Laser Burglar Alarm -- Need Help

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

Anonymous321 said:

But where does it go then? Does it try to go to the siren but is not enough to make it ring?

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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).

So then here, the resistor (which you said is controlling the current in this case) supplies enough current for the siren?

Click to expand...

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.

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...hmm...

Click to expand...

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.

And also, do you know approximately how much voltage the laser (or any light source) has to provide to the photocell to turn the transistor on?

Click to expand...

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

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

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

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

I'm off to bed now.
Best of luck to-morrow ( i think its to morrow)

Let us know how it goes.

Cheers, John

Ah. Thank you again John and Torben. What you have said makes sense. And thanks for the compliment, John. I must admit though that it's not easy for me either to understand. I really have to think about it. For now, I think that, just to prevent my head from exploding anytime soon, I'm going to go study some math. Then I'll go back to understanding this alarm and I should be able to get it all. When I do, I'll make a post explaining how each of the components work.

Well, there's only the photocell and how when electromagnetic radiation--laser light-- is applied to it, the electrons are liberated from the cell (the photoresistor lowers its resistance), thus showing the photoelectric effect.
Then the transistor, which acts like an amplifying switch (I hope I got this right). I'm going to read what you two wrote again after math studying.
Then the resistor, which controls the current.

Yeah, I know I should go to bed. But I have a math test to study for too. Also, I took a power nap (these are awesome!) after I came back from school so I'm not exactly too sleepy.

i am definitely going to bed now,
my cat has come back in, and she is shouting at me (in a cat voice)
i dunno why.
maybe cos i should be in bed

Goodnight see ya later, John

Sounds like you're getting it. I'll still be online for a while. If you have more questions tonight I'll do my best to help.


Torben

Alright, good night (morning?) John. Please apologize to your cat for me.

And, ok Torben, thanks.

Well I think I know it.

So? What happened? Teacher like it or roast ya?


Torben

Mmm ...

Maybe there was someone there with a Higg's Bosun in a sock,
maybe the laser slipped and caught the tutor in the eye ...

Well...let's just say that he did ask about the flow of current/electrons. Sniff. Gotta go for now. Another math test tomorrow -_-;

Dang. Hope you remembered that electrons flow from negative to positive.

Anyway, I hope it went OK overall. Good luck with the math test.


Torben

dudes.........cud anyone out there gimme some walkie talkie circuits

deepak george said:

dudes.........cud anyone out there gimme some walkie talkie circuits

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Please don't hijack other people's threads; start your own.

I suggest googling "walkie talkie schematic".


Torben

Hope you remembered that electrons flow from negative to positive.

Click to expand...

Hi Torben,
Yes this came up earlier,

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?

Click to expand...

This was the comment from Anonymous, and she has quoted the direction correctly, i dont see that
this should invite any problems. There seems to be no doubt as to direction.

However,
it seems the tutor did ask about ... " " the flow of current/electrons " "
and this was followed by " Sniff " , and an unhappy face.
followed by ... Gotta go for now.

So, maybe that bit didn't go well.
But i hope the rest went alright.

I would be very interested to find what was asked about "current/electrons",
and of course what was given as an answer.

*******************

And i hope the Maths exam went well for you.

Cheers, John

Hi John,

I'm a little confused. Why did you copy out half of Anon's email for me? English is my first language; I had no trouble understanding it.


Torben

Hi Torben,

I was agreeing with you.
And confirming that she mentioned it correctly before.

It wasn't just for you, i copied out a bit from before cos this thread has a lot of watchers,
and many of them may not look back to earlier posts.

I would still like to know what was asked about "current/electrons"
from the tone of Anonymous' post, that bit may not have gone well.

I think we both know that some people will bring up aspects of the well known "electric flow"
alternatives, sometimes just to generate "lively discussion".
Very often this can just confuse people.

John