Favorite simple circuits for demo, play, world domination?

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ks_physicist
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Favorite simple circuits for demo, play, world domination?

Postby ks_physicist » Tue Feb 26, 2008 5:21 am UTC

I recently ran across the simple FET electrometer circuit, and building it was the best $3 or so I've ever spent at Radio Shack.

I plan on buying a hundred FETs in bulk from Digikey or Mouser and get to work building dozens more. I'll use it in class for labs and demos, and to play with at home.

So, what simple electrical circuits do you think are the bees knees--the wasps nipples--etc.?

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Solt
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Re: Favorite simple circuits for demo, play, world domination?

Postby Solt » Tue Feb 26, 2008 9:42 am UTC

I've always wanted to build a DC stepup circuit that can go from, say, a 3 Volt battery supply to a 300 Volt capacitor. Then shenanigans can ensue. World domination is also a possibility.
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Re: Favorite simple circuits for demo, play, world domination?

Postby K^2 » Tue Feb 26, 2008 10:49 am UTC

Solt wrote:I've always wanted to build a DC stepup circuit that can go from, say, a 3 Volt battery supply to a 300 Volt capacitor. Then shenanigans can ensue. World domination is also a possibility.

You need an AC stage. Simplest way to do this is to use a two position relay driven by a small AC generator to alternate polarity of your DC supply. This square-wave AC can easily be stepped up with a transformer. Finally, get a rectifier bridge to get back to your DC. Final DC output will be slightly oscillating because of the inductance of the transformer, but it will work just fine for charging a cap. You might want to connect a resistor across the low-voltage end of the transformer to reduce/eliminate the sparks at the relay.

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Bruce
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Re: Favorite simple circuits for demo, play, world domination?

Postby Bruce » Tue Feb 26, 2008 11:15 am UTC

I would suggest using solid state switching, a real relay is not going to last so long being put through that kind of punishment.
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Re: Favorite simple circuits for demo, play, world domination?

Postby hyperion » Tue Feb 26, 2008 11:36 am UTC

ks_physicist wrote:I recently ran across the simple FET electrometer circuit, and building it was the best $3 or so I've ever spent at Radio Shack.

If it turns out they have those transistors at school, I am going to make one.
Or three. :D
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Re: Favorite simple circuits for demo, play, world domination?

Postby LikwidCirkel » Tue Feb 26, 2008 10:57 pm UTC

Solt wrote:I've always wanted to build a DC stepup circuit that can go from, say, a 3 Volt battery supply to a 300 Volt capacitor. Then shenanigans can ensue. World domination is also a possibility.
Buy a disposable camera with a flash. The flash circuitry does exactly that. It's really fun to short-circuit the leads of the cap together when it's fully charged. It would certainly hurt a lot if you touched it to your finger.

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Re: Favorite simple circuits for demo, play, world domination?

Postby hyperion » Tue Feb 26, 2008 11:07 pm UTC

LikwidCirkel wrote: It would certainly hurt a lot if you touched it to your finger.

It does
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Re: Favorite simple circuits for demo, play, world domination?

Postby Micron » Tue Feb 26, 2008 11:17 pm UTC

Take a look at what you can do with a 555 timer chip. When I started playing with electronics there were many projects I wanted to build but did not learn how to make a fairly reliable clock signal for far too long. With just a couple of parts you can make something blink or tick with decent accuracy.

You might also want to start playing with digital logic circuits since determining their behavior is sometimes easier than considering an analog device. The 74LS family of components are a standard set of chips which provide a number of useful simple logic circuits you can combine as you please.
Grab a breadboard, an old AT computer power supply, some switches, LEDs, and 74LS chips can you can put together any number of nerdy toys. Building your own 4 bit adder might be a good place to start once you have some idea how the parts work.

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Re: Favorite simple circuits for demo, play, world domination?

Postby Solt » Wed Feb 27, 2008 12:49 pm UTC

K^2 wrote:
Solt wrote:I've always wanted to build a DC stepup circuit that can go from, say, a 3 Volt battery supply to a 300 Volt capacitor. Then shenanigans can ensue. World domination is also a possibility.

