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'First PIC project...'
2000\01\24@112731 by Russell Farnhill

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face
Hi all,

I want to make myself a UV exposure box for making PCB's.
I need to design a PIC timer board that is capable of
counting down any where from 9mins & 59secs, I want to drive
3 7-Segment displays and be able to set the timer by
turning a pot which is processed by an ADC on the pic.

I was thinking how to drive the display and wanted to know
if it sounded ok or not ?

First common all the three disp-segs data lines and connect to one
of the ports, then take the 3 commons and connect them to separate
port pins, then just cycle through each display by outputting the data
to the port and pull the relevant common pin to ground for a short time,
then repeat for each display in a loop.

Is this the usual way of driving more than one display ??

Thanks,

Russ...

2000\01\24@120658 by James Paul

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Yes, it's called multiplexing.  You only have to tweak the
timing between digits to get minimum current draw, while still
all of them appearing to be on at the same time.  Not difficult,
but may take a few tries to get it just the way you want them.

                                             Regards,

                                                Jim



On Mon, 24 January 2000, Russell Farnhill wrote:

{Quote hidden}

spam_OUTjimTakeThisOuTspamjpes.com

2000\01\24@210440 by Byron A Jeff

face picon face
>
> Hi all,
>
> I want to make myself a UV exposure box for making PCB's.
> I need to design a PIC timer board that is capable of
> counting down any where from 9mins & 59secs, I want to drive
> 3 7-Segment displays and be able to set the timer by
> turning a pot which is processed by an ADC on the pic.

OK.

>
> I was thinking how to drive the display and wanted to know
> if it sounded ok or not ?
>
> First common all the three disp-segs data lines and connect to one
> of the ports,

Make sure you have current limiting resistors between the port pins and
the displays. Also make sure that the displays are common cathode.

> then take the 3 commons and connect them to separate
> port pins,

Now this is going to be a problem. You have to balance the fact that you
need sufficient brightness in the LED's, and the limited amount of current
that a port pin can sink. This unfortunately is a no win situation without
some help.

The solution isn't too difficult. Simply put a transistor between each common
port pin and the display. Connect the collector to the common cathodes, the
emitter to ground, and the base to the port pin through a 2.2K resistor
or so. Then the transistor can sink sufficient current to brightly light the
displays.

When I had a lot of displays to do, I'd use a 7445 decade decoder because
each of the 10 pins pin can sink up to 80 ma.

> then just cycle through each display by outputting the data
> to the port and pull the relevant common pin to ground for a short time,
> then repeat for each display in a loop.

That's the idea of multiplexing. Be sure to turn the driver pin off when
changing displays.

>
> Is this the usual way of driving more than one display ??
>

Yes.

BAJ

> Thanks,
>
> Russ...
>

2000\01\25@003506 by Dwayne Reid

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>Hi all,
>
>I want to make myself a UV exposure box for making PCB's.
>I need to design a PIC timer board that is capable of
>counting down any where from 9mins & 59secs, I want to drive
>3 7-Segment displays and be able to set the timer by
>turning a pot which is processed by an ADC on the pic.

Do you want to do it the easy way or the easier way?  I do a lot of projects
that require both / either lots of ins or outs.  I use shift registers - it
takes a few more parts but the board layout tends to be easier, as is the
software.

If you are driving LED displays, choose common anode types.  Connect the
segments and dp to the 8 outputs of a 74hc595 SR thru 100R resistors.
Connect the anodes of the displays to a 4V supply - I use lm2575 simple
switchers but you can use anything you want.  Or - you could use 150R
resistors and run the displays from your 5V supply (resistors run hotter).

You cascade the shift registers and need only 3 lines to talk to them.  You
can also use 2 of those lines for other purposes, such as reading switches
or reading from a parallel / serial SR.

dwayne


Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 16 years of Engineering Innovation (1984 - 2000)

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2000\01\25@084356 by Keith Causey

picon face
Also reset the segment drive lines befor switching to the next digit
otherwise you will get a 'ghost' of the previous digit on the display. This
is just a bug I encountered when I tried using 7-seg displays.

{Quote hidden}

2000\01\26@002611 by paulb

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Byron A Jeff wrote:

>> First common all the three disp-segs data lines and connect to one
>> of the ports,
> Make sure you have current limiting resistors between the port pins
> and the displays.

 So far, so good.  Use Port B for this, include the decimal as the
eighth bit.

