john1

Why is there a connection between the collector and the -ve of the siren?

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Connecting the siren straight across the battery, would make it sound, but to control it
we put the transistor in line with it.
In this case the transistor collector/emitter junction controls the current for the siren,
by being placed in the circuit between the siren neg and battery neg.

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Briefly:
Because the collector is the output connection of the transistor, and in this case it
feeds the negative connection on the siren.

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So basically, why is everything connected in the pattern that it is? I mean, somethings, I can make out.
next post ...

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john1

So basically, why is everything connected in the pattern that it is? I mean, somethings, I can make out.

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well,
that arrangement is only one possible arrangement, using the transistors and stuff that you have.
That one was chosen for simplicity and basic straightforward construction, bearing in mind that
you would have to be able to answer some questions about it.

Other more involved circuits were mentioned, and at one stage i was wondering if single transistor
would be able to do it.

The basic concept is that the light affects the cell,
and the cell controls the transistor,
and the transistor controls the siren.

The cell would not have the strength to control the siren,
even if it did it would be "backwards",
that is low resistance (more conductive) when light falls upon it,
so the transistor 'reverses' the logic of the effect.

So theres two effects to cover,
one, the ABSENCE of light has to operate the siren,
two, the current needed by the siren is more than the cell can control. (in this case)

[hope that covers it]

John

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john1

Hope i havent just made it look more complicated than it is ....

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Anonymous321

No, you haven't made it complicated. I just need some clarification. Sorry for the relentless questions..

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

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

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...

Yeah, true. I just want to be able to understand everything properly so that I'll be able to relay it without hesitation.

May you please explain (in the circuit) how? Like not detail, just in the circuit. The emitter and base I think I can see--is it by the photocell?


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?

I'll read that last post about why everything is connected like that now.

john1

heres a picture of my cat:

Attached Images

Paranoid32.jpg (315.3 KB, 3 views)

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Anonymous321

Hmm ok. And the resistor just simply controls the current.

john1

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?

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Well, its light really, i would say ordinary sunshine should push that sensor
to maximum conductivity.

Fortunately those little lasers have a pretty narrow beam, so the light
intensity is pretty high in the 'spot'.

At the other end however, most sensors dont start to respond until its
far from being dark, usually it needs to be better than even gloomy,
before a change is noticed.

Unless they are more sensitive than they used to be.

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Anonymous321

Lol. Ok I get the point--I'll stop asking questions. But you have explained it all to me now. So thank you.

john1

And the resistor just simply controls the current.

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Well sort of.
In this case it sets the range of current that the transistor will operate within,
the cell controls the output current depending on the light.

The resistor is chosen, so that the base current is not too much for the cell to control,
and also that it has to be enough to turn on the transistor when the cell is dark.

Hope that makes sense ... the cell has direct control, the resistor sets the range.

*********

I hope the questions you face are not so in-depth.

John

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john1

Woah there, not at all.
I just thought you could do with a laugh.

You ask whatever questions you want,
i will try to answer as best i can

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john1

The concept of the emitter being in both the controling current path,
and also the output current path,
seems to need further clarification.

I will attempt to show how this occurs on the diagram.
back soon.

but i feel its a bit late where you are,
do you want to continue, or to leave it at that ?

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Anonymous321

Yeah ok, it makes, sense. Ty.

Ah ok, ty. Yeah, I definitely can do with a laugh. I'm thinking too hard.

Other than that, may you please just give brief explanations to the questions I had in the post with the 5 quotes.

Yeah, me too. And also, my project is about alarms and I have 4 methods to describe (the laser method being one of them). So I'm probably also going to be asked about the other methods (i.e. ultrasonic, microwave, PIR) too. I think that since I chose the laser alarm method to be my model, I will probably be asked how it all works and like the general electron flow. If I mess up, he'll just continue asking.

Anonymous321

Well, it's only 1:20AM here. I think it's 6:20AM where you are so I don't know how you're managing to stay up and answer my questions. If you want/have to go, please do so. If not, I think that if you could give me brief one line explanations to those questions in the post with 5 of your quotes, I'll be good. It's not only the laser alarm that I'm going to be asked about and I have a math test to start studying for as well. Once again, I really appreciate your help. Thank you.

Torben

I don't know if John already covered this, but remember that current flow is described two ways: "conventional flow", where we refer to current flowing from the positive terminal of the battery to the negative terminal; and "electron flow", where we refer to the fact that the electrons actually flow from the negative to the positive terminal. From a practical standpoint it doesn't usually matter which you use as long as everybody in the conversation is using the same convention.

Whichever one I suggest you use in class, the teacher will prefer the other one.


Torben

john1

Okay ...

heres the first part of the explanation concerning the operation of the
cell, and how the resistor sets the current range,
and the cell can alter the current in the emitter/base junction.

Attached Images

final-circuit3.jpg (53.4 KB, 4 views)

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