You need an AC stage. Simplest way to do this is to use a two position relay driven by a small AC generator to alternate polarity of your DC supply. This square-wave AC can easily be stepped up with a transformer. Finally, get a rectifier bridge to get back to your DC. Final DC output will be slightly oscillating because of the inductance of the transformer, but it will work just fine for charging a cap. You might want to connect a resistor across the low-voltage end of the transformer to reduce/eliminate the sparks at the relay.


I'd rather do it without a transformer and rectifier. There are simpler (and cheaper) ways but they require things like inverters and FETs, which I never seemed to have sitting around.
"Welding was faster, cheaper and, in theory,

produced a more reliable product. But sailors do

not float on theory, and the welded tankers had a

most annoying habit of splitting in two."

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Re: Favorite simple circuits for demo, play, world domination?

Postby Bruce » Wed Feb 27, 2008 9:03 pm UTC

You need a transformer. No option.
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Re: Favorite simple circuits for demo, play, world domination?

Postby Minerva » Thu Feb 28, 2008 3:15 am UTC

I've always had a special place in my heart for Cockroft-Walton Multipliers.

They can be finger sized, or house sized.
You can clean dust out of the air, create negative ions in the air, or smash nuclei with them.

Need a 20V power supply from a 5V rail for an EPROM programmer? no problem - use a 555 to drive a few CW stages.
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Re: Favorite simple circuits for demo, play, world domination?

Postby ks_physicist » Thu Feb 28, 2008 5:36 am UTC

Doesn't the CW need the voltage to swing negative for part of the cycle in order to work?

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Re: Favorite simple circuits for demo, play, world domination?

Postby Bruce » Thu Feb 28, 2008 6:51 am UTC

It would require a change in voltage - on/off would suffice.
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Re: Favorite simple circuits for demo, play, world domination?

Postby ATCG » Thu Feb 28, 2008 6:57 am UTC

ks_physicist wrote:Doesn't the CW need the voltage to swing negative for part of the cycle in order to work?

No. The theory behind this and other charge-pump circuits is explained nicely here.

Briefly, in a simple voltage doubler the two plates of a capacitor are rapidly switched, one between two voltage levels and the other between a charge source and a charge sink, in two repetitive phases. In the first phase, one plate is connected to ground and the other plate is connected to the positive supply. Charge fills the capacitor until the voltage across the cap is equal to the supply voltage. In the second phase, both plates are disconnected from the supply and the plate that was formerly at ground potential is connected to the positive supply. The voltage across the cap remains equal to the supply voltage so that the voltage on the plate previously connected to the positive supply rail is now at twice the supply voltage relative to ground. This plate can now be connected to a sink (generally another capacitor) through a diode or other switch and charge transfered from the switched capacitor to the sink. Alternating phases transfers charge from supply to output in much the same way that working a pump handle moves water from a low to a high level.

Tiny surface-mount packages that integrate just about everything but the bootstrap (switched) capacitor are readily available. In fact, entire charge-pump back-bias generators (including capacitor) are integrated into nearly every modern MOS integrated circuit. These keep the device substrate at a negative voltage without burdening the user with having to provide a negative voltage supply. (In the bad old days having to provide +12V, +5V, and -5V supplies was the norm for MOS ICs.)
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Re: Favorite simple circuits for demo, play, world domination?

Postby alterant » Fri Feb 29, 2008 2:19 am UTC

ks_physicist wrote:I recently ran across the simple FET electrometer circuit, and building it was the best $3 or so I've ever spent at Radio Shack.


Hello there;
Well, I'm new to the forum so I had better introduce myself in the introductions thread after this, I suppose. I am a beginner at electronics and I tried building this circuit, which I can see would be quite cool. I'm having trouble with it, though, probably because I don't have the transistor that is being asked for. I only have access at the moment to two kinds of transistors:
--> NPN-type switching transistors (2761617)
--> PNP transistors (2761604)
Obviously I don't expect anyone to do my homework for me, but do you know offhand if it absolutely has to be a JFET? of that precise type?
Thanks and I'd love to see other circuit ideas here :)
-Ian

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Re: Favorite simple circuits for demo, play, world domination?

Postby ATCG » Fri Feb 29, 2008 2:40 am UTC

alterant wrote:
ks_physicist wrote:I recently ran across the simple FET electrometer circuit, and building it was the best $3 or so I've ever spent at Radio Shack.

I don't have the transistor that is being asked for.

[Does it] absolutely [have] to be a JFET?

