# School project question

Discussion in 'Electronic Design' started by BZ, Oct 25, 2004.

1. ### BZGuest

Hi everyone,

I hope someone can help me with this. My daughter is doing a
school project in which she will make a diagram of an atom
using LED's stuck through a foam board, and power the LED's
with 9-volt batteries. I'm an electronic no-nothing, and have
some very simple questions.

First some description of the project: it's a foam board from
Wal-Mart (basically two pieces of poster board with 1/4" of
lightweight plastic foam in the middle). She will have a
cluster of diodes in the middle to serve as the nucleus of the
atom, surrounded by other diodes representing electrons. She
plans to stick the wire leads of the diodes directly into the
foam board from the front and make all the connections in the
back. There will be three separate circuits so that protons,
neutrons, and electrons can each be lit up separately. Each
circuit will have its own rocker switch on the front and will
be powered by a 9v battery. We have some 270 ohm resistors
and figure on using two in series in each circuit.

My questions are:

1) Will sticking the bare wire diode leads through the foam
board create any kind of fire hazard? I'm concerned about
bare wire on paper, but have no idea whether a 9v battery will
generate enough heat to create a problem.

2) The diodes are rated 3v, 20 mA, and she plans to hook up
as many as 22 diodes in series in one of the circuits. Will
one 9v battery do it, or will she need 1 battery for every
three diodes in series?

3) Can she get away with twisting and taping the leads and
wires, or is this something that really should be soldered to
get a reliable connection?

4) Is there anything else I should be asking but am not?

Any help is appreciated. I'm trying to avoid having us spend
many hours constructing this thing only to find out it will
never work.

Thanks!

2. ### Ken TaylorGuest

No, there will be no problem.
Essentially you need to wire up a circuit with the battery, a resistor to
limit current and an LED(s). The value of the resistor (270Ohm) is about
right for one LED off a 9V battery. To power several LED's, each should have
it's own resistor of the same value. Twenty-two LED's is going to be too
much current draw for one battery - it won't last long before the terminal
voltage on the battery won't do the job. You may want to look at either
using a number of batteries powering say, three LED's each, or just getting
a wall-wart.
You'd get away with the twist-and-tape method, until the first movement (or
the first attempt to get a grade) at which time you'd be likely to have a
failure. It'll happen at the worst moment, belive me! Solder them, once
you've ascertained that the circuit works. Make sure that whoever solders
them uses a small iron, the right solder (NOT plumbers' solder and acid!)
and gets the LED's wired correctly.
Best of luck!

Ken

3. ### meGuest

no, but the battery will get hot if you try what you have planned.
Hook in parallel LED and resistors that are connected in series. kinda
like this.

| | | |
R R R R
| | | |
L L L L
| | | |

One battery will not work (well or long). Use plug in power adapter
(wall-wart) rated 9 Vdc and 400-500 mA (or greater).
You could get by using small wire nuts to connect everything together,
and still have pretty good reliability.

4. ### Spehro PefhanyGuest

a.k.a. "Foamcore".
A 9V alkaline cell should not create a problem, but try to
prevent shorts.
Connect them in series pairs, with the 270R resistors in series
with each pair, across the battery.

(view in fixed-pitch font such as Courier)

/
9V x -o o--x--[270R]-----|>|----|>|--x- 0V
| switch | LED LED |
| | |
| x--[270R]-----|>|----|>|--x
| | LED LED |
| | |
| x--[270R]-----|>|----|>|--x
| | LED LED |
| |
| .... etc... |
| |
| |
| |
| / |
x -o o--x--[270R]-----|>|----|>|--x
| switch | LED LED |
| | |
| x--[270R]-----|>|----|>|--x
| | |

you some hours of life from a good quality alkaline cell,
with the LEDs getting significantly dimmer toward the end.
If you put 20 strings (40 LEDs) across one battery, the life
will be very short, but you can use more batteries

Suggest they be soldered. You can buy a soldering iron
inexpensively from Radio Shack etc. Get the solder from
RS or another electronics place- DO NOT use plumber's solder-
it has an acid core which can cause problems. It takes little
time to pick up the skill and it's a good one to have. Try
asking the counter help at RS to demonstrate soldering a
couple of wires together. They likely have someone there
who can do that for you.

Best regards,
Spehro Pefhany

5. ### CFoley1064Guest

Subject: School project question
1) No fire hazard, but I'm not sure the leads will be long enough to go
through a foam board. You might be better off punching a couple of holes 0.1"
apart into a piece of cardboard, and then gluing the LEDs onto the cardboard
with the leads poking through the other side. After gluing, bend the leads
down. That will also help with stability when moving the display.

2) I can sense a problem coming with your power source. A 9V battery has a
very limited A-h (amp * hours) rating. Not only that, but you generally get
very poor results with 9V "transistor" batteries if you ask them to pull more
than 100 mA. Since the universal science fair rule is that you can't plug
anything in for your display, I'd recommend getting a 6V lantern battery
(available at Wal-Mart or any hardware store) to power your project. If you
notice the LEDs starting to dim before showtime, buy another battery for a
spare.

