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Topic: Need help with large thermochromic matrix design (Read 2 times) previous topic - next topic



I'm doing a feasibility study for a large (27 x 46) matrix display, but despite trawling the web for a few days I can't find any examples of circuits I can base my design on. I have a very crude knowledge of shift registers (I have a simple 8x8 LED matrix working happily in a breadboard) but I can't find any clues on how to build exactly what I want.

The problem is I DON'T want to use LEDS! :smiley-eek:

I have a tonne of 10ohm SMD resistors and want to mount these in a matrix under a sheet of thermochromic film. The resistors warm up nicely with the tiniest voltage through them and show up as a rainbow spot on the thermochromic sheet. I'm guessing there'll be sneaking paths in the matrix and resistors around the selected one may 'illuminate' at quarter brightness, but as this display is for artistic purposes it's not much of an issue. That, and I don't really want to spend £100 on 1242 SMD diodes!

I have a handful of ULN2003As, and have been trying to introduce these into my typical LED matrix circuit to see if I can get more power in, but applied to the anode or cathode side, the Arduino just crashes and resets. I'm getting the extra voltage from using an external supply and pinning the ULN2003A's COM to the Arduino's Vin, which has worked fine in other projects.

Any input would be very greatfully received,



How much heat will be produced in total?  temp?  as first thing that came in my mind was: "Fire!!"
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)


Hey Thomas,

First impression - this sounds like a freaking awesome idea! Nice one.

When it comes to addressing the matrix array, don't worry about it. The biggest issue with LEDs is going fast enough that you don't see the flickering, but this won't be an issue with you as, I'm guessing here, the heat will take a little time to decay away.

Two thoughts strike me instantly - the heating effect in resistors is all about I2R :: Power = Current * Current * Resistance, the voltage is secondary. Also remember that resistance may change significantly (and non-linearly) depending on the resistors (check your datasheet with a fine toothcomb for absolute limits too)

Second thought: What is the active temperature range of the thermochromic film? Is it going to look the same in an museum gallery at 10 degC as it will on a tradeshow exhibition floor at 35 degC ?

As for the crashing issue, maybe some schematics could help us. Sketch it on a napkin and take a photo for us.



Since you have UNL2003s, how about this (I can draw up a schematic when I get home).
Picture as a 3x3 array to start, for discussion sake.
       QT1  QT2  QT3
QS1  R1    R2    R3
QS2  R4    R4    R6
QS3  R7    R8    R9

You have 3 transistors (which are the ULN2003 output pins) across the top of the array, and 3 down one side.
Side 1 (tops) of resistors R1, R4, R7 connect to collector of QT1.
Side 2 (bottoms) of resistors R1, R2, R3 connect to collector of  QS1.
and so on to complete the matrix.
Emittors are at ground (which is the ULN2003 design anyway).
R1 heats up when QT1 is off and QS1 is on.
R4 and R7 would also heat up if QS2 and QS3 were on respectively.
To heat up R2, R4, R8, then QT2 would turn off and QS1,2,3 on respectively.

Multiplexing is the key to control of course.

Need resistor between Vsuppy and collectors of QT1,2,3 so they are not shorting supply to ground when turned on.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.


If you had ULN2803's you could put one the output of a shift register like 74HC595, and each shift register/ULN2803 could control one 4x4 area.
Then your display could be as many strings of 4x4 blocks arranged as rows, or columns, or larger squares, or however you wanted to arrange them (contiguous, spread out, different sized clumps, etc).

But the building block could be the same: shift register, driver, resistors, and power/ground/3 control lines (data, serial clock in, load signal)
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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