john1

Hi Torben,

Yes, good point.
I have mentioned this already,
my solution is simply not to refer to direction at all,
simply to call it current flow.

As in an earlier post, i mentioned that surprisingly there is still an element of
difference of opinion over this silly issue.

If pressed, then clearly the issue should be mentioned,
but otherwise i have avoided scrupulously any mention of direction.

The explanations fortunately dont rely on any current direction.

Cheers, John

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john1

My preference is for 'electron flow' as i suspect are most people nowadays.
Hopefully it wont come up.

John

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Anonymous321

Hi Torben.

Yeah, hopefully the teacher doesn't ask.

john1

Having said that, my latest entry includes current direction !!

Just cant win can ya

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john1

Anonymous, you put that like you knew all about it.

.

did you know of this curious issue ??

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Anonymous321

I knew about that conventional current was from positive to negative and that electron flow was from negative to positive. But beyond that, I did not know. I did not know it was an issue either. I mean, they're two different things, right?

john1

Fair enough.
I tend to stick to electron flow.
Just be aware, that sometimes people seem to argue for nothing.

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Anonymous321

Hey John, one last thing:

So the transistor is not used as a switching device anytime? I thought that this (a switch) was what we were using the transistor as.
And if you already answered it, whoops, and sorry.

john1

my latest offering, and i wish i could set this out without arrows.
but never mind.

So long as you get the gist of it, i'm sure it will be fine.

Unless your tutor has a background in electronics,
it is unlikely he will have any particular opinion about current direction anyway.

So long as what you say makes sense, i'm hoping theres no reason for
direction to come up at all.

Just drawn this for you ,see what you think.

John

Attached Images

final-circuit4.jpg (48.2 KB, 5 views)

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Anonymous321

Ah ok. Sorry! Hopefully this is the last question: so the first diagram you had--is that current circulating when the light is on the photocell? Then the second diagram's current illustrating when the light is off the photocell?

john1

switching device ...

Transistors which are refered to as 'switching devices' are really just
normal transistors, maybe not meant to have 'good' characteristics,
but still normal.

Its just a term,
they still turn on and off like ones that arent called switching types.

**********

In this particular case,
if the laser were partly obscured, like fog or smoke, or a fly in the way,
then it is quite possible that the transistor could be half open, or half
closed. Or in between.

But not likely enough to mention,
i only mention this to help you see that its just used as a current amplifier.

Hope that hasn't confused you at all.
I am most impressed with your level of comprehension so far.

Your questions are sensible, and show some understanding of a subject
which is very difficult for most people to grasp.

John

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Torben

Not really; it's just that when the transistor's base has too little current flowing into it, no current will flow between the collector and emitter. Since the collector and emitter are between the buzzer and v-, this means that no current can flow over the buzzer (the collector and emitter are like two sides of an open switch).

Yes. The 47k resistor is connected between V+ and the transistor's base, allowing 176uA to flow into the base, sufficient to drive the buzzer.

No, it's still switching. It's just amplifying so much that it acts like a switch. Normally there is no current flowing into the base, since the current is shunted to ground because the LDR has low resistance when it's lit up. When the light is removed from the LDR, its resistance goes up, and current now has a harder time getting through it. That current now flows instead in the base of the transistor, which causes the transistor to allow a much larger (amplified) current flow across its collector-emitter. We just dump so much current into the base that the transistor overloads ("saturates", actually) and allows as much current as possible to flow over its collector-emitter.

Actually, the light source isn't providing any voltage at all. It just reduces the resistance of the LDR, allowing more current to flow over it (instead of into the base of the transistor, which would make the buzzer buzz).

Think of it this way: the transistor is like a switch installed between the buzzer and ground. The switch is controlled by current into the base. The 47k resistor between V+ and the base can provide enough current to do this, but if the LDR between the base and ground has little enough resistance, it "robs" this current from the base of the transistor and shunts it to ground, so the transistor shuts off and the buzzer can't buzz.


Hope that made some kind of sense. I'm a little tired.

Torben

john1

Not quite ...

that first one showing the split current path through the cell and the junction,
was to show that the current path is made up of the two parts.

The proportion of the current going through the junction,
can be seen to be dependant on the state of the cell.

If the cell goes higher resistance, then more goes through the junction,
see what i mean ?
and vice-versa,
Ok

John

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john1

There ya go,
Torben has described the way the cell 'bypasses' the junction,
so hopefully that will be clear now.

It all seems so much explaining for just a little junction ...

My cat is not impressed, she's gone outside now.

John

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john1

It must be pretty late where you are.
Did you put some sleeving on that transistor leg ?
I think you should get to bed now.

John

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