🚀 go-pugleaf

In article <1990Mar19.154615.22362@tree.uucp> mirandor@.PacBell.COM (W. Le Roy Davis (1-23-90)) writes:
>I am looking for the circut for a very inexpensive three event detector.
>I work with the Boy Scouts, and they have this "pine box derby"

I couldn't send mail, so here goes:

I seem to have a no-frills circuit that uses 6 2-input nand gates, 6 resistors,
and 6 LEDs. This is assuming that your trigger switches stay on once they are 
triggered (ie, they don't just provide a pulse). If not, you will need three 
latches to capture the trigger inputs.

The basic idea is to compare pairwise, who beat who. To compare A and B, 

      ---------------------------------------------------------    
      |                                                       |
      |     |-------------|                    |-----------|  |
      |-----|             |   LED to VCC       |           |  |
            |  NAND       |--------------------|  NAND     |--- LED to VCC 
GND-^^^^--|-|             |     GND---^^^^--|--|           |
   100ohm | |-------------|          100ohm |  |-----------|
          |                                 |
          |                                 |
           /                                  /
          /                                  /
          |                                 |
          |                                 |
         VCC                               VCC

Normally, both NAND gates are high, since the trigger switches are open and
the corresponding inputs are low. (the resistors have to be low enough to 
ensure this condition). Suppose A beats B. Then A's Nand gate output goes 
low. This disables B's NAND gate. 

The gate of the winner is low.  All you have to do now is compare the
three pairwise.  The winner is the one with two wins, the second has
one win, and the third none.  For convenience, you could hook up all three
LEDs of a contestant near each other. Or you can get to be as fancy as you
want - put in digital displays, colored lights etc depending on whether a
contestant had 0, 1 or 2 wins. Not too complicated.

This is cheap - at the expense of assuming ideal components. The
disadvantage is that if the cars are within a fraction of a
microsecond of  each other, you could get contradictory results. A
small price to pay for a simple, ckt, I think.

Shaji Bhaskar
shaji@sbcs.sunysb.edu