4 digit 7 segment display


I've just nicked the 4 digit 7 segment display from my alarm clock and now i need your help.

All the LEDs share a common +ve lead. (common anode or cathode???)

Now here are my questions.

1) Can I wire it so the +ve to say plus 3V and then all the -ve to the Digital Pins?

2) If so, my code would be the opposite right? eg for the blink sketch to get one of the lights to turn on it's be digital output LOW, and then off would be HIGH.

3) Can the arduino do this? Bad for it?


a) common anode

1) Not a good idea. Too much current can be drawn and kill the arduino. You want to use a transistor driver. If you have isolated V+ pins for each digit, you would use a PNP transistor to drive each anode with software multiplexing.

2) A transistor driver will usually invert logic.

3) You can likely make this work... but you should "buffer" the pins, as stated above.

NOTE: some alarm clocks LEDs have all LED pins isolated. Meaning that it is not multiplexed (so you have MANY LED pins) have you verified the LED wiring? Are the digits isolated?)