5x7 LED matrix driver

"Jon Kirwan" schreef in bericht news: snipped-for-privacy@4ax.com...

If I understand your schematic well, you're driving each LED individually. By muxing you only need 5+7=12 lines and 12 transistors, maybe 5 more for driving the high side. You select one digit at a time together with all segments you want to light for that particular digit. Off course the correspoding segments of the other digits are selected as well but they do not light as those digits are not selected.

petrus bitbyter

An entire row is enabled at a time, just to be clear.

Muxing in the usual way would drive the LEDs in a ROW (0 to 7 of them) in parallel to each other. This isn't good as some LEDs will hog the current. I want to know that if I set 100mA as the peak current, that each LED will ACTUALLY have 100mA as its peak (100% duty cycle) current.

I know that what I've done in the schematic has been done in commercial products, because I've contracted to do work on them before. OSRAM built 16x16 RGB modules, self-contained, for OEM sales which actually used the technique I show (with lots more transistors, but the idea is right.)

I'm just hoping for a simpler method to achieve precision (5% worst case variation during a row-enable) current control of each LED and not have them all with individual BJTs... but I suspect that isn't possible. Going to RGB, which I intend on also doing, only adds 14 more transistors per color, do 28 more for a total of 92 BJTs for 105 LEDs. So it scales better than it looks.

Jon

I use a Constant Current Sink (not Source) to power a Common Cathode LED matrix.

The Constant Current Sink pins are controlled via a shift register. Then I use a 1-of-8 demuxer to select which "column" to use.

So, each column is powered by one state of the Constant Current chip, then I disable all current, set up the next column in the shift register, then move to the next column and enable current on the rows that need it. This means each LED has its own independent current controller.

BTW, I do this 16x60Hz so that I basically use PWM for intensity control (16 levels of brightness) and the entire display updates 60 times a second.

The design I illustrated in the link I provided also uses constant current sinks. But in contast to what you write below, I enable or disable all at once.

Okay. So if I follow you on the above, unlike the design I illustrated, you have individual control of each of the current sinks. Then you simply enable the high side switch. So you have, in this way, individual control of each current because you can enable/disable each driver.

So if I instead redesign the discrete circuit to permit individual control of each current sink (more transistors per, plus a shift register), then I could go to a common anode structure.

Thanks, I'll consider that one more carefully. I think it's something I didn't take time to consider and I need to do that. Very much appreciated criticism.

Jon

Hmm. This makes the single programming resistor circuit a little more complex, using discrete design. Looks like a constant current sink chip with an external setting resistor and up to 8 outputs and a shift register is the way to go, then.

I suspect that there are constant current chips with built in shift register enables. Know of any commonly used ones?

Thanks, Jon

I know some custom ones...

Last year I did some chip designs for billboard video ;-) ...Jim Thompson

I'm pretty much finding this $1 part:

TLC5916/5917, 3-5.5V operation, 5ma to 120mA, max 17V LED

Looks okay for my goals.

Jon

For reference, I used 3 of TI's tlc5917 [1] chip as the current sink, and a single 54HC238 [2] chip as the column selector. My MCU was an ATTiny84 [3] (basically, a scaled down Arduino core), and the LED matrix I used was equivalent to the one available from SEEED studio [4].

I did tester and optical calibration systems for OSRAM's OWM parts.

But they are unobtainium for me.

What do you think of:

TLC5916/5917, 3-5.5V operation, 5ma to 120mA, max 17V LED

Seems more than decent. 3% between LEDs and 6% between LEDs on different ICs. I could live with that.

Jon

Yup, that's the one I used. I replied to my earlier message with the list of components I used for my project (sans resistors and the 20MHz oscillator).

I'm dedicating one micro per digit, to modularize this, for now. So I have enough pins and don't need to worry about more. And yes, I'm seeing the 5916/5917 as the best option right now. ($1 each in small qtys.) MUCH less wiring results, so worth the extra part cost over lots of half-penny BJTs.

Did you feel a need for the special error detection in the

Thanks, Jon

Yep. That's a pretty standard matching. ...Jim Thompson

Yes, given a boosted current mirror design, with differing numbers of active current mirror sinking BJTs enabled, that seems about the best I could expect with a simple BJT design (I computed about 4% variation by hand _without_ taking into account variations in discrete BJTs.) On an IC, I assume they have more transistors (to improve on it) and better uniformity (to also improve on it) so that 6% across ICs seems like a comfortable design margin.

Jon

Yes, and the Shift Register+Latch aspect of it give you some advantages as well. My MCU only needs 6 output pins to drive 192 LEDs. 3 for Column Select, 1 for shift data, 1 for shift clock, and 1 pin which alternately updates the latch and enables output.

The 74238/54HC238 has a way to do output-enable with either high or low input, depending on which pin you connect to. I tied the "Latch" pin on the 5196's to each other and to the "Output Enabled" pin on the 54HC238. So that disabling the display latches the data. This seems to work really well, but I'm pretty newbie so there might be some problem lurking I don't know about

As far as "part cost", if you're hand building something, include the time-cost of assembly and trouble-shooting.

No. As a matter of fact, I think I have the 5916. I was quoting the part number based on the URL I used here :-).

I could see the error handling as more of a "professional" set up, where I would want to alert an operator of a failure so they could replace the module. Nothing I'd be working on.

Good luck. Let me know how it turns out.

Yeah, there is that of course. And board space required for all those holes and traces. It's much more attractive this way.

Still.... if I were doing up a LOT of these as a commercial product ... I can get the BJTs at 0.003 and .004 per in to-92

-- probably cheaper in smt. They are very common and will NEVER go away. Multisource everywhere. Part cost would be low, but I suspect placing and board space would make up for it. So yeah, I can't win with that crazy approach.

However... doing up discrete BJT current drivers that are individually controlled, rather than the way I did it, would cost more BJTs per... but instead of 35 BJTs for each LED, it would be 7 sets of controllable drivers. So just the basic idea you pointed at would also reduce the discrete count, too.

I'm glad I sat down and learned something doing that up in the first place, at least.

Okay! I think I save 20 cents or more that way. (I'm finding them at 85 cents each in ones as opposed to 1.04 for the '17.

Yeah. Same here.

Okay. I guess here, then.

The makerbot ABS module is very pretty, by the way. Makes a very nice looking unit. And weather proof besides.

Jon

"Jon Kirwan" schreef in bericht news: snipped-for-privacy@4ax.com...

So I understand you to activate one or more whole rows at once. But as you're multiplexing I suppose you activate only one column at a time. This way only one LED of each row can light, all others of that row are in other columns. So why not tie all cathodes of the LEDs of one row together using only one transistor to drive them? You can still control the brightness of every single LED. For instance you set the rate of column activation to 5ms and vary the row activation from 0 up to 5ms depending on the brightness required for that particular LED.

petrus bitbyter

No. I can operate more than one LED at a time with the schematic I laid out. Anywhere from 0 to 7 of them. So I only need to do 5x scanning for the display.

I think Dennis identified my stupidity, already, pointing out the use of a x8 controller like the TLC5916. I had avoided setting up completely separate and individually controllable current sinks in the silly idea of reducing BJT count, while actually doing the opposite. Although it takes more BJTs to do individually controllable current sinks, it takes a lot less of them to do the job. So it's a win. I just missed it while taking my first shot at this. Which is why I posted, to get my head bashed in. Dennis did that perfectly. Made absolute sense the way he wrote so clearly to me.

Jon

Sorry!!! That is Daniel!!!!

Jon

Just ordered 25 of the DIP from arrow at .81 each, plus about $5 shipping, which brings them to about $1 each.

Jon