Re: TECH: Electronics help needed

You may only be getting .49ma through the pullup resistors (on the
Zaccaria board) but you are probably getting a lot more through the
3081 because you are driving a lot of current (from the 74CH238)
through the base of your row transistor to the emitter. You need
series resistors from the 74hc238 to the transistor bases. You don't
even need the pulldown resistors.

Rick



On Dec 4, 5:40 pm, David Gersic <usenet_spam_t...@zaccaria-
pinball.com> wrote:
I'm stumped. I've been working on a design for a board here to simulate
having a switch matrix plugged in. This is for my test bench, rather
than having to probe connector pins with wire jumpers to test the switch
matrix function of a board being repaired.

The board functions as designed, but doesn't quite work. I've figured
out why it's not working reliably, but my self-taught EE knowledge isn't
up to the task of figuring out what the problem is and how to solve it.

Here's the schematic for Zaccaria's switch matrix:

 http://www.zaccaria-pinball.com/temp/ZSwitchMatrix.png

The matrix lines are pulled high by the resistors (R94-R101) on the
columns side of the matrix. The 3081 at IC38 is used to strobe (Low)
the rows side. The CPU then reads (via IC25 and IC15) the columns and
can tell a closed switch vs. open by the High or Low signal on the
columns, translated to a data byte on the data bus.

Here's a section of the schematics of my switch simulator:

 http://www.zaccaria-pinball.com/temp/Stamp-Matrix.png

I'm using a Stamp to control the two 74HC238s. The 16 BC548 transistors
are connected to the 8 row and 8 column lines from the CPU board, so
that I can close any single switch by turning on the appropriate row
and column transistors to complete the circuit. RN1 and RN2 are 8x10K
resistor packs.

The results are imperfect, though. Some CPU boards this works fine with,
but on others, the CPU doesn't see switch closures happening at all.
From what I've found, so far, it seems that the problem is that the
row strobe signal is being weakened.

Here's what the signal across the switch matrix looks like on my 'scope
using a jumper wire to close one switch:

 http://www.zaccaria-pinball.com/temp/img_4495.jpg

Here's what the signal across the switch matrix looks like, using my
board to simulate a switch closure:

 http://www.zaccaria-pinball.com/temp/img_4494.jpg

The first signal is a good High +5V signal with good strong Low 0V
pulses. That's what needs to be there. The second signal is a good
High signal, but the Low pulses are only about 2.5V. My assumption
is that this is just strong enough to work on some boards, but not
strong enough to work reliably on all.

With all of that as setup, the question is: what am I missing here?

Everything I've read on transistors suggests that the Collector to
Emitter should be conducting with almost no voltage drop, given that
the Base is switched on. I'm not seeing a voltage drop (+5V High
signal either way), so I think that's ok. I have not, as far as I
can tell, built a voltage amplifier here (+5V is what I'm seeing).

I checked with the jumper wire, and I'm getting .082mA through
the switch matrix. With my board instead, I get .049mA. So I also
haven't built a current amplifier.

So why is the 3081 no longer able to pull this +5V signal to 0V if
there are a couple of BC548 transistors in the circuit, when it can
if they aren't there?

--
| David Gersic                            http://www.zaccaria-pinball.com|
| Every person constructs their own bed of nails.                         |
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.

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