7Seg Driver IC - Page 1
Forum section: electronics / projects

author: RoboTechEd
author_rank: Regular Contributor
timestamp: April 28, 2015, 03:31:25 pm
content: Hi All,I am designing a circuit that will have a mcu control a series of four 7 segment displays (hopefully alphanumeric, but I cant find any to meet my spec) through i2c. The only issue is, all of the ICs I have found to do this are super expensive! It would actually be cheaper in many ways to get a dedicated MCU (atmega 328) just to listed for i2c and multiplex all of the pins continuously.Does anyone know a cost effective chip that is i2c controlled that can control 4 7segs at full brightness? My current favorite option is two (PCA9685PW,112). It certainly isn't cost effective, but I like the simplicity of the design, and the PWM control could be useful.Edit: I wasn't clear enough in the original post with the specs/reasonings. I posted them below in a response, but will post them up here as well.-Sourceable: I would like the IC to be easily/reliably sourceable (digikey, mouser, farnell, etc). This isn't due to pride in this situation, it is because I am trying to design this as close to production capable as possible, just incase. If I was able to procure off ebay/aliexpress I would get an HT16K33 or something comparable.-i2C Driven: This is because the main MCU will likely be doing other things (possibly time sensitive), and I would like it to be relatively hand's off. I would like an IC to control the muxing/latching.-Relatively small: This isn't super important because I can likely make room for additional ICs, but I would like it to be as size efficient as possible. I have a limited space to work in (and a battery), and the less space used allows for more battery/more expandability in future revs.-Not Multiplexed: This is because I dont want to lose brightness of any of the LEDsThanks!

author: Rolo
author_rank: Regular Contributor
timestamp: April 28, 2015, 03:44:15 pm
content: Don't know if space is an issue but you can take a look at the WS2803 led driver. This has 18 outputs that can be individually dimmed by PWM using the build in current source. Just 2 wires to connect and can be chained for more outputs. No multiplexing needed.It's not I2c so one MCU extra could serve as I2C slave and controlling the display's. The WS2803 is cheap and easy to implement.I used this concept in my "Lumitron" module :

author: paulie
author_rank: Frequent Contributor
timestamp: April 28, 2015, 03:58:22 pm
content: Alpha displays with serial interface can be had on Ebay for a buck and change:http://www.ebay.com/itm/1pcs-MAX7219-dot-matrix-module-Arduino-microcontroller-module-DIY-KIT-M67-/371314934585?pt=LH_DefaultDomain_0&hash=item5674184339Multi digit 7 seg for about 20 cents per:http://www.ebay.com/itm/301440672688?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITIt's amazing how many fear the chinese sources. Probably due to prejudice/bigotry or misplaced sense of patriotism. Fortunately they do end up paying for that in the end. Big time. Wages of sin.... Or maybe just too lazy to search and want everything on a silver platter.

author: ajb
author_rank: Super Contributor
timestamp: April 28, 2015, 04:11:59 pm
content: If you're just looking to save a few GPIOs, then any old shift register or I2C expander will do. A single 12-bit IO expander will be able to drive the eight segment lines plus four common digit drivers in one chip, it'll be up to you to multiplex the digits of course. If you're trying to offload the multiplexing entirely, then driving every segment individually may well be cheaper than a dedicated display driver. Of course either way, you lose the built-in decoding that some of dedicated display drivers have, but that's not particularly hard.Quote from: paulie on April 28, 2015, 03:58:22 pmAt least for me, if I decide I don't want to use an eBay special it's not fear or bigotry at all--it's usually some combination of not knowing exactly what I'll get, not wanting to wait a couple of weeks for shipping, or wanting some confidence that I'll be able to build a design again some years from now without having to jump through a bunch of hoops because I can't find that particular eBay assembly that I designed it around anymore. Maybe you don't care about the last point for hobby projects, but there are plenty of reasons other than fear or bigotry, and that's aside from supply chain concerns inherent to a commercial project.

author: paulie
author_rank: Frequent Contributor
timestamp: April 28, 2015, 04:27:42 pm
content: The majority here are probably hobbyists so that is what I usually reference. But you are right. There are other reasons, mostly the shipping time. This is why it's a good idea to buy more than one piece (aside from the fact you save a ton of money,).BTW note that the 74hc595 approach typically ends up costing double MAX7219 modules. Another reason to avoid shift registers is the need for resistors and more port bits used up. Also no intensity control and no segment decoding. Basically a holdover from the old days perpetrated by tarduino noobs.

author: RoboTechEd
author_rank: Regular Contributor
timestamp: April 28, 2015, 04:41:09 pm
content: Thanks everyone for the responses. Here are some more details. I will edit the original post with the same info shortly.-Sourceable: I would like the IC to be easily/reliably sourceable (digikey, mouser, farnell, etc). This isn't due to pride in this situation, it is because I am trying to design this as close to production capable as possible, just incase. If I was able to procure off ebay/aliexpress I would get an HT16K33 or something comparable.-i2C Driven: This is because the main MCU will likely be doing other things (possibly time sensitive), and I would like it to be relatively hand's off. I would like an IC to control the muxing/latching.-Relatively small: This isn't super important because I can likely make room for additional ICs, but I would like it to be as size efficient as possible. I have a limited space to work in (and a battery), and the less space used allows for more battery/more expandability in future revs.

author: mariush
author_rank: Super Contributor
timestamp: April 28, 2015, 05:03:20 pm
content: Any reason two TLC5928 wouldn't work for you? 16 channels each, two of them would give you 32 channels which would be enough to drive your 4 x 7 = 28 segments plus maybe some dots. Even one tlc5928 would work I guess, with some multiplexing. They're cheap when bought in quantity.Seehttp://uk.farnell.com/texas-instruments/tlc5928dbqr/ic-led-display-driver/dp/1697163You could also use some attiny or pic as inbetween, you just send through i2c the four digits and then small micro talks to the led drivers.

author: RoboTechEd
author_rank: Regular Contributor
timestamp: April 28, 2015, 05:13:56 pm
content: Quote from: mariush on April 28, 2015, 05:03:20 pmThanks for the link. That does look cheaper/better than the (PCA9685PW,112) I was planning on using. The only issue I see is that another MCU would be needed to interface with these. Adding another ATTiny would put it at about the same price (a bit cheaper) as the chips I originally looked at (in large QTY), but would have an additional chip. I guess I will need to spend some more time looking at the datasheet for it and weighing the pro's/consEdit: I do like that you can control all of the outputs currents with one resistor. Could be useful for dimming etc