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Posted: Sat Jul 19, 2008 8:12 am
Here is the best circle I could make up using 28 20x10 blocks. At almost 42 feet across by 33 feet high, it's going to be a monster!
More to follow.
Posted: Sat Jul 19, 2008 11:31 am
Here is a block diagram of a possible display module and PC-interface.
Each module would have an individual select line and a common data and clock line would be fed to all modules.
Disadvantage of having 200 pixels on a single module is that in order to write to a single pixel, all 200 pixels must be written to as well. This is because the MAX335 is in effect a shift register and so connecting 25 of them in series produces a 200 bit shift register which must be clocked through with 200 bits of data. You could split the MAX335 into smaller groups, but that would increase the number of select lines needed to access each module.
It would be possible to connect every MAX335 (all 700 of them) to for a 5600 bit shift register, but now you would need to write to the complete display in order to change the state of just one pixel.
Hope you got a large budget for this, you will have to spend over $2000 in MAX335 chips alone!
Posted: Sat Jul 19, 2008 7:31 pm
You're right, It'll be pretty a big screen, I have a decent budget but the LCD film cost much more than I expect, oh well...
thanks a lot for the diagrams! I'll try it this week and keep you posted.
I Think I'll distribute the 28 panels as shown in the attached image, the more square the screen is the easier it is to install it on the wall with the curtain wall system (and cheaper).
you actually can find more info about the project at
What do you think about this idea someone gave me?
Use a 15V center tapped transformer and connect the center tap to the common (GND) for the section. Each end of the winding is 7.5V in opposite phases. I would rectify the 7.5VCT into + and - approximately 11VDC (unregulated). These go to the +V and -V pins on the MAX335. One end of the 7.5VAC winding goes to one terminal of each (all) pixels. The other end of the 7.5VAC goes to each (all) of the COM pins of the MAX335. Each of the NO pins on the MAX335 goes to the other terminal on each pixel.
If the pixels turn out to be accidentally driven by capacitive leakage, you would drain that off with maybe 10K across each pixel. Maybe a 100nF capacitor in series with every pixel just to block all incidental DC.
how should I rectify the 7.5VCT Rob? with a full bridge and a transformer?
thanks a lot again Rob I do really appreciate your help.
Posted: Sat Jul 19, 2008 9:51 pm
Don't ground center tap and rectify the 15VAC transformer for DC supply! Use separate Â±15VDC and +5VDC supply as in Viking's drawing.
You would have two "commons" (DC ground via center tap, and return from panels) which must be connected, even by accident; or big puff, bye-bye lots of stuff. Besides, you've enough panels to keep separate supplies "fully employed". More also, switching supplies for DC is more energy efficient and don't create power factor problems.
At 60Hz a 100nF cap will too much impedance. Save a bunch of grief, and extra parts, by using the 15V transformers only for AC to the panels.
All IMO of course.
[edit, added later] The MAX335 can switch plus or minus 15V. (peak analog voltage can't exceed DC supply) Unforunately, the 15VAC of the transformer is a little more than 21V peak. 10VAC would work with MAX335 and newer generation film (have you purchased the 15V film yet?). Or you could ground center tap and use two switches per pixel (still need separate DC supply). Or try to find a higher voltage switch (MAX4800 (maybe))
Posted: Sun Jul 20, 2008 12:34 am
I strongly advise keeping the screen supply separate from the DC supplies.
The +/-15V supplies need only supply about an amp maximum (15W+15W) switching supply and the transformer will only be around 20VA and if you use a toroidal, it will be light and have a slim profile.
From your layout, it looks like you will not have as many pixels as I first thought, which is a good thing!
I still canâ€™t find any specification on the LCD. For instance, is the 15VAC RMS or Pk-Pk. What about capacitance?
I will put up a suggested circuit diagram when ready.
Posted: Sun Jul 20, 2008 3:31 am
Here is a quick sketch.
It would need additional decoupling and probably a local data buffer if the number of MAX335 is large. You will note that the transformer is 10VAC to limit the peak voltage to less than 15V, and a bi-directional clamp has been added to ensure ac supply never exceeds the maximum voltage. Iâ€™m still not sure if this device will drive the displays, we may need to go to a switch with a higher maximum voltage or use an alternative switching device. The data sheet for the MAX335 is a but vague on the maximum switch current, it is somewhere between 1mA and 30mA. And we need to factor in the capacitance of the display as well.
