Laser Burglar Alarm -- Need Help

[Anonymous321]

Thank you, Torben! Not only for the compliment but also for your help earlier! Yes, the presentation. That'll be something. I hope he doesn't try to burn me that bad. I'll have to prepare everything...in a power point presentation!

 

[john1]

I'm just glad it worked.
Was starting to think that a single transistor would not do it.

Hope it goes ok, and i hope the tutor doesnt give you a hard time over it.

Thanks for your support Torben.
Goodnight all.
John

 

[Anonymous321]

Yes, thanks once again, John!

 

[Torben]

Hey Anon,

I'll be online for a few hours yet if you have anything you want to work on before calling it a night.

PowerPoint? Argh. I feel your pain. Well, your presentation should certainly beat 2 balls in a sock.


Torben

 

[Anonymous321]

Hey Torben, you must be awake for some time then if you got back from a softball game? I'll try drawing the general schematic of this thing (I'm not very good at drawing schematics--in fact I only started with my first post on here, unless I did some in grade 10 which I can't seem to recall...and I have no special program..I use paint) and show you. Of course, I'm not forcing you to stay awake like I practically did with John. But hopefully this schematic doesn't take too long.

 

[Anonymous321]

Oh, ah, you helped me before I asked for it. Thank you. Yeah, hopefully it can beat the sock quarks. But honestly she didn't even deserve the relentless questioning by my teacher. I mean...quarks..how is one supposed to make a working model, or even any type of model, for those?
I, myself, had 3 ideas rejected by him before he accepted one (I had to call him 2 days before our handout--a poster explaining our topic--was due to confirm!).

 

[Torben]

John's a trooper for sure. It's not late for me yet. By the time I go to bed John will probably be back online.


Torben

 

[Anonymous321]

Well this is the schematic that I was able to draw on paint. It's not the best and I tried to explain all the connections. I can't believe that this is all that there is to it. I feel silly. I believe I have some questions too but I'm a little too tired to articulate them right now. I will for sure put them on...maybe a little later or after I've back from school.

 

[Torben]

Can you post a quick photo of the circuit from above with all unused wires removed etc?

The end of the battery symbol with the long line is the positive end.


Torben

 

[Anonymous321]

Yeah sure, I was just about to. My batteries for the camera died though...like I bought it ~5.5 hrs ago and the batteries run out.. But I'll find some others and post it up.

"The end of the battery symbol with the long line is the positive end."
Touché. I'll fix that too. Sorry. But I'm still calling it V1!

 

[Torben]

Cheap batteries suck. I guess some would say you were lucky to get any pictures out of them at all.


Torben

 

[Anonymous321]

Yeah. This camera did help me out too.
Ok so a picture of my circuit and my revised schematic are up with this post.

 

[Torben]

In the picture the circuit looks OK. I know you don't have a lot of experience drawing schematics but don't worry--that will change with time. I don't see the resistor in your schematic but between the text and the photo I think I've worked it out. I've attached how I might draw what I see in your photo of the circuit.

What sorts of questions do you think the teacher is going to hit you with?


Torben

 

[john1]

Hi Torben, Hi Anonymous,

Had a good kip.
Had a look at that latest picture.

I think that resistor wire goes a bit too close to the collector.
Maybe you could put a bit of sleeving on it ?
Or if the transistor wire is thinner, maybe a bit of insulation from
those red wires would fit on to that collector wire, just to save it
from touching accidentally.

(going to look at your little diagram)

John

 

[john1]

Hi,

Just a tiny alteration, and i put the resistor in

John

 

[john1]

Hi Anonymous,

Notes/comments ...
Ive changed "NPN" to transistor, cos you never know what kind of questions may get
raised by someone.
I thought, keep it as straightforward as possible.

If you get asked 'Which bit is the transistor' thats fair enough,
but what if you get asked whats NPN,
or what does N.P.N. stand for,
Or why use an NPN instead of a PNP,

Or any kind of related questions that might throw you.
I thought best just call it a transistor.

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

The battery,
You may have noticed Torbens drawing of a battery is a bit longer that yours,
thats cos the long and short lines are in pairs, each pair represents a 'cell'

So a nine volt battery has six cells. (they are 1.5v each on that battery)

Now that doesnt really matter, and unless your tutor has a background in
electronics or higher physics he would be very unlikely to pick that up.
So i shouldn't worry about that.

If you care, an easy way is to draw the first and last cells with a dotted line
joining them up. That is common practice, but i dont think you need to worry
about that.

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

The resistor.
If someone says 'whats that for?' ... 'why use that value?' ... 'How do you know
its the right way round?'

Or stuff like that, ...
.Its there to turn the transistor on, 'conducting'
.That value? ... tried a few values, that one was suitable.
.Its the same either way round.

***DONT CLARIFY by saying ... unlike some things, which are different the other way
round ... BECAUSE that may well produce another question, like, 'such as ..' or
what bits are direction sensitive, why are they like that, and so on.
you dont want opportunist questions coming at you.

***********

How long will the battery last?
Dunno, this is only a demo,
a proper burglar alarm would be run from the supply,
and would only use a battery as back up.

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

Any questions you think might come your way,
let us know, we'll try to suggest an answer that does not invite another question!

Cheers, John

 

[Torben]

Yeah, the battery symbol is just the one which Eagle added when I selected '9V battery clip'. If I was drawing this by hand I'd use the abbreviated symbol John mentioned.

John's right about that resistor wire coming too close to the transistor leg. Maybe it just looks close because of the angle but better to take his advice and be safe.

By the way, anon, if you are interested in doing more circuit design etc. after this project is done, there are a couple of free packages available on the net which might help.

One is Eagle, which is an excellent schematic capture (drawing) program which includes a PCB layout module. Version 5.0 just came out, and a free version is available for free download: http://www.cadsoftusa.com

Another is LTSpice, which you can also use for drawing schematics, but it is also a simulator, so you can get some kind of idea of whether a given circuit will work even if you don't have the components to actually build it. LTSPice is also called LTSpice/SwitcherCad III: http://www.linear.com/designtools/software/switchercad.jsp

These obviously won't be any help for the current project but over time they might come in handy if you keep up with electronics.

Were there any students presenting today? Any more questions from the teacher which might indicate what he'll hit you with?


Torben

 

[john1]

Hi Torben,

Its been about 18 hours,
We may not hear from Anonymous again,
but i would like to know how the presentation goes.

John

 

[Torben]

Hi Torben,

Its been about 18 hours,
We may not hear from Anonymous again,
but i would like to know how the presentation goes.

John

I hope we do. Even if it's a horror story about the teacher's response. I'm hoping s/he's just really busy right now.


Torben

 

[Anonymous321]

Hey John and Torben. Thanks for the responses and clarifications. I've read your suggestions. I can't believe I missed drawing the resistor in! However, I do have more questions and I hope you guys will be online to answer them before I have to present it, or rather, go to sleep.