Large 7 segment display module MAX7219 or shift registers? - Page 1
Forum section: electronics / projects

author: adriansmith31
author_rank: Contributor
timestamp: April 14, 2024, 10:32:23 am
content: I was wanting a large 7 segment display module that has a digit height of 2.3" or larger but there does not appear to be such a product available. Just those cheap MAX7219 based displays which are up to 0.56". So I designed my own but went for a chain of power logic shift registers instead of using a MAX7219 with external drivers. I thought this might be a suitable idea to make and sell for hobbyists. What do you guys think of this idea? Wasting my time, good idea but with improvements?I made a video showing a prototype I made herehttps://youtu.be/eKuvKOkwBKwwhich could be improved by putting the shift registers on the rear of the panel along with the power circuitry allowing them to be daisy chained.Thanks,Adrian

author: xvr
author_rank: Frequent Contributor
timestamp: April 14, 2024, 12:05:43 pm
content: Looks good. The same on SMD components will be betterI'd developed something like this as part of my Basics of Embedded Design course (but it never was built in hardware)https://drive.google.com/drive/folders/1AarnyRSlBZl0I_KwDBsOc403BPMy-cp-?usp=sharing(lecture 1)Video on Yourtube channel (but in Russian, sorry)

author: adriansmith31
author_rank: Contributor
timestamp: April 14, 2024, 01:35:56 pm
content: I'll check that out, thanks

author: mariush
author_rank: Super Contributor
timestamp: April 14, 2024, 02:10:26 pm
content: I would have used something like IS31FL3749 :https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3749-QFLS4-TR/22042042https://www.lumissil.com/assets/pdf/core/IS31FL3749_DS.pdfIt's 4 x 24 channel , 60mA per channel, tolerates up 20v, recommended <=16v .. it's SPI so easy to connect with others... but footprint is hard for hobbyists.Something easier even for hobbyists, IS31FL3265A or IS31FL3265B ... 18 channels, i2c, 60mA per channel, supports 12v or more..https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3265A-ZLS4-TR/12675529?s=N4IgTCBcDaIJIGUDMBGAYgGSWAbAVgEEQBdAXyAAlternatively, something like IS31FL3733B would also work, it's a 12 x 16 driver, with 42mA per led :https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3733B-TQLS4-TR/12675547https://www.lumissil.com/assets/pdf/core/IS31FL3733B_DS.pdfYou have 6 digits, so you could have 2 current sources per digit (you could easily have 8 digits in total), and there's 12 sinks so you could use one for each segment ... you could easily have 4 leds per segment running at up to 42mA .... and you'd have plenty of spare leds for ":" if you want to make a clock, or apostrophes, or C/F (could be a 7th modular digit with custom segments or other markings on the side.With red or yellow/green leds, it can do 2 leds in series easily as it only needs less than 0.5v headroom voltage, so if powered with 5v, 4.5v is enough for 2 2.2v leds in series.

author: adriansmith31
author_rank: Contributor
timestamp: April 14, 2024, 05:50:53 pm
content: Thanks for your suggestions, I might look into those if I design another. This was built to a price and I got the shift register chips in bulk cheap. They were new, old stock not RHOS compliant but good enough for hobbyist projects.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 15, 2024, 10:09:48 am
content: MAX7219 is a quite horrible IC. The "original" Maxim variant is horribly over the top expensive, and the Chinese clones are sensitive to data corruption. You have to add a pullup (or was it pull down) resistor to one of the input pins to make it start up reliably. If it powers on in a faulty state, then the only remedy is a power cycle. But the worse thing about the MAX7219 is it's lack of decent brightness control. You can hardly dim a display with it. It just has a few brightness levels, which are also linearly spaced in current.Also, max7219 is 5V only, and those big LED displays may have multiple LED's in series and may need a higher voltage. (I have some that just start emitting some light at around 10V).For those reasons, the display I designed (but have not built yet) is also designed around shift registers.Also, if you make a video about your project, it's always nice to show the schematic. The schematic tells much more about what's in a circuit and how things are connected.I also agree with xvr. I also prefer to work with SMT. The display circuit I designed is for an 8*8 matrix display which can be stacked in two directions, and therefore the PCB can not be bigger then the display itself. And even with THT parts, all the electronics could have been put quite easily on the backside of the PCB. This makes the display more universal, (for example when mounting in a front panel, buttons can be placed around it), and smaller PCB's are also cheaper.But don't take this comment too negatively. It works, and it works to your specification / fills your needs, and that is the main thing.

author: adriansmith31
author_rank: Contributor
timestamp: April 15, 2024, 02:39:50 pm
content: Yeah no worries. I'm aware of the expensive maxim original chips and the bad Chinese fakes - I've done a couple of videos on them. I also did make a large high current display using external drivers on the segment and digit pins which worked fine.I'll show the schematic on my blog and also I've made it open source and put on a couple of sites such as maker.pro and given the schematic and example code.It's not shown on the video for this project though.