>  Also make sure that the displays are common cathode.

 I beg to differ.  You want common anode, drive the segment cathodes
LOW.

{Quote hidden}

 NPN transistors as implied here are generally more readily available.
The *simpler* approach I am suggesting however, using common anode
displays, is to use NPN transistors as common emitter drivers, base
driven directly from port pin (no resistor), emitter to digit anode and
- note this - collector connected to unregulated supply if practical.

 *Many* advantages here:  Waste no current driving bases, notably if no
segments are active, transistors do not saturate so minimises ghosting,
base resistors not used = fewer components on PCB, and the LED current
need not be supplied by the regulator, so a (smaller) low-power
regulator can be used.

 You lose about 0.8V instead of 0.3V in drop across the transistor but
this won't matter at 5V.

> When I had a lot of displays to do, I'd use a 7445 decade decoder
> because each of the 10 pins pin can sink up to 80 ma.

 OK, but consumes more power than a working PIC, even when idle!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 So, you use eight port B lines for segments (the DPs are quite useful)
and three port A lines for digit drive.  A fourth port A line gives you
your control output and the fifth multiplexes to your eight control
buttons using eight diodes (or you can cheat and use resistors if you
don't want combination button presses) to the port B lines.  Just switch
on port B pull-ups, pull the button select line low, and read port B.

 You don't need shift registers for this application - three
transistors, some diodes, eight 150 ohm resistors for the segments and
it's done.  But when you *do* need expansion, shift registers are the
way to go!
--
 Cheers,
       Paul B.

2000\01\28@082244 by Russell Farnhill

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Eric Wrote:

>Hi, Russell,

>I have done the first part of your project - making a drive circuit for
>two 12" UV tubes.  I need to try if two UV tubes are enough for
>processing a decent size PCB, like 4"x6"?  What do you think?  Also, I
>am using pre-coated photo-resist boards and I wonder if it will work
>for  boards with resist lamination layer?  Do you know?

I was thinking more along the lines of four tubes as this gives better
even exposure across the board, but saying that I have been using a sunbed
and just placing the board underneath sandwiched between two pieces of
glass.
Ive always used pre-coated boards as it saves time and messing about
coating boards your self, plus I think you need to bake them in an oven
if you coat them with spay on resist.

>For the timer side, I will be using a 16F84.  I wonder why you want
>three 7-seg displays?  I believe, two 7-seg units for minutes display
>(00-99) is all you need and we can get away with a 1P3T switch for
>seconds.  I think for UV exposures, it is always better to over expose
>than under.  Therefore, up to the seconds counting is not really
>needed.  I will accept a 15 seconds resolution, that is, 0-15-30-45
>seconds.  A 1P3T switch will talk to the PIC yet we see its position
>setting, serving two purposes.  What do you think?.

I wanted three digits and even considered four as to make it a universal
timer module rather than just for the UV-Box. I do think more resolution
than 1 min is needed though, 15sec sound ok to me. Not sure what a 1P3T
switch is ???? I was just going to use a potentiometer and use ADC.

>How do you make out on the UV hardware set up?  Just interest to see
>what others are doing if you would like to share.  I use a 9V walmart
>and a small transformer to step it up.  Two 12" tubes takes 800mA really
>bright when substituted with daylight tubes.  I have purchased a Burr
>Brown OPT101 light sensor thinking that I can standardise the UV output
>from my setup, I will have the measurement shortly.

I want to fit all the electronics/PSU inside the UV-Box and just
have a 3pin power connector to plug a main cable into, this way its a
fully self contained unit. I also considered putting normal tubes into
the box as well, then you could switch between UV/Normal light and the
Box can also double as an artwork box as well!!.

Ive been making small single sided PCB's for quite a while now and
have had good results. Now I want to start making some double sided
boards and making them through plated as well. here's an excellent link on
making PCB's http://www.netcomuk.co.uk/~wwl/pcbs.html


P.S. do you have any schematics for driving the tubes ??


Thanks,

Russell.

2000\01\28@105653 by Marcos Sassahara

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<x-flowed>Friends,

First of all I want to apologize for sendind comments in the middle of your
discussion. I am a hobbyst and know how terrible is to create a good PCB
lay-out on a PC tool like tango for example and don't be able to end the
process in a more professional way, I mean, have a good artwork but have to
make the board manually. Won't be fastastic if you could print the lay out
(using a common deskjet printer) over a kind of paper able to transfer the
drawing directly to the board? No UV or any other method involving light or
Chemistry or silk screen?

