I've been working on a project that will read the temperature off some probes and display the temperature locally (and log it to a database), I have everything working with one d'oh exception. I purchased one of those pre-assembled LED controllers, based on the MAX7219 driver that would in turn control a mic2981/82yn to power my larger 7 segment display. My larger 7 segment common cathode digits (Specifically the Lite-on LTS 1723P). I tried to find the common annode version of this 7 segment display, but no luck.
After reading around I can see that the MAX7219 cannot drive common cathode digits, so I was wondering if anyone knew of a good pre-assembled driver like these
that are all over eBay that I can use as a substitute OR if there is a work around for this? One thing I was considering was using some PNP transistors in place of the MIC2981 driver. That way when a segment was told to light up on the MAX7219 a ground path would be created on the base pin of the transistor and current would be allowed to flow to the appropriate annode pin for that segment. And on the flip side a simple ULN2803 could be used to provide a ground path for the common cathode of my 7 segment digit when the MAX7219 selected a specific digit.
I'd prefer a pre-assembled solution since I'm ready to be done in the most expedient manner, but am I on the right path with the PNP transistors idea if the pre-assembled option doesn't work?
After reading around I can see that the MAX7219 cannot drive common cathode digits,
Absolutely and totally wrong and foolish advice! Even allowing that you have confused and unwittingly exchanged “common cathode” for “common anode” in your description. The MAX7219 can drive either form of display equally effectively.
The actual problem is that you need more than 5V to power the drivers for those displays as their nominal operating voltage (except the decimals) is 4.5 to 5.2 V.
You need to use TPIC6B595s, one per digit with individual segment resistors and a 9V supply. Exactly as you would if the MAX7219 was able to provide sufficient voltage and you needed to drive common-anode displays, you need to generate the number font yourself.
Hi, if I can suggest an alternative, have a look at the saa1064 driver chip. Designed for common anode displays with higher forward voltages. Like the max chip, but unlike '595-based circuits, this chip does the multiplexing for you and keeps the circuit simple. Connects to the i2c bus.
Hi, if I can suggest an alternative, have a look at the saa1064 driver chip. Designed for common anode displays with higher forward voltages. Like the max chip, but unlike '595-based circuits, this chip does the multiplexing for you and keeps the circuit simple.
True enough, but in fact what I suggested, using TPIC6B595s, one per digit also completely removes the need for multiplexing - I just though I should make that clear.
I have a strong suspicion (having bought some from China) that the TPIC6B595s are more readily available. (I may also be referring to cost - if you are going to make it just once on stripboard, ease of assembly and PCB real estate are less significant.)