Re: About some parts on logical circuits

On Wed, 23 May 2007 15:11:56 +0200, "Dr Gang" <dr.gang@xxxxxxx> wrote:

Since I was aware that it was a quite heavy current, I told the guy so that
he'd pick up a fitting transistor. He looked up in his transistor bible and
gave me a rather big one with a holed package so that I can attach an
heatsink if needed.

A thick piece of aluminum strip will often do it. You may get by with
one per channel (all 8 or so drivers on one strip). Use heat-sinking
compound like the stuff used on CPU heatsinks. And a mica insulator if
necessary.

Here's another idea: Run the LEDs off the transistor's collector. That
would eliminate the voltage threshold problem, but you'll still need
to make sure the JK can source the current.

Why would a branch going from the collector to the LEDs' network take in
consideration what's going on in the emitter branch ?

The base current controls the main current.

Base current flows from base-to-emitter. Main current flows from from
collector-to-emitter. (The from-to direction is arbitrary depending on
NPN, PNP, etc, but you get the picture).

So injecting a small current into the base will cause a larger current
from emitter to collector. The larger current flows through both the
emitter and collector, so you can put the LEDs in series with either.
There are differences in the way that the circuit works, depending on
which you opt for.

Since the current that you inject into the base is going through the
emitter, and so is the collector current, the emitter current will be
the sum of both. Let's say the transistor has a beta (gain) of 10. A
base current of 1ma will turn on 10ma of collector current (ideal
conditions). The base current = 1ma. Collector = 10ma. Emitter = 11ma.

Regular bipolar transistors are current amplifiers, so the input works
on current. FETs and tubes do much the same, but the input is voltage
(output is still current).

This is simplified.

The logic is quite simple : pushing channel one temporary switch sets a JK
flip flop to high while resetting the 2 others through OR gates (so that
pushing channel 2 OR 3 turns off channel 1 for instance).

Again, without seeing a clear picture, I won't comment on circuit
specifics. (I'm not inclined to untangle your drawing if you aren't).
But have you considered what happens if you step on more than one
switch? Since you're controlling amp signal paths, you may want to
make sure that you can't induce oscillation if more than one path is
turned on.

I was considering suggesting that you look into a keyboard decoder
like the 74c922 or 74c923, but you'd have to run two lines for each
switch. Maybe there are other similar keyboard encoders.

That general scheme would work in that you would pick the appropriate
input to turn on only one of the output lines (binary pins 1, 2, 4, or
8). Chip info here: http://www.elexp.com/tips/74C922.pdf
Not sure if anyone makes an equivalent chip that ground-refs the
switches.

Also you don't know what the logic condition will be on power-up, do
you?

Are you perf-boarding this? Easier than trying to do an actual circuit
board, especially in that you may need to modify it. If it's a
perf-board, leave lots of space so you can retrofit more circuitry if
you run into problems.

In any case, it's cool to see that you're taking some initiative. But
how about drawing a clear version of the circuit and posting that?
.

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