 Volt of the other 12V power supply. Now it appears to work fine without a darlington pair - just the one transistor acting as a switch.

We're both newbs so thanks a lot for helping us out!