Saturday, November 8, 2008

Better LED Rails, FTIR Attempt

Earlier this week I ordered a piece of acrylic 12" x 24" x 1/4" from It is about 6mm thick, where my LEDs are 5mm in diameter. I also got a new laptop charger off of craigslist to run the LED rails. Here are the specs of the charger:

Since it is 19vts (19.43 volts so says my voltmeter), I needed longer chains of LEDs in series (12 instead of the previous 8), and higher ohm resistors (12 instead of 1), all according again to

Again, here are pics of my previous LED rails taken off:

And what the LCD looks like without any LED rails on it:

It's basically the LCD crystal taped to a piece of thin acrylic with both masking tape for securing it down, and electrical tape on top of that to create a solid black frame.

I went to Home Depot and got some aluminum L beams to mount the LEDs more accuratly in and cut them to size:

I measured out the height needed to drill the LEDs at on the beam:

And then drilled the rails, insulated them with electrical tape, put the LEDs in, soldered them together, along with wires running to the new power supply, and put the power supply into the case.

(That's the laptop power supply hiding behind the power strip in the corner)

And below in a Side DI setup, which is the same as FTIR, minus the acrylic:

Then I put some screws in the floor to hold the backlight in place:

I then began work on polishing the edges of the sides of the acrylic. I was told to get 800, 1000, and 2000 grit sandpaper and some polishing compound, but Im on a budget so I only got 800, 1500 and the polishing compound.

Initially I just hand sanded the edges, but the curve of my palm caused the edges to be curved instead of flat, so I wrapped the sandpaper around a deck of cards as a sanding block and that worked well. The edges came out a lot smoother.

Here is a video showing the initial performance of the FTIR setup:

It works okay, but I have to press harder than Id like to get the blobs to show up, so I posted to the nuigroup forum to try and get some answers on improving it. There also is very little capability of dragging, since applying pressure and dragging is hard to do.

What works really well is removing the piece of acrylic and using it again as a Side DI setup. The higher amount of LEDs and the more even distribution of them makes it for a perfect setup in Side DI. Also the height of when the blobs appear from a falling finger can be reduced to about 5mm, which is really good, but still not as good as a working FTIR setup, so the quest continues to find the best and most practical setup when on a budget and using an LCD.


  1. How you solve the backlight problem for your FTIR setup?

  2. Backlight problem? I used the same backlight from the LCD to light it up.

  3. Are you using a compliant surface of any kind?

  4. Not in the video above and not I havnt tried to make one for an LCD FTIR setup as it is known to be difficult to not have the LCD image become dimmer or fuzzy with one on top.