From: Bryan Levin <blevin@netcom.com>
Subject: DSD mod: simple upgrade for LED display
Date: Tue, 13 Sep 1994 10:31:06 -0700 (PDT)

Hello, all you DSD kit builders:

I came up with a simple circuit that improves the readability of the led
sample rate display.

In the current setup, you have 2 leds to indicate 3 states (32,44,48).  If
both leds are off, you have to infer that the samplerate = 44.

In the new scheme (below), you can see the display in terms of individual leds
for each sample rate.  Ie, there is a separate led for each freq.  As an added
bonus, when all leds are off, this is the same as no 'locked-on' condition
(no valid spdif signal on the currently selected input).  With this scheme you
only need 3 led's to show samplerate and locked-on; same as the current DSD 
except that (IMHO) this is a bit more readable.

Each led is driven by NAND gates and so is buffered from the actual
receiver/GAL signals (less line loading).  You also get brighter leds and
don't need resistors for current limiting.

The only part needed is a single 7400 (quad 2-input NAND gate) chip.  This is
available at radio shack (luckily enough - its one of the few TTL chips they
still carry!).

The ascii schematic is shown below.  To get a (more readable) postscript
version, FTP to my public area: ftp.netcom.com, then go to the
'blevin/dat-heads' area under '/pub'.  (I posted instructions on how to do
this a few digests ago).  I also drew up a rough PC board layout - its also
in my FTP area.  Its not drawn to scale, but the tracing does work
(I etched it by hand with a resist pen and etchant - also available at R/S).


Enjoy!

--
.bl

# web: <a href=" ftp://ftp.netcom.com/pub/blevin/index.html " Bryan Levin </a>



          4
       +---|---\ 6                 48
fc0----|   | ~* o--+-------------|<-----+
       +---|---/   |                    |
          5        |                    |
                   |  1                 |               12
                   +---|---\ 3     44   |  220  11 /---|---+
                       | ~* o----|<-----+-/\/\/---o ~* |   |-----erf
                   +---|---/            |          \---|---+
                   |  2                 |               13
          9        |                    |
       +---|---\ 8 |               32   |
fc1----|   | ~* o--+-------------|<-----+
       +---|---/
         10


+-------------- symbols --------------------+
|                                           |
| 7400 NAND gate:     LED:                  |
|                                           |
|   ---|---\            cathode     anode   |
|      | ~* o---             ----|<----     |
|   ---|---/            (notch)             |
|                                           |
|                                           |
+-------------------------------------------+

NOTES:

  - This circuit requires only one 7400 chip (quad dual-input NAND gate)
    Vcc is pin 14; ground is pin 7.  (Connect a 0.1uF cap from 7 to 14).

  - The numbers near each gate refer to the chip pin numbers.

  - The inputs to the left-most NAND gate come from signals named "fc0" and
    "fc1".  These can be found on pins 22,23 of the GAL device, on the DSD.

      You can tap into these by removing LEDs D1,D2 from the board, and running
      wires from the anode (non-notched) sides to the proper pins on the 7400.

      The NAND gates that connect to fc0,fc1 have both of their input pins 
      tied together (this may not be clear on the ascii picture, above);
      ie, on the upper left-hand gate, fc0 is connected to pins 4 & 5.

  - The input to the right-most NAND gate comes from pin 1 of U1 (the receiver
    chip).  You can tap into this signal at the base of Q1, on the DSD.

  - Only one LED will illuminate at a time, based on the input sampling freq.
    (But, if there isn't a valid signal being received, all LEDs will be off.)


-- 
.bl

[ed] I added a 220 ohm resistor on the 7400 pin 11 for LED current limiting.