Using existing pinball switch to fire external circuitry ...

W

Wes

Jan 1, 1970
0
I would like to utilize the closure of an existing switch in my
pinball machine (Bally KISS) to trigge
r external circuitry. My initial
idea is to have a spinner switch create a pulse that would serve as a
clock for a decade counter (4017
).

So I researched the fundamentals of the switch matrix and how it is
scanned by U10 on a -35 MPU. The
manual shows the 2 wires (poller/send + switched return) on the MPU J2
connector to be at pins 4 and 14, respectively.

So initially, I figured that I could simply tap off these 2 wires and
feed them into an AND gate, usin
g the AND output for the clock pulse into the 4017 counter. BUT, here
is what I have found, and I'm h
oping someone can help me accomplish the goal. I'm thinking a
comparator or some sort of A/D converte
r is needed, but I'm inexperienced in the application/use of linear
ICs.

When I measure the voltage of the strobe/send wire (J2 pin 4), I do
get the expected pulse and I was a
ble to clock a 4017 directly with this send. Obviously, this doesn't
isolate the closure of the spinn
er switch, though.

The voltage at the RETURN (J2 pin 14) is where I start to lose it. I
expected it to be near 0V when t
he switch is open, and near +5V when the switch is closed. But what I
find is that this pin seems to
be strobing, and when I measure the voltage, I get around +3.2V when
the switch is open, and around +3
..4V when closed. I WAS able to see that this .2V delta is consistent
with other switches in the same
column of the matrix, so ....

I do seem to have almost what I need - I'm thinking I can determine
(theoretically) that the switch mu
st be closed if:
1) the send wire is "high"
2) the return wire is relatively "highER" (the .2V delta)

So, I THINK if I could just translate the +.2V delta into a logic
"high", then I could feed the AND ga
te as I originally thought I could do. Any insight into how I could
make this work would be greatly a
ppreciated.

I do realize that there are other (possibly better) ways to accomplish
this end goal - i.e. an isolate
d parallel switch, an opto-decoupler - BUT I'm intrigued by the idea
of tapping directly into the swit
ch matrix, and if I could get a handle on it, it would open up a world
of ideas for modifications with
easy installation (piggy back off the MPU J2 connector).

Thanks! -Wes
~

R

Rich Grise

Jan 1, 1970
0
[email protected] (Wes) wrote in message
I would like to utilize the closure of an existing switch in my
pinball machine (Bally KISS) to trigge
r external circuitry. My initial
idea is to have a spinner switch create a pulse that would serve as a
clock for a decade counter (4017
).

So I researched the fundamentals of the switch matrix and how it is
scanned by U10 on a -35 MPU. The
manual shows the 2 wires (poller/send + switched return) on the MPU J2
connector to be at pins 4 and 14, respectively. ....
Thanks! -Wes

Well, you've figured out that it's strobed (or scanned) - this is a
Good Thing. So what you want to do is sample that particular
switch's return line _while_ it's being strobed. And since the
spinner is so much slower than the scanner clock, you don't want
to gate the clock pulses with the spinner switch - clock a D
flip-flop with the strobe line, and get its input from the other
side of the switch. (possibly conditioned). That should give you
a reasonable approximation of a rectangular wave which shows
the current state of the spinner switch.

Good Luck!
Rich

W

Wes

Jan 1, 1970
0
Thanks Rich! This has helped me out tremendously, although I still
can't get the desired result.

Your mentioning that the switch contact is much slower than the strobe
frequency made me see that I was on a horribly wrong track. Since
closure of the switch will only "make a copy of" the strobe signal on
the return line, by feeding an AND gate with these 2 lines, all I'll
get is the same strobe at the out of the AND - not at all what I want
to clock the 4017.

But I just don't understand what I'm seeing - the return line is
pulsing, whether or not the switch is closed. I would have thought
that as long as no switch on that return line was closed, its voltage
would be "low". And I would have thought that if ANY switch on the
return line was closed, it would pulse at the rate of the send for the
closed switch. Not the case - that return line just pulses away
regardless of the switch's state...

When I hooked up the D FF as you suggested (clock with send, use
return line of switch for data), the Q output just pulses away whether
or not the switch is closed (it's a nice steady pulse). That seems
really weird to me - since we're tracking the return line ONLY when
the send line is going high, it seems like a steady pulse wouldn't
happen.

Here's one thing that occurred to me: since the FF will clock on the
rising EDGE of the send pulse (thus transferring the D input to Q
output), and the D input on the other side of the CLOSED switch is
just a "copy of" the send/clock, won't the D input then be in sort of
a transient state (going from low to high just as the clock is), thus
making it a bit unpredictable as to what will get transferred to Q?

Why would that return line be consistently pulsing? Could it be just
be the effect of the pin being READ? I would have thought since it's
an INPUT pin to the PIA, its being read wouldn't affect its voltage.

See, this is how I understand the switch matrix works:

There are 8 sends and 8 returns. Each send drives a column of 8
switches, and each return "reads" a row of 8 switches. Then in a
round robin fashion, this algorithm happens:

make only first send high
(1 closed switch on send line will cause
high to appear on 1 return line)
make only second send high
(1 closed switch on send line will cause
high to appear on 1 return line)
etc....

Could it be the sequential READING of the return lines that makes it
pulse?

- Wes

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