EMRFD Message Archive 7512

Message Date From Subject
7512 2012-05-27 06:43:11 Ashhar Farhan electronics workshop : crystal set reprise
this might be off topic, but it is not. it is about experimental
methods in very primitive electronics, and i need help. but i am
getting ahead of myself...

i am running a children's summer workshop for a week (it is summer
here). on the first day, we make the mandatory lemonade battery and
hooked up two and three of them to light the LED. it was a huge hit,
thanks to the galvanized nails which had to slip inside the lemonade
class of each kid, then the hands go in to fish it out, then the juice
spills onto the table, then it gets onto their hair, shorts and
face... it was good fun to send them back home to dunk every liquid at
home between the copper board and the nail and report back the voltage
and current. we wrote the norton and thevnin equivalents the following
day while wiping ketchup and beer stains off the VOM. (ketchup is bad,
but beer is a pretty good electrolyte : from lab notes.

the second day, we learnt to use the wire stripper, use it to short an
AA cell, and discover how hot it can get. then we put it all into a 6v
battery case, measured the voltage, ran the fat battery directly into
another shorted wire but saw the compass needle jump. we saw that we
could bunch the wires together and increase the distance at which the
compass needle jumps until we came up with the idea of a coil. at this
point, we stuck the coil to the back of a paper cup, held a neodymium
magnet behind it, and connected it to someone's ipod player and heard
the music out of it. we found that maxwell dude had worked this out
into a very complicated set of equations, that we looked at on the
screen for a minute and gave up on him. he should have gone out and
played with some wire and batteries instead.

the third day, today, we tried our hand at soldering. we all got
pieces of copper clad boards about 3 inches by 1-1/2 inch. It has line
cut along it's side by a dremel. we solders the battery's snap
connector to the two islands across the cut. some of us solder a LED
directly and blew it up, some soldered it in reverse and found on joy.
but after a few hundred blown LEDs, we finally figured that we needed
to control the current and got an LED with a 1K resistor to glow. This
was pretty much the same as the lemonade light up but it was done with
style and solder. we also used the VOM to detect if we hadn't placed a
solder blob across the islands.

i plan to lead them to understand what a capacitor does (charges and
discharges like your ipod, just a few billion times faster). play with
a resistor, an LED and a capacitor on this board,figure out how the
current and voltage vary during charge and discharge of a series R and
C using really really slow (about 10 seconds time constant). next, we
will introduce a transistor into the picture and see how it can be
used as a 'fan regulator' to control the current flow. how very little
current in the base (just touch the base lead with your hand) leads to
LED lighting up, or charging or discharging. then, put two of them
back to back and we have a flip flop!

that is as far as i have planned. now i am at cross roads. i know that
a lot of people here have elmered kids.

yesterday, i assembled a very simple crystal set from the odds and
ends of my shack. i wound a fat coil around a CD-ROM case, attached it
to a variable capacitor, used a 9v wallwart power supply transformer
to step down the impedance to that of an ipod style earphones. it
worked so well that i was surprised. then, i decided to take away the
transformer and try it directly off the 1N34. that worked too, though
about 10db quieter (nothing was measured on the scope, as a matter of
primitive purity). i noticed that the CD ROM coil had far more
inductance than usual as the 800KHZ All India Radio was tuning up at
the low end of the capacitor. then, replaced the capacitor with two
copper clad scraps with a page from The Economist in between. an
overlap of about 2 inch square tuned up the AIR station again. I
replaced the 9V transformer with a big 1/2 inch steel bolt having a
500 turn primary and 50 turns secondary resulting in disaster. I could
barely make out the audio trace. however, it is really a lot of
winding. i wonder if the kids will have the patience. Twenty of them,
trying to get 50 turns of hookup wire around a CD-ROM case, sitting
next to each other is asking for trouble.

my question is where do i take them from here? i can think of turning
that two transistor astable into an RS bistable. I thought of doing a
regenerative receiver. But I am focused on making them understand what
they are doing. I,myself, don't understand how exactly the
regenerative receivers work, I don't want them to make something that
they will not understand fully well. I have been toying with the idea
of the primitive DC receiver of EMRFD (the one with two transistors,
anyone who has not made it is chicken).

please!! need suggestions.... i have another four days to fill up (of
two hours each)

- farhan
7513 2012-05-27 07:42:47 Russell Shaw Re: electronics workshop : crystal set reprise
7514 2012-05-27 07:50:00 Russell Shaw Re: electronics workshop : crystal set reprise
7515 2012-05-27 08:01:46 k5nwa Re: electronics workshop : crystal set reprise
A simple 2 or 3 transistor audio amplifier, a
speaker and microphone and a cardboard box and
they will have a blast with a homemade megaphone.