It absolutely has to be a FET. A junction transistor won't cut it. The circuit as given depends critically on the (for all intents and purposes) infinite input impedance at the gate of the FET.

I'd try to match the characteristics of the given device as closely as possible, but you don't need precisely that part.
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Re: Favorite simple circuits for demo, play, world domination?

Postby OneFish » Fri Feb 29, 2008 5:57 am UTC

Bruce wrote:You need a transformer. No option.


Really?

I always liked this ckt:

http://en.wikipedia.org/wiki/Voltage_multiplier

I think that even if there's an issue with the amplitude of your initial AC source you could cascade several diode ladders with active switching between them and get to 300V without a transformer.
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Re: Favorite simple circuits for demo, play, world domination?

Postby Bruce » Sun Mar 02, 2008 5:51 am UTC

Yes you are quite right, AC is required a transformer is not.
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Re: Favorite simple circuits for demo, play, world domination?

Postby hobbesmaster » Sun Mar 02, 2008 8:05 am UTC

Bruce wrote:Yes you are quite right, AC is required a transformer is not.


Define AC, some of those stunts are DC all the way. (at least, with the definition of DC you're likely to use around power/drives people)

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Re: Favorite simple circuits for demo, play, world domination?

Postby Bruce » Sun Mar 02, 2008 8:30 am UTC

Switched DC becomes AC IMO. Anything with a changing potential.
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Re: Favorite simple circuits for demo, play, world domination?

Postby hobbesmaster » Sun Mar 02, 2008 8:34 am UTC

Bruce wrote:Switched DC becomes AC IMO. Anything with a changing potential.


Then there is no such thing as DC in the real world.

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Re: Favorite simple circuits for demo, play, world domination?

Postby genewitch » Mon Mar 03, 2008 9:38 pm UTC

wireless transmission is fairly easy to design in a circuit. Transistors and capacitors making a NAND gate logic circuit is an awesome way to introduce logic circuit design, as well as the fact that you can create EVERY other type of logic gate using JUST NAND gates.

Let's see, what else is fun to play with.. timer circuits (the 555 was mentioned i believe), flip-flop ICs, stuff of that nature. I'm really a huge fan of communication circuits so i'm biased (no pun intended). building saltwater capacitors is cheap and interesting, five gallon bucket, a roll of aluminum foil, salt, water, and some wire coat hangers and you can zap stuff.

Electromagnets are fun to play with too.

If you have a classroom of students that have camera phones, playing with infrared LEDs can be a fun experience as well, at least in my opinion. (it's like magic, a lot of people don't get it, so it's cool to explain).

a push pull amplifier, oscillators, a simple audio amplifier using a couple of transistors... it all depends on how basic or advanced you want to get. I'd be willing to help you come up with more if you'd like, drop me a line :-)

good lord, i just remembered THIS: http://video.stumbleupon.com/#p=u291ko2fly it's a little power supply that uses electromagnetic radiation in the vicinity to power small devices. One of the coolest, cheapest most awesome things i've seen in a while! And i see lots of cool stuff, believe you me. (i'm sure you can tune it with a little playing with capacitance/resistance values, yes?)

hobbesmaster wrote:
Bruce wrote:Switched DC becomes AC IMO. Anything with a changing potential.


Then there is no such thing as DC in the real world.


I'm not exactly sure what you mean... is there a such thing as "AC"? I was always under the impression that electrons moving in one direction along a path of conductors was considered "DC", whereas electrons that "changed directions" was considered "AC".

as i said, i'm completely clueless as to what you're trying to imply :-(
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Re: Favorite simple circuits for demo, play, world domination?

Postby hobbesmaster » Tue Mar 04, 2008 1:17 am UTC

genewitch wrote:I'm not exactly sure what you mean... is there a such thing as "AC"? I was always under the impression that electrons moving in one direction along a path of conductors was considered "DC", whereas electrons that "changed directions" was considered "AC".

as i said, i'm completely clueless as to what you're trying to imply :-(


"Anything with a changing potential" is what he said. Theres a decent coverage at wikipedia about what DC means in various contexts. Continuous current is the definition you gave, and includes things like rectified AC. It can get a little confusing as the definition will change depending on context.

DC motors are generally driven by switched DC of some sort. Choppers and phase control on rectified AC are common, both are considered DC drives. It'd be kinda confusing otherwise. ;)


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