Common Anode displays. what would you do?
TC
Posts: 1,019
Hello all,
I bought some Common Cathode 7-segment displays from eBay. After getting them, I realized they are Common Anode. I got my money back, and the seller told me to keep them for my troubles. But now I have a problem. What do I do with 100, single 7-segment common anode displays? Every idea I have had used common cathode displays.
Any ideas?
Thanks
TC
I bought some Common Cathode 7-segment displays from eBay. After getting them, I realized they are Common Anode. I got my money back, and the seller told me to keep them for my troubles. But now I have a problem. What do I do with 100, single 7-segment common anode displays? Every idea I have had used common cathode displays.
Any ideas?
Thanks
TC
Comments
-Phil
With high-side master switching, you would need a p-channel mosfet and 7x Prop pins could do the low-side.
A quad buffer for the highside could be OK if each segment is set for 5mA
http://www.mouser.com/Search/Refine.aspx?Keyword=SN74ABT126
http://en.wikipedia.org/wiki/Open_collector
I have these 100 displays now, and everything I have worked with uses some kind of multiplexing the digits. But all the examples I can find is where all the common anodes are tied together, and all the digits are driven at the same time.
I understand that might not be that big of a deal, but it is a waste of pins and current. for example, 8 digits, 20mA per segment (just a random value), all segments "on" = 1.28A
I am more along the lines of asking, What would you do with 100, 7-segment common anode displays?
-Phil
Do you want to use them all 100 at the same time or 2-3 or 4-10 of them?
Thats the thing, I only needed 20 common cathode displays. So I ordered 50 from eBay, waited 2 weeks for them to get here, found out they were wrong. The seller told me to keep them and they would send the correct ones. Waited another 2 weeks, got the replacements in, still the wrong part. I am sitting on all these displays, and I have no idea of what to do with them. I guess I could give them away.
I do admit some thought is involved, but it is worth learning that you can be creative. If you just always copy schematics from the internet, you really are not learning electronics.
Of course, if you already built boards and bought chips for the other configuration, it is a much nastier problem. You need the parts to fit the rest of what you already have.
Swap npn for pnp or vice versa. Or swap n-channel for p-channel.
I thought those good old fashioned 7 segment driver chips expected common anode anyway.
I have done some "common anode" projects. But I have found they were more expensive (more parts, more board space) then common cathode. that is why I work more with common cathode. This "hobby" has broke the bank more than once. but I keep doing it, and I learn and work around doing things on the cheap.
The displays are the final part that is needed for the thing I am working on. I have included the renderings from OSHpark. the 2 rows of pins are for the displays, 5 for each board. The MAX7219 is driving 64 LED (5 displays, 4 3mm, and 20 rectangle). And I spent a lot of time making sure that no traces, or vias can be seen from the front( I did not want to see them ). Trying to do this with common anodes would of been imposable, with what I wanted.
Intersil has Display Drivers for common anode:
http://www.intersil.com/content/dam/Intersil/documents/fn31/fn3160.pdf'
I too ran into this problem and have bought these chips but have not wrote as program for them as yet.
Thats not a bad driver. From what I can tell it is not serial.
No it's not. Eats up pins. That has been my problem with this projects. It's be impossible to find a serial drive for common anode drivers.
That is why I have always used common cathode, there are more drivers out there made for common cathode that does not use a lot of pins.
http://www.mouser.com/Semiconductors/Driver-ICs/LED-Lighting-Drivers/_/N-7zhqfZscv7?Keyword=TLC59213&FS=True&Ns=Pricing|0
I would do this trick to save pins for 8x8seg LED display:
Total use 9 Prop pins.
Set 8 as output with 1-of-8 as high going the above IC, pulse 9th pin clk signal low-high
Quickly set the same 8 pins as low or inputs and these are shared with low side of all 8seg LEDs.
Wait 1/200sec
clr is connected to clk, so now set it low and repeat the above to move on the next letter 1/8 duty this way
and very minimal visible bleeding if any.
Could dedicate a 10pin for full on/off control for clr for absolute no bleeding.
These days, there should be little difference in price and parts count, and I'd expect common anode to come in cheaper, as it allows Open Collector/Drain segment drivers, and they are made by the shipload.
eg TPIC6C595
Maybe impractical with the MAX7219, but that MAX7219 is seriously expensive.
With the right choice of serial interface drivers, like 2 x TPIC6C595, and ~8 small SMD parts, and you would be done.
You could use a PLD, but if you have a Prop talking to this, handling scan rates should be simple.
The SAA1064 has been around for donkey's ages and works really well driving 4 digits from the I2C bus, you can cascade drivers. Here's my Tachyon Forth project page.
EBay is not without risks, especially at the low end of cost. The vendor may have a high rating because people pretty much don't want to bother with returns and refunds of petty expenses. But specifications can be very sloppy.
I think I would deal with the risks.
20 displays
ebay.com ~ $8.95USD + free shipping
digikey ~ $27.30USD + $4.99 shipping
Mouser ` $18.00 + $4.99 shipping
I dont know of RS Solutions, so I can not compare there parts.
I would love to get the parts from a real distributor, but with home, school, food, car, gas, medical ,kids, dogs, etc bills. I just dont have the money to buy high quality parts. And please, i mean no disrespect by saying, It WOULD hurt to buy from Digikey, Mouser, etc.. You have no idea how hard it is for me to scrounge up money just so I can enjoy my hobby. That is why some projects have been lost because I can not afford the parts. It took me 3 months to order those boards from OSHpark.com ($30USD)
I am going to have to try that.
I might just use the displays for my reflow oven project. It is going to be a chalange because I am using a tired old BS2 for it.
I've been thinking about your displays a bit and I do think you could still drive them with a MAX7219. You'd need to heavily modify the software to do so. Instead of driving one display at a time like normal, you'd drive one segment at a time on all displays at once.
I haven't used the MAX7219 with 7-segment displays so I don't know if you use the "decode" register or not. If so, you wouldn't be able to use this feature of the chip. The software would have to tell the chip which segments to light up when. Still I don't think it would be too hard.
Of course it wouldn't work with your current board.
Hey! BS2's don't get tired, they get better with age!
I do understand that money is tight, but there is indeed a limit to what you can do by scrounging for cheaper parts.
In most cases, EBay is dumping excess parts that are either obsolete for new manufacture or the wrong item. If you do feel that it is the only way to go, do the following:
A: Design and test your creation.
B. Buy parts from EBay and such
C. Order the board manufacture after you have all the parts in stock.
As it is, you have a board designed for one thing, and parts for another configuration. So it seems that Digikey and Mouser are your main alternatives. Sometimes Futurlec that these items as well, but the are slow and some people here distrust their quality.
I don't use EBay as there is nobody giving free shipping to Taiwan, and in many case they won't even take an order from Taiwan.