Re: School project question

From: Ken Taylor (ken_at_home.nz)
Date: 10/25/04 

Date: Mon, 25 Oct 2004 18:09:15 +1300

Hiya. Nice project (see comments....)

"BZ" <not@available.now> wrote in message
news:1A%ed.10764$p87.416@fe2.texas.rr.com...
> 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.

No, there will be no 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?

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.

> 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?

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.

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

Best of luck!

Ken

> 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!
>

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