Ken,

There are no strobes on the '154, they are output enables.  I have used the
'154s for running an LED Clock in a manner similar to what you are proposing
(although I used a PIC 16C84 rather than a Stamp).  Because the '154 is NOT
latched, I'm not sure it is the part you want to use.

If I was going to use a straight PIC (I'll explain why I'm making this
distinction below), I would use the following circuit (please excuse the
ASCII graphics):


                 4      __'154__________
             +---/-----|A-D (Selects)   |
  _PIC__     |         |                |              ____'154______
 |4 O/Ps|----+  Gnd----|G1-G2 (Enables) |             |              |
 |      |              |                |             |       O/P "0"|-- To LED
 |      |              |         O/P "0"|-------------|G1         "1"|   .
 |      |              |                |     Gnd-----|G2         "2"|   .
 |      |              | Remaining O/Ps |--To         |           "3"|   .
 |      |              |                |  Next       |          etc.|
 |      |   4          |________________|  '154s      |              |
 |4 O/Ps|---/----+------------------------------------|A-D (Selects) |
 |______|        |                                    |______________|
                 |
                 +----- To Nex '154s


You are basically cascading the most significant 4 address bits from the
first '154, to select the next '154, which has the LED you currently want to
display.  With this circuit, you could address up to 256 different outputs
(LEDs).  This circuit uses only 8 PIC outputs (which is what the STAMP uses)
and I don't think you should have any fan-out problems to the 14 '154s for
each "radii".

Because this circuit does not have any latching, you are going to have to
cycle between the 14 different '154s and select the appropriate LED for each
"radii".

A good rule of thumb for cycling LEDs, is to cycle at 500 Hz (there won't be
any flashing visible to the eye and the LEDs will be nominally pretty
bright).  This means, for 14 different "radii" of LEDs, you will have to
cycle at 7 KHz.  I don't think this is possible for any STAMP (although I
could be wrong).

7KHz should be possible in a PIC running at 4MHz and a simple Interrupt
Handler looking up from a table (one entry/byte for each radii) containing
what each radii should be displaying.  You want the maximum time for the LED
to be displayed, so this should be in assembler, running as fast as possible.

Now, if you were going to latch the values for the LED's, this should be
possible for the STAMP, but the logic will be a lot more complex.

A few comments:

1.  In your original note, you indicated that you would use two '154s to
drive a grid of LEDs.  '154s are totem pole output (not Open Collector), so
if you were to use a "Grid", you would get a column lighting up at one time.
This may be what you want, but I don't think so.  In any case, you would be
exceeding the drive/sink capabilities of the '154s (Each LED typically
requires 20mA+).  Instead, the LED circuit that you should be using is:

  '154 Output|-----~^~^-----------|<|------- +5V
                   220Ohm         LED

The resistor in series will make sure the current available to the '154 and
LED won't exceed it's drive capabilities.

With the selection circuitry above and the LED Drive shown here, you will
require 15 '154s (14 for the radiis and one to select the radii).

2.  How are you going to wire this?  If you look at the LED circuit above,
you will have to make 6 connections (3 wires) between the +5V Rail, the LED,
the Resistor, and the '154.  Multiply this by your 140 LEDs, you will have
to do 840 connections!

You should be thinking about designing a small PCB for each of the "radii".
I would probably say that the LED "Drive '154' should be on this PCB, so the
only wiring required is +5V, Gnd, the '154 Select, and the 4 Bit Selects.
When doing this, you should probably design the PCB to use a ribbon
("Scotchflex") cable to carry the signals (avoid having to wire everything
individually).  The LEDs should be spaced so they poke out the bottom/top of
your "Saucer" at the correct locations.

This really isn't that onerous or expensive if you use a PCB Proto-house
such as AP Circuits in Calgary (I've used them for three of my hobby
projects with excellent results each time).  And with the time and effort
you save, you really won't mind the extra minor costs of such things as the
scotchflex connectors.  One Christmas, I made a half dozen led-lit XMas
trees and I thought I'd go blind with all the connections I had to make!
I've never gone back to doing my own wiring in those types of situations.

3.  I've copied this reply to PICLIST to see if anybody has any additional
comments.

If you have any questions, please drop me a line,

Myke
>
>I'm designing a UFO that has 140 LED's on it for halloween. It's
>aproximately .50m is diameter. The 10 of the LED's will be placed along 14
>seperate radii.
>
>I want to have the effect of both outward pulsing circles and spinning arms.
>
>
>What I'd like to use two 74154's to drive a grid of led's . One on the anode
>one on the cathode switching a series of transistors.
>
>On the 74154 there is 4 inputs 16 outputs and two strobes.
>
>Could some one suggest how I can use this chip? Where do the strobes go?
>
>========================================================================
>Ken Owen, B.A.                        176 Blucher Ave.
>President                             Thunder Bay, Ontario.
>Norlink Communications & Consulting   Canada, P7B 4Y7
>(807)767-5055
>
>
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Do you ever feel like an XT Clone caught in the Pentium Pro Zone?