At 08:43 AM 5/27/2012, you wrote:
>
>
>this might be off topic, but it is not. it is about experimental
>methods in very primitive electronics, and i need help. but i am
>getting ahead of myself...
>
>i am running a children's summer workshop for a week (it is summer
>here). on the first day, we make the mandatory lemonade battery and
>hooked up two and three of them to light the LED. it was a huge hit,
>thanks to the galvanized nails which had to slip inside the lemonade
>class of each kid, then the hands go in to fish it out, then the juice
>spills onto the table, then it gets onto their hair, shorts and
>face... it was good fun to send them back home to dunk every liquid at
>home between the copper board and the nail and report back the voltage
>and current. we wrote the norton and thevnin equivalents the following
>day while wiping ketchup and beer stains off the VOM. (ketchup is bad,
>but beer is a pretty good electrolyte : from lab notes.
>
>SNIP



>my question is where do i take them from here? i can think of turning
>that two transistor astable into an RS bistable. I thought of doing a
>regenerative receiver. But I am focused on making them understand what
>they are doing. I,myself, don't understand how exactly the
>regenerative receivers work, I don't want them to make something that
>they will not understand fully well. I have been toying with the idea
>of the primitive DC receiver of EMRFD (the one with two transistors,
>anyone who has not made it is chicken).
>
>please!! need suggestions.... i have another four days to fill up (of
>two hours each)
>
>- farhan
>

Cecil - k5nwa

Never take life seriously. Nobody gets out alive anyway.
7516 2012-05-27 08:19:58 Ashhar Farhan Re: electronics workshop : crystal set reprise
I want to stress on measurements as a way to understand the circuits
that they build and come to an understanding of how it works.
Experimentation, observation and measurements have to be an integral
part of it.
Very often, we manage to gee-whiz a noob without explaining it all.
While fun, I hope to help kids 'see' how it all works together.
Crystal radio will still require them to understand how the inductor
and capacitor keep tossing energy between each other...

Emrfd, is at the heart of this approach. The first chapter lays this
foundation. Those who have missed it are well advised to reread it
again.

On 5/27/12, k5nwa <k5nwa@sbcglobal.net> wrote:
> A simple 2 or 3 transistor audio amplifier, a
> speaker and microphone and a cardboard box and
> they will have a blast with a homemade megaphone.
>
> At 08:43 AM 5/27/2012, you wrote:
>>
>>
>>this might be off topic, but it is not. it is about experimental
>>methods in very primitive electronics, and i need help. but i am
>>getting ahead of myself...
>>
>>i am running a children's summer workshop for a week (it is summer
>>here). on the first day, we make the mandatory lemonade battery and
>>hooked up two and three of them to light the LED. it was a huge hit,
>>thanks to the galvanized nails which had to slip inside the lemonade
>>class of each kid, then the hands go in to fish it out, then the juice
>>spills onto the table, then it gets onto their hair, shorts and
>>face... it was good fun to send them back home to dunk every liquid at
>>home between the copper board and the nail and report back the voltage
>>and current. we wrote the norton and thevnin equivalents the following
>>day while wiping ketchup and beer stains off the VOM. (ketchup is bad,
>>but beer is a pretty good electrolyte : from lab notes.
>>
>>SNIP
>
>
>
>>my question is where do i take them from here? i can think of turning
>>that two transistor astable into an RS bistable. I thought of doing a
>>regenerative receiver. But I am focused on making them understand what
>>they are doing. I,myself, don't understand how exactly the
>>regenerative receivers work, I don't want them to make something that
>>they will not understand fully well. I have been toying with the idea
>>of the primitive DC receiver of EMRFD (the one with two transistors,
>>anyone who has not made it is chicken).
>>
>>please!! need suggestions.... i have another four days to fill up (of
>>two hours each)
>>
>>- farhan
>>
>
> Cecil - k5nwa
>
> Never take life seriously. Nobody gets out alive anyway.
>
>

--
Sent from my mobile device
7518 2012-05-29 13:03:41 Gary, WB9JPS Re: electronics workshop : crystal set reprise
Ashhar, you are doing a great service to those kids. Here's a few random things I did with the kids from down the block. If this isn't learning by experiment, I don't know what is...

