Led Dispay with mutiple grounds? - Page 1
Forum section: electronics / beginners

author: noah4546
author_rank: Newbie
timestamp: November 05, 2014, 02:14:15 am
content: If you look at the image. When i want a certain digit I have to change the ground not the vcc. Why. And what is the type of technology called.

author: free_electron
author_rank: Super Contributor
timestamp: November 05, 2014, 02:27:47 am
content: common cathode display. nothing weird about that.

author: noah4546
author_rank: Newbie
timestamp: November 05, 2014, 02:30:38 am
content: Quote from: free_electron on November 05, 2014, 02:27:47 amWell, how do I use it with an arduino to display time. (Not asking for code)

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 05, 2014, 02:32:05 am
content: Set it up to sink current into the Arduino rather than source current from it. 1s will shut it off and 0s will turn it on.Hint:10110111 xor 11111111 = 01001000

author: noah4546
author_rank: Newbie
timestamp: November 05, 2014, 02:44:29 am
content: what I meant was swapping through grounds

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 05, 2014, 02:52:04 am
content: You mean multiplexing? I'm not following your terms.If so: hang series resistors off pins 1-7. Supply current from two arduino output pins to 8 & 9. Sink the current from the 1-7 resistors into 7 arduino output pins.You can only turn on one digit at a time, so you switch between the two digits nice and fast so your eye doesn't see them blinking.I'm assuming there's enough output pins on an arduino. I've never used one so I don't really know.

author: sleemanj
author_rank: Super Contributor
timestamp: November 05, 2014, 04:11:28 am
content: Quote from: noah4546 on November 05, 2014, 02:14:15 amThe picture you included appears to be a common anode display if you intended that the red lines indicate positive voltage and the grey ones ground. So if you found you have to do the opposite, you have a common cathode display.In terms of multi-digit 7-segment displays, the "common" refers to the "digit selection pin", you can see why that is if you think of it as only a 1 digit display, like this...Which is the "common" pin in that picture, clearly it's pin 8, so if you have to put positive to 8 and negative to one of the others to light a segment, then it is common anode. If you have to put negative to 8 and positive to one of the others to light a segment, then it is common cathode.Either can be driven with a micro controller directly, with shift registers, with discrete logic, with 7-segment drivers.... many ways to skin a cat!The MAX7129 is a popular way to drive common cathode 7-segment displays.

author: noah4546
author_rank: Newbie
timestamp: November 05, 2014, 02:44:33 pm
content: Quote from: sleemanj on November 05, 2014, 04:11:28 amThat makes sence.Thanks for the help.

author: denelec
author_rank: Regular Contributor
timestamp: November 05, 2014, 10:48:34 pm
content: Multiplexing is used to minimize the number of pins needed to drive a multiple digit display.The drawback is that only one digit is lighted at each time.You must adjust the frequency of the scanning so every digit appear lighted without flicker.Multiplexing also decrease the apparent brightness of the display. You increase the led current in consequence.