Related to this is the question of how to wire the LEDs. If you use a 6V
battery, you will need a current limiting resistor to avoid smoking the LEDs.
If you put two in series, you're depending on the bulk resistance of the LED to
limit current -- usually a bad idea. For a 6V battery, with a 3V LED at 20 mA,
you should go with a 150 ohm 1/4 watt resistor. You can either go to radio
shack for these or, if you have the time, order from mouser.com. You can order
100 ea. of their 660-CF1/4L151J for \$0.03 USD ea.

Mouser doesn't have a minimum order size, but while you're there, you might
also want to get your neutron/proton/electron switches there, too.An
inexpensive toggle switch which should work for you is their 108-1MS1T1B1M1QE.
You'll need 3 ea. at \$1.90 ea. When you mount them on the cardboard display,
put a small metal panel in front to provide rigidity (a thin piece of aluminum
would work well here).

3) If you want to be Mr. Wizard for your kid (one of the best feelings in the
world), you want it to work reliably. Use a soldering iron and rosin-core wire
solder to do the job. If you don't know how, ask around. You'd be surprised
how many people do.

4) Here's how to wire up your project (view in fixed font or M\$ Notepad):

.--------o-------------------o--------------------.
| | Neutrons | Protons | Electrons
| .---o-------. .---o-------. .---o-------.
| | | | | | | | | |
| | | LEDs | | | LEDs | | | LEDs |
| V ~ V~. . . V~ V~ V~. . . V~ V~ V~. . . V~
| -~ -~ -~ -~ -~ -~ -~ -~ -~
| | | | | | | | | |
| | | | | | | | | |
| .-. .-. .-. .-. .-. .-. .-. .-. .-.
+| | | | | | | | | | | | | | | | | | |
6V--- | | | | | | | | | | | | | | | | | |
- '-' '-' '-' '-' '-' '-' '-' '-' '-'
| | | | | | | | | |150 ohms
| '---o-------' '---o-------' '---o-------'
| | | |
| o o o
| '\ SW1 '\ SW2 '\ SW3
| \ \ \
| o \ o \ o \
| | | |
| | | |
'--------o-------------------o--------------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

You'll find that the LEDs are polarized -- they only light up one way. Usually
one side of the LED has a flat to indicate the cathode, and one lead is longer
than the other. Test on one (with a series resistor) before you wire 'em all
up to find out what works, and then wire them all the same way.

You should try to find 20 or 22 gauge solid insulated wire to do the wiring in
the back of the cardboard. Depending on the size of your project, you may need
50' or 100' of wire. Keep the wires tidy and close together. After you've got
the whole thing working, use spots of hot melt glue to hold the wires in place.
Put another piece of cardboard in back of the first to protect the wiring
during shipping to and from class.

If you have any other questions, feel free to email.

Good luck. This is one of the best parts of being a dad. Enjoy the moment,
and let her work some things out for herself (be sure to supervise if she's
using the soldering iron!)

Chris

6. ### xrayGuest

Spehro,

Shack people might be a bit optimistic.

RS is ok, but in my local one, I'd be surprised if any of the people
have ever soldered anything.

7. ### Spehro PefhanyGuest

It probably depends on the store. I've actually seen them soldering
things on the counter, in my local RS, once (to make up a patch cable
or something). It's worth asking anyhow. ;-)

I just looked at a couple of sites that claim to tell people how to
solder, and they offer silly (clean the wire with acetone) or outdated
and thus harmful advice (like filing the tip of the soldering iron
first!).

Best regards,
Spehro Pefhany

8. ### Rich GriseGuest

I just had a thought - somebody at the school probably knows somebody
who knows how to solder!

Cheers!
Rich

9. ### Frank BirbacherGuest

Hi!
Instead of emailing people should post to the newsgroups so that
others can read responses, too, like me

Frank

10. ### Don PearceGuest

Provided you don't want them mega bright, this should be no problem.
Make it a decent sized battery - like a lantern cell with screw
terminals.
Better to get some "chocolate block" screw connector strips. You could
solder, but probably not worth learning just for this.
You could ask why the model of an atom needs individual leds - the
Bohr model is long discredited, and a much better model would be one
with frosted glass over the front, and concentric circles of diffuse
light behind it.

d
Pearce Consulting
http://www.pearce.uk.com

11. ### Country LoonGuest

She will need a quantum noise generator for the led and a dimensional
transporters so that the LED can jump from energy state to state.If it is
observed then the electron (LED) will change state.Quite difficult.The
electrons in an atom do not rotate like planets around the sun.

Tom

12. ### N. ThorntonGuest

no, they dont get hot.
no, no.

yes, or screwed, or crimped, or possibly just croc-clipped.

If the LEDs are 3v rated you'll need them in pairs not 3s. Theyre not
usually 3v rated though, what colours are you using?
You'll need either a bigger battery, or switch over to low current
LEDs, typ 2mA each.

NT