Saturday, September 5, 2009

Allen organ project - lighting the LEDs

Last time, I breadboarded the LED controller and got its serial communications working. Now it's time to fire those LED's.

When the controller is idle, all the LED's are on, as if there is no card in the slot. (Among other things, this convention allows me to continue programming the organ with cards, if I wish.)

Breadboard LED control - powered on

Wow, those LED's are bright.

Now, let's look in detail at what happens when a card is inserted and removed.

[Update: I was wrong about the quadrature signal. The next post explains.]
  • First, the paper blocks all the lamps (LEDs are all off). This condition persists until column 6 lines up with the photoresistors.
  • Next, for each group of three columns, the row 9 hole, and the data in rows 0-6, come into view. LED 9 and the data byte are lit.
  • The row 8 hole comes into view early, because its photocell is displaced half a column. LED 8 lights.
  • The data column leaves the photocells (LEDs 0-6 and 9 go off).
  • The timing column leaves the photocell (LED 8 goes off).
  • About 1 1/2 column of blank space (all LEDs off) passes over, and then the cycle repeats for the next fifteen data bytes.
Once all this is done, the LEDs stay off for a short pause (the card blocks the light from all of them, andt then the entire cycle repeats, reversed in time, as the card is withdrawn. Finally, all the lamps are lit as the card exits the reader. has the code baseline that does this. With the clock rate slowed down to where my human eyeball can track what's going on, the data look right.

Now to move the thing onto the card reader and try it out on the organ!
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