Posted: Mon Jul 21, 2008 9:42 am
Hi Rob and dyarker,
Thanks a lot,
I haven't got the film yet so I think you're right dyarker, I should go with the NPD-300 (a little more expensive but lower voltage).
Thank you so much for the diagram Rob, I really appreciate your help. I'll need 25 max335 chips for panel, and each panel will get a RS422 serial line from the image processor.
I asked the guys from Maxim about the max voltage of the chip and they replied that:
"According to the datasheet, this device is able to Accepts Â±15V Analog Swings. However, make sure consider the power sequencing factors mentioned under "ABSOLUTE MAXIMUM RATINGS", in page 2 of the datasheet."
Also I'm pretty sure it's 15VAC RMS since the film comes to on 120V to be directly connected to an AC outlet.
this is the tech specs I have for both films but I asked the fabricator about the capacitance of the film.
Posted: Mon Jul 21, 2008 10:01 am
If it is RMS, then peak-to peak will be about 21V.
So +/-15V swing (Peak-to-peak) will be 10.6V RMS.
So this is the MAXIMUM voltage that can be fed to the display if using the MAX335.
It may be easier to abandon the MAX335, and use a shift register, driving a output driver. Then there is no question of current or voltage limitation and possible reliability issues with the MAX335 would be eliminated and your choice of LCD material would not be restricted.
The driver would still have to be bi-directional, and will still be quite a complex arrangement, which is why the MAX335 was such a elegant and efficient arrangement.
Posted: Mon Jul 21, 2008 10:11 am
I'm afraid that if I use a triac with Optoisolators connected to the shift registers I may get too much electrical noise.
The NPD-300 uses 10V so still I'll be able to use the MAX355, right?
what about using MAX4800? is a Low-Charge Injection,8-Channel, High-Voltage Analog Switches.
Posted: Mon Jul 21, 2008 11:42 am
A triac is actually an excellent solution, is bi-directional and should not cause any noise problems. You only get these problems when you switch large loads abruptly, especially in the middle of a mains cycle and usually when you switch the triac off. In this application, we can limit the rate of change of current to such a low level that there should be next to no electrical noise generated. If we needed to, we could implement zero voltage switching and eliminate noise completely.
If the film voltage is 10V RMS, then the peak voltage is 14.1V, which is just within the capability of the MAX335. I will look at the MAX4800, I think it can switch several 100 volts.
Will also look at the triac idea and do some simulations.
Posted: Mon Jul 21, 2008 12:04 pm
the LCD film fabricator sent me this reference.
http://www.allegromicro.com/en/Products ... /index.asp
is a 32-Bit Serial-Input Latched Source Drivers with Active Pull-Downs
it seems a feasible solution too.
I order some max355 and max4800 so I'll start trying pretty soon and let you know how it goes.
Posted: Mon Jul 21, 2008 12:16 pm
Yes, that device would be ideal, if going for the discreet pixel switching option.
Lots of possibly device similar to this one exist.
Posted: Mon Jul 21, 2008 12:44 pm
I think at this point I have to do some research before I ever ask, because I don't even know what discreet pixel switching means.
I thought that these chips were also a Shift register controlling an array of Darlingtons so you could use it kind of the similar way of the Max355 But I guess I'm so wrong.
So you see any advantage of one system over the other?
Posted: Tue Jul 22, 2008 3:46 am
You misunderstand. By discreet, I mean that the switching element is not part of the shift register as it is with the MAX335. You would have a shift register as one device and connect that to 'discreet' switching elements (e.g. triac's). The resulting circuit would operate in the same way as the MAX335.
I think the MAX335 is dead, as you will need to use a ac voltage considerably higher than the minimum needed to fully switch a pixel. If you operated very close to the minimum voltage needed to switch the pixel, then you will probably get an uneven look to the display, some pixels will be fully switched, some 95% switched, etc.
Posted: Tue Jul 22, 2008 5:24 am
I see what do you mean, the A6818 chip doesn't support AC on the switches so I'll have to use triacs anyway, right?
I wonder if I should try to use the max4800 directly.
thanks so much Rob