🚀 go-pugleaf

touch@dsl.cis.upenn.edu (Joe Touch) writes:
>
>
>RS-latches?  JK-flip flops?  A whole C-64?  Geez, folks, talk about
>doing things the hard way!!
>
>Here's a way thats SIMPLE, CHEAP, AND *SCALABLE* (easy to extend to an
>arbitrary number of inputs, if you balance the OR into balanced trees)
>
    Talk about deja-vu! About 20 years ago when I was still in high, I
was posed with this same problem by one of the teachers there. One of
the local radio stations had a program called Scholastic Tournament in
which students from various high schools competed. The teacher at my
high school wanted an arbitrator built so that the students on our
team could practice.

    Not having much design experience, I asked my father for help (he
was an engineer at the radio station at that time). He came up with a
circuit that used Silicon Controlled Rectifiers (SCRs) and a
transistor.  It has been a long time and I can't remember many
details, but I can at least describe how it worked. Any EE worth the
cost of the paper his degree is printed on should be able to figure
out the fine details of the circuit (I'm a programmer now and my
design skills still aren't up to much).

    The system consisted of a lamp for each person with an SCR connected
in series with it. The cathods of all of the SCRs were tied to ground 
and the side of the lamps not tied to the SCRs were connected together.
This lamp common was then tied to a low value resistor which was then
connected to B+.

    At this point things get real hazy. The transistor circuit was
used to supply the gate current for the SCRs. It was set up so that if
one of the lamps was turned on, the voltage drop across the resistor
in series with the lamps would reverse bias the transistor and turn it
off.  Since the transistor was the source of the gate current for the
SCRs, turning it off would prevent any other SCRs from being
triggered.

    Resetting was accomplished by putting a switch in series with the
lamp common. Opening this circuit would turn off the SCR that had been
turned on and thus turn the transistor back on, allowing another SCR
to be triggered.

    The circuit was very simple and very cheap. Once the transistor
circuit was constructed, adding additional players consisted of merly
adding another SCR, lamp, switch, and what resistors were required for
that portion of the circuit. And try as we might, we could never get a
tie. The switching action was fast enough that there never could be a
race condition.

    Eventually the radio station hired me to build one for them as
well. For them I also added sound effects. This consisted of two
different sonalerts connected to the lamps through diode OR circuits.
I connected half of the lamps to one sonalert and the other half to
the other sonalert. This allowed a listener at home to be able to
determine which team was first.

    Probably the nicest feeling I had occured a short while back. I
was searching across the radio dial for something to listen to and I
chanced upon the same radio station broadcasting the same tournament.
When a player pressed a button, I could distinctly identify my old
arbitrator, still at work twenty years later.