Charge up a BIG electrolytic and crowbar it with a screwdriver. Sparks are fun. (Wear safety glasses and use a long screwdriver, please.) Explain in terms of 1/2CV**2. Then discharge thru a resistor while watching with a scope. Explain with T=RC.

Interrupt dc current thru a BIG inductor, say from a power transformer winding, and watch it spark. Explain in terms of 1/2LI**2, and how inductors like current to stay constant.

Try putting those big L and C parts in parallel, drive with a signal generator and look for resonance. Also, try shock-exciting, and see it ring down. Nice, low frequencies.

Explain resonance analogy with a big pendulum. Vary length and mass (L and C) and see what happens. (Use math if appropriate for your kids' ages.)

Build a transformer. Wind a whole bunch of wire around some kind of coil form; make two separate coils. Drive one with an oscillator, watch the other on a 'scope or meter. Move them near and far, and orthogonally. Explain in terms of mutual inductance, and bring up frequency response. Note that this is kind of radio system.

Resonate the above transformer with a cap across one coil. Explain in terms of selectivity and how it relates to the crystal set.

If you have an LC meter, use it to measure some parts. Then make your own capacitor. We laid out aluminum foil with Saran wrap between layers, on a large table. Got a few hundred pF. Explain in terms of C = e A/D. Imagine how big one Farad must be.

Take apart a junk lawnmower engine!!! Take it all the way down, see how the ignition system works, how gas and air mix in the carburetor, how the piston and valves are timed, and most of all, GET DIRTY. Every engineer needs to know how an engine works, for heaven's sake. My dad did this for me in second grade. I took it to school, with posters, for show-n-tell. Clearly, I was doomed to be an engineer.

73,
Gary, WB9JPS
7520 2012-05-29 15:44:34 R Wall ML emails Re: electronics workshop : crystal set reprise
Hi Ashhar,

I’ve added a pdf file in the groups file section called “Capacitance Charge Voltage Demo Prac.pdf” Have a look and see if that helps.

Roderick Wall, vk3yc.

7521 2012-05-29 21:14:46 Ashhar Farhan Re: electronics workshop : crystal set reprise
gary, thanks i am almost doing exactly that.

i am not really sure if this thread belongs here. so, i will keep it
brief. this is the syllabus that i came up with (we are on day 5)

day 1 : concept of charge, static electricity, how to move charge around
(current) using the cafe lemonade and lighting up an LED
day 2: concept of magnetism with a small compass and nedymium magnet balls
that automatically align north-south, the effect of high current on the
compass, increasing magnetism by bunching the wire together as an inductor
and then sticking it behind a paper cup, hold a neodymium magnet behind the
cup to make the speaker.
day 3: learn to solder make an ugly circuit of an LED and a resistor. make
resistors out of pencils by sharpening it on both sides, learn to use the
VOM
day 4: watch a capacitor charge and discharge (RC low pass). study the
phase difference between current and voltage. use a transistor with and led
and resistor in the collector lead. touch the base and the +ve supply (all
on 6v batter pack) to light up the LED. wire two of them back to back to
create a bistable - a single bit memory.
day 5: evolve the bistable into an astable. attach to a CD4040 to study
binary count
day 6: crystal radio with CD ROM case, wire, capacitor made by sandwiching
paper between two copper scraps. move and hold them with clothsline peg to
set the station. (btw, we have a powerful local AM station : ipod variety
headset works directly)
day 7: LM386 audio amp!
day 8: oscillator, and diode mixer added to the LM386 to make a direct
conversion radio for 7 MHz.



7523 2012-06-05 00:29:00 Mark Re: electronics workshop : crystal set reprise
Looks great Farhan!

Day 7 may be more interesting to start with a one or two transistor audio amp just to show them transistor action and also all the items packed into that little LM386. You could also use the discrete amp to introduce them to schematics (so they could just point to the parts and how the schematic symbols imitate the physical bits of the parts.)

Kids like action so for the next day you could introduce them to generating rf signals (and a little rfi hihi) via a spark gap transmitter and then a simple receiver to pick up the signals -- they could then send each other morse code! Or shoot off a canon like Jagadish Chandra Bose (a little National history isn't a bad idea!)

LED's also make good photodiodes so you could play with that idea too (solar energy?).

Electrical generators (permanent magnet motors) demonstrate magnetics and energy producti