Re: About some parts on logical circuits

On Thu, 24 May 2007 16:29:47 +0200, "Dr Gang" <dr.gang@xxxxxxx> wrote:

The base current controls the main current.

Ok but let me raise another objection, even if you're right about all this
of course :

If? Hey, I've been trying to help you here. Think about this: Active
devices like transistors and tubes need to have some way to
amplify--to turn a small input voltage or current into a larger
output. I stated how that works in the most simplistic terms I could
think of. You should probably read Wikipedia on BJT's:
http://en.wikipedia.org/wiki/Bipolar_junction_transistor

The LED network has one side going to the power supply, the other to the
ground, in between you have resistors and LEDs.

Are you talking about symmetry between emitter and collector?
Simplified again: You have a battery, an LED, and a switch--connected
in series (a loop). It doesn't matter whether the switch is connected
to the + terminal of the battery or the - terminal, right? It's going
to control the LED either way.

That is somewhat analogous to connecting to the emitter vs collector.
The base current simply turns on the switch.

But the analogy (and symmetry) stops there. If you're wondering how
things are different when you connect the LEDs to the emitter vs the
collector:

The base current goes from base to emitter. The base is virtually
isolated from the collector. To understand that you will have to read
the Wiki entry and probably more. (This eventually gets into quantum
physics)

But the idea is that the emitter circuit is sort of inline with the
base. If the emitter is 'biased' at over a volt (which it will be if
the LEDs are in series) then the voltage feeding the base has to be
over a volt to push current into the base. (Actually the base-emitter
junction itself takes about .6 volts to start conducting, so that is
also added to the input voltage requirement)

If you had a bunch of LEDs in series, hooked onto the emitter,
eventually the JK's output voltage would not be high enough to push
the base into conduction--the LEDs would effectively be in series with
the input to the base. Not so if they're hooked onto the collector.

That's about the best I can do for now. Sounds like you want to read
up on the theory.

So, and it's coherent with other posts here, not to have insufficient
current issues at the flip-flop output, I should use MOSFETs rather than
bipolar.

There are other ways around it but MOSFETs may be the simplest
solution.

There you go : http://dr.gang.free.fr/test/Switching%20simplifi%e9.jpg
The cap right after the ch.1 momentary switch (they're all momentary of
course) is there to let a transient pulse set the amp to channel 1 when
powering up before setting to 0v very quickly and not have any effect
anymore.

OK, good. The circuit is easier to see. Remember the pull-down
resistors at the switch outputs (logic inputs).

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 could be a very good idea. It'd simplify the PCB considerably;

The 74c9xx's unfortunately need to connect to both sides of the
push-switch. Awkward if you're running long footswitch lines. Ask on
electronic groups whether there are other similar chips with
'ground-referenced' switch lines.

I don't have a perfboard, I'll get one next time I go to the electronic
store.

And sockets for the chips.
.

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