I made several experiences trying to reach this but I failed. My last try
was to use a matrix printer as a plotter and print the lay out
directly over the board but the result wasn't good.

About the circuit, I don't think it is a good idea to use a potentiometer as
reference. You know, you will have a digital device to see the time but an
analogic one to set it. For the timer I would like to make the following
suggestion.

The 4017 is a sequencial counter of 10 outputs. The first three outputs can
be used to drive three displays. The pic will RESET the
4017 to make the first display active. A pulse in the CLOCK pin will change
the output to the second display and a new pulse to the third.
To set the initial time, one push button can select the display to be
configured and other one can increase the number displayed. One start
counter should be present. The necessary PIC ports will be:

7 - displays segments
1 - 4017 reset
1 - 4017 clock
1 - Increase push button
1 - Select display push button
1 - Start Counting / Config mode switch push button
1 - Load

Counter will display 99.5 (99min 50sec) to 00.0 in steps of 10sec.

Regards.



{Quote hidden}

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2000\01\28@181036 by Graeme Zimmer

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> I was thinking more along the lines of four tubes as this gives
> better even exposure across the board, but saying that I have
>  been using a sunbed and just placing the board underneath
> sandwiched between two pieces of glass.

Doesn't normal glass completely block UV?

.................... Zim

2000\01\28@182525 by TIM

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no... my light exposure box   consists of 4  - 15 watt tubes uv radiating
(got  a bluish tint)   1-1/2 inch apart...approx 4 inches high over the
window plate glass. which under it i have the film which is dark so dark
one could view the eclispse with it and see nothing,,,i expose for about 30
seconds and it is done .....

{Original Message removed}

2000\01\28@191341 by Tim Hamel

picon face
Yes...glass DOES block UV. If it didn't, then EPROMs wouldn't require quartz
glass. Unless of course, you've discovered a new way of defeating this? I
could be wrong, but that's what we learned in physics.

My 1.9 cents,

Tim Hamel

In a message dated 1/28/00 3:25:57 PM Pacific Standard Time,
@spam@stm800KILLspamspamCITY-NET.COM writes:

> no... my light exposure box   consists of 4  - 15 watt tubes uv radiating
>  (got  a bluish tint)   1-1/2 inch apart...approx 4 inches high over the
>  window plate glass. which under it i have the film which is dark so dark
>  one could view the eclispse with it and see nothing,,,i expose for about 30
>  seconds and it is done .....
>

2000\01\28@203445 by Sean Breheny

face picon face
Hi Tim,

You are correct about glass ATTENUATING UV, but I don't think that a think
sheet of glass blocks it ENTIRELY. Also, I think the UV wavelength used for
PCB exposure is considerably longer than that used for EPROM erasure, so it
might go through glass better than the short wavelengh UV.

It seems to me that what is probably happening is that the glass cuts the
power considerably, but since the other Tim :-) has a full 60 watts of
power (of which a considerable fraction is probably longwave UV, because we
are talking flourescent bulbs), enough gets through to expose the film.

Just a guess...

Sean


At 07:11 PM 1/28/00 EST, you wrote:
{Quote hidden}

about 30
>>  seconds and it is done .....
>>
>
|
| Sean Breheny
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| Electrical Engineering Student
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2000\01\28@212510 by Andrew Hooper
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UV will penetrate plain glass and clear plastics, if you want to
prevent this then you can get small pieces of glass from welding
companies these are used as filters in welding helmets.

A good example of UV light penetrating glass is the sunburn you
get on your arms when driving, the light will be part UV and part IR.
yes the glass will filter a small amount but not enough to stop your
EPROM's being erased.

if you want to cover the UV lamps in your EPROM eraser then use
thin 2mm to 3mm polycarbonate.. this is what I used, my eraser used
a Philips UV tube, cant remember the model as it was so long ago
Behind the tube I used tinfoil to reflect as much as the light as possible
I also put a small CPU fan in the housing as I found there was a bit of
heat buildup after long periods of operation.

Here is an interesting fact, If your looking for a filter for your IR.
project get an old exposed film and use the piece that is not
developed on the end's, it works pretty well.

{Original Message removed}

2000\01\31@182841 by Bill Pierce

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<x-flowed>I don't know about glass but Plexiglass really blocks UV because it is
damaged by it. They put in UV block to keep the material from degrading in
the sun.

Bill


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