Anonymous321

And of course you'll see me again! I'm just really busy now-a-days with school and such.
He didn't "destroy" anyone today. The key really is to know your topic and model down pact, I would say. Because if he catches you being unsure of something, it'll just go downhill for from there.

john1

Hi,

Torben thought you might be a bit busy.
Nice to see you anyway.
If you do think of any questions you might get asked ...

And please let us know how it goes,
Cheers, John

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Anonymous321

Yeah, I'm just preparing them. Or rather thinking about of all of them so I can ask in one go. And yeah, of course, I'll let you know how it goes.

john1

will it just be the tutor that asks a few questions,
or would it be anyone ?

(dunno anything about this sort of presentation)

John

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Anonymous321

Anyone can ask. But it's mainly the tutor who asks us. 10 min presentation, then 5 minute questioning period.

john1

Hi Anonymous,

Ive just noticed that i have put the resistor in the wrong place on that little diagram.
I'm very sorry about that, i hope that is not a problem with your stuff,
I feel a bit silly about that, i have re-positioned it on the diagram
to show where it really is.

John

*****

Attached Images

final-circuit2.jpg (26.0 KB, 4 views)

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Anonymous321

Yeah, I was beginning to wonder. But it's not a problem. Thank you. Questions still coming...

john1

Thank goodness for that,
i would not want you to turn up with an error in your stuff.

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Anonymous321

Ok, I'm a little confused as to how everything works.

What exactly is a photocell? Wikipedia says that a photocell “is an electronic component whose resistance decreases with increasing incident light intensity. A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.”
So when light shines on it and gives it current, does the current flow from the emitter to the –ve of the 9V battery (i.e. the ground) and so the circuit stops (no current flows over to the siren)?
And then when the light is blocked, the 9V battery supplies current to the siren (b/c the +ve of the 9V is connected to the +ve of the siren)?
Why is the photocell connected to the emitter and the base? And not to somewhere else?
Why is there a connection between the collector and the -ve of the siren?
The resistor is supposed to resist current, isn't it? How does it turn the transistor on? Why is it connected from base to +ve of siren?

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

john1

thinking ...

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john1

easy ones first,

The resistor is supposed to resist current, isn't it? How does it turn the transistor on? Why is it connected from base to +ve of siren?

***

Well, its called a resistor because it has resistance.
It does resist current flow, but thats not its only function.
In this case its job is to control the current.

The degree of resistance is chosen to control the current, so that it suits what is wanted.
In this case it is to manage the transistor control current via the 'base',
such that the photo-cell can operate the transistor with the light from the laser.

Various values were tried, this value was suitable. Lower 'resistance' would mean more current,
Higher 'resistance' would mean less current.
The photo-cell works it satisfactorily using this value.

***

I'm trying to keep this in easy-to-understand terms,
I hope this comes across alright.

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

How does it turn the transistor on?
next post

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Anonymous321

Oh ok. Yeah, this came across perfectly. Thank you.

john1

How does it turn the transistor on?
************

The resistor is positioned in the circuit to allow some current to flow through
the photo-cell and the base of the transistor.

At rest, with light on the cell, most of this current passes through the cell,
the base contributing very little towards the current through the resistor.

If the light is interrupted,
then the photo-cell changes and becomes less conductive, (higher resistance),
then the current through the resistor passes mainly through the base of the transistor,
and the photo-cell contributes very little towards the current through the resistor.

[i have tried to avoid reference to current direction, in some places there are still
disputes about how that should be expressed. I favour'electron flow', but i thought it
best to avoid it altogether, if you get asked about that it can be a headache]

Briefly:
The transistor in this case is used as a current amplifying device,
the output current being controlled by the smaller base current.
The emitter is common to both current paths.

[i doubt you would be expected to explain in great detail, but that should cover it]

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Why is it connected from base to +ve of siren?
next post

Thank You. (I do try)

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john1

Why is it connected from base to +ve of siren?

Thats the supply positive point.
it just happens to be the same connection.

The resistor goes from the base to the battery positive,
the siren also has a connection on the battery positive.

*********

What is a photo-cell ?

Ive looked at Wikipedia, redirected to photodetector, and yes its a bit technical.
They've tried to cover everything that could be a photo sensitive device.

For most purposes you only need to know about two basic types,
Ones that change resistance when light falls on them, like you have in this case,
Photo-resistive.
And ones that generate a little voltage when light falls on them, Photo-generative.
like camera retinas, and sunlight energy sources.

The type you have used in this unit is 'photo-resistive', it changes its resistance
when light (laser beam) hits it.

Briefly:
A photo cell is a device that responds to light,
how it responds depends on which type you choose for your purpose.
Photo-cell is a bit of a general term for many types.

[hope that covers it!]

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Why is the photocell connected to the emitter and the base? And not to somewhere else?
next post

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john1

Why is the photocell connected to the emitter and the base? And not to somewhere else?

Because the Emitter/base junction is the control point of the transistor.
The small current through this junction controls the output current.

[ The collector and the base both share the emitter in their current paths.
The base current and the collector current are separate currents,
but they share the emitter.
dont know how far you want to get into this ... ]

Briefly,
cos thats the control point.

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

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