Controling 28 1 digit 7 seagment led displays with arduino - Page 1
Forum section: electronics / general

author: FlevasGR
author_rank: Regular Contributor
timestamp: April 02, 2015, 07:06:20 pm
content: So, this is my scenario. I'm having 28 1 digit 7 seagment displays and i want to make them work. Each display has 10 pins. 2 of them are connected to ground so you dont have to worry about then. You're left with 8 pins which is linked with each seagment. If you do the math you have 28x8 = 224 seagments which you need to control. In my case there are 217 seagments (pins) since im using the dot seagments in only one display. In my case what can i do with my shift registers?I'm having 20 of those:http://www.ebay.com/itm/130806676456?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITand 10 of those:http://www.ebay.com/itm/321626159436?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITI'm really confused. What should i do?Please note that if you buy the 4 digits display instead of having 4 x 8 = 32 pins you have only 10 pins. So basicaly i might need to buy new displays :/

author: PeterFW
author_rank: Frequent Contributor
timestamp: April 02, 2015, 07:18:40 pm
content: Quote from: FlevasGR on April 02, 2015, 07:06:20 pmThat is a bit bad, those are common cathode segments.If they were common anode there are plenty of drivery out there, the next best cheap solution would be your standard 595 shift register.But they only can source a tiny amount of current but if you got better 7segment displays they will be happy with ~4mA per segment.Just wire each segment to a shift register, put a resitor on each output and you are done.If you want less drivers, you have to multiplex the segments, but then you need an additional driver for the rows/colums.Neither of your chips can handle enough current for a matrix display.

author: FlevasGR
author_rank: Regular Contributor
timestamp: April 02, 2015, 07:41:05 pm
content: A guy told my about the Max7219 chip which does some magic and can handle many displays

author: PeterFW
author_rank: Frequent Contributor
timestamp: April 02, 2015, 08:12:57 pm
content: Quote from: FlevasGR on April 02, 2015, 07:41:05 pmOh, yes, i forgot about those. Last time i checked they were horendously expensive for what they do.But i had a look again, five recycled china ones are on ebay for $5 with shipping, that would be a cheap and easy solution without much soldering or a big an cumbersome board layout.The MAX7221 has a proper spi interface and should do nicely. The Max7219 as well if you do not want to put annything else on the SPI bus.And both are common cathode drivers, current sources rather then sinks.You could also go for a complete display, if you just want something working, quickly.These things have become incredibly cheap:http://www.ebay.com/itm/32X08-3208-Green-LED-5mm-Dot-Matrix-Display-Information-Board-HT1632C-/160871007689?pt=LH_DefaultDomain_0&hash=item2574a8c5c9

author: dannyf
author_rank: Super Contributor
timestamp: April 02, 2015, 08:17:10 pm
content: A combination of1) wire all 7 segment lines to a shift register - or to 7 IO pins;2) wire all 28 digits to 4 shift registers - or to 28 IO pins;3) write your code.

author: dannyf
author_rank: Super Contributor
timestamp: April 02, 2015, 08:18:41 pm
content: A static display - 1 digit per shift register - is also possible

author: FlevasGR
author_rank: Regular Contributor
timestamp: April 02, 2015, 08:30:47 pm
content: Quote from: PeterFW on April 02, 2015, 08:12:57 pmThe 7219 also has a module with 7 seg displays like this:http://www.ebay.com/itm/-/310634882296?roken=cUgayN&soutkn=4wXicHI could get this. Also about the SPI i really need it for the ethernet shield.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 12, 2015, 12:53:18 pm
content: What I would do is divide the displays into four groups of seven. In each group, all like segments are tied together. The segments are driven by a 74HC595. Then an additional device per group is used to control the common cathodes, perhaps a TPIC6C595. The displays are multiplexed, one in each group on at a time. The shift registers can all be chained together so that's only 3 IO pins used from the arduino. Even the coding is simple once you work out your scheme and timing for display updates.The 74HC595 can drive an individual segment per output, just use an appropriate value resistor. I like to keep the current at ~4mA. However, don't use a 74HC595 to sink the cathodes. That will be current from all seven/eight segments and it's too much for that device.

author: westfw
author_rank: Super Contributor
timestamp: April 16, 2015, 06:00:02 pm
content: I would be tempted to get a Holtek HT1632 chip. This usually shows up in Chinese dot-matrix modules, driving 8x32 or 16x24 dots, but they should work fine for 28 individual 7segment displays. Cheap, too (especially compared to the Maxim chips!)

author: daflory
author_rank: Contributor
timestamp: April 17, 2015, 11:35:27 am
content: Multiplexing that many displays by hand is a real pain. (I did mine with wire-wrapping, and if I do it again I'll order a custom board). You might want to consider getting the sets of four with the digits internally multiplexed, as you mentioned. Remember that if you need the display to be as bright as possible, you should maximize the duty cycle by splitting them up as Dragon88 suggested.