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[Reply #15 on:]

they explode due to too much voltage being applied (25v on a 9v source is fine) or having its polarity switched

[Reply #16 on:]

If you only use low capacity batteries they will just get warm and not explode when the polarity is reversed.

[Reply #17 on:]

If you only use low capacity batteries they will just get warm and not explode when the polarity is reversed.

I've never intentionally tried it, but couldn't they potentially vent and leak (I don't consider venting to be the same as exploding, although exploding is the same as venting - just in a violent and quick manner!)?

[Reply #18 on:]

9v batteries have an extremely low max current thanks to a very high internal resistance. They can't dump current nearly fast enough to cause (much) damage.

[Reply #19 on:]

If you only use low capacity batteries they will just get warm and not explode when the polarity is reversed.

that clears it..  Next point..

Any interesting small project ideas? I was thinking of a small IR/LDR based intruder alarm (kids will have fun putting it up in their rooms) any other such suggestions which kids can relate to and apply in their own worlds?

[Reply #20 on:]

pwillard had a great basic circuit

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1270733826/3#3

but that is more topics to cover

something else would be like





 :-?

[Reply #21 on:]

555-based "music synthesizer" ?
You can even replace the cap and convert it to an LED blinky, thereby demonstrating all sorts of thing about caps and sound...

[Reply #22 on:]

Do you think kids would understand 555(10-13yr olds)? i'm just thinking of a few logic ics...anybody got any experience in teaching 555 to kids(personally all the modes might look daunting for kids)? I was thinking of a buzzer and a led to be moved around a wire.. you know the kind of stuff you find in exhibitions...some kind of a loop... any improvisation suggestions on this?

[Reply #23 on:]

Do you think kids would understand 555(10-13yr olds)?

Well, we didn't analyze it at the transistor level till senior year of college, but if you've gotten them to understand resistors, capacitors, and switches, all you need to add is the function of a "comparator" (which is pretty obvious.) to understand basic operation of the 555.  Use something like http://www.ecircuitcenter.com/Circuits/555_Timer1/555_timer1.htm as the basis converting the flipflop/transistor to a back-box "signal controlled switch (and leave out the spice/etc)    The uninterested will be happy to treat the chip as a black box, and the few who are interested will leaving feeling like "holy crap, I sorta understand how the insides of a chip work!"

You could even "construct" a giant 555 timer using a kid or two to  watching meter and throw switches to replace the chip contents.  A fine, fine, use for those $2 harbor-freight meters...

Another possibility is a binary to decimal or decimal to binary converter (switches to lamps, using a diode array), but that's less useful...

[Reply #24 on:]

Reversing a polarised electrolytic can lead to an explosion, over voltage can lead to an explosion.  Using non-polarized caps of a suitably high rating is probably the sensible approach.  Kids will try and blow things up, trust me!!

And stick to low total energies, way below 1J is a good guide, otherwise you'll have to make them wear eye protection from the sparks when they short them out.

As for the understanding, the water analogy or similar is good because there's nothing to see in a capacitor.

Using analog volt and ammeters is crucial - avoid DMMs.  Oscilloscope could be useful for demonstration (but not hands-on).

[Reply #25 on:]

I was thinking of a small IR/LDR based intruder alarm (kids will have fun putting it up in their rooms) any other such suggestions which kids can relate to and apply in their own worlds?

This reminds me of a project back when I was in grade school (5th grade); my friends and I were constantly missing things (pencils and paper, mainly) from our desks (assigned seating, old-school lidded desks), we suspected the teacher.

I ended up rigging an alarm using my 150-in-one kit from Radio Shack and a custom trip-wire like switch to the desk lid; it could be turned off if you knew what to do by carefully openning the desk, reaching in, turning it off with another switch.

Imagine our surprise when, during recess (do kids even get recess anymore?), the alarm sounded very loudly and our teacher ran out calling my name to come shut the thing off. I turned it off, looked at her, then went back out to recess. No other words were exchanged that I remember.

I was never sent to the principal.

Our desks were not messed with the rest of the year.

 

[Reply #26 on:]

Why not make a Leyden jar. Really easy to make and tons of fun!

The best Leyden jar (or capacitor) I made was an aluminum pie tray with pen tube stuck to it. It stores electrical charge. Just put it on an old TV and turn the TV on and off. Then touch the pie tray to be shocked. (Harmless)

After teaching that electricity can be stored, point out that "these" (you point to electronic capacitors) do the same thing by storing electricity. And do this experiment with the electronic capacitors:

[Reply #27 on:]

Why not make a Leyden jar. Really easy to make and tons of fun!

I haven't done this myself. So its a tricky thing to teach someone what I haven't done.

I'm planning to add a DC motor with diode to charge a capacitor and power up an led(perhaps a simple example of generation + storage)

I ended up rigging an alarm using my 150-in-one kit from Radio Shack and a custom trip-wire like switch to the desk lid; it could be turned off if you knew what to do by carefully openning the desk, reaching in, turning it off with another switch.

This sounds cool... Will include this.. I believe these are the sort of things kids would love to do. Perhaps a touch sense light as well...

[Reply #28 on:]

Using non-polarized caps of a suitably high rating is probably the sensible approach.

Ok I'm unable to get these in the local stores. Whats the alternative. I was wondering if a diode on one end should protect polarity reversals?

[Reply #29 on:]

Forget the 25v capacitor, go for one with a higher voltage rating, say 64 or 100.  With a 9 volt battery you can do what you like regarding polarity, the capacitor will survive and more importantly, will take a charge.  If you introduce a diode to prevent reverse polarity charging, you add an element of "doubt" to those poor grey cells that you are trying to excite.

If you really want to play ultra safe, get an ac rated one from a washing machine or an old fluorescent strip lamp.  These are generally around 4 to 8 microfarad and rated for 400volts.  An added benefit is that they are of reasonable size, have decent metal lugs and are virtually kid proof.  (Give them a small wire-ended one and someones bound to pull its "legs" off !!)
jack