Electricity question borne out of idiocy

[dogjones]

Some friends and I have been playing with a 'shocking autopsy' game. The (ostensible, layman's view) design is as follows:

Box, with picture of human on top. The human has various holes in it - these are lined with a silvery metal, and into each hole fits a plastic object.

A wire comes out from the side of the box which culminates in a pair of tweezers, basically two thin strips of metal.

Object of game: use tweezers to remove plastic objects from holes. If the tweezers touch the silver lining of the holes, you get an electric shock.

What jolly good fun it is!

However, we, of inquiring but ignorant minds, are stumped as to how it works. At first, we thought it was simple enough, a circuit is completed when you touch the sides of the holes which gives you the shock. But the weird thing is, occasionally, after someone has been shocked by touching the side of a hole, they pick it up again, and even though the tweezers are nowhere near the hole, they get another shock, of what seems to be about the same strength.

If we hold just one of the tweezer prongs and touch that to the side of a hole, there is no shock. It doesn't matter which prong.

Basically, one has to hold both prongs (the prongs themselves don't have to touch, so long as your hand is in contact with both of them), and touch either prong to the metal lining of a 'hole' - in order to get a shock - and then occasionally get a secondary shock without touching the lining of the 'hole'.

So how does the circuit work? We've flung ideas out like static electricity, paths of least resistance, the earth... but the exquisite truth of it is, none of us know what the hell we're talking about.

Can anyone shed some light on this, preferably in simple terms, but if people want to go all esoteric and assume prior knowledge that would be cool too!

Many thanks
Dog

 

[Mr. Scott]

Can anyone shed some light on this, preferably in simple terms, but if people want to go all esoteric and assume prior knowledge that would be cool too!

There are at least two ways to implement a game like this:

Implementation #1:

Connect a high enough voltage (let's say 100 volts DC or AC) between the silver lining (foil) and the tweezer prongs through a pair of resistors. One side of the voltage source (let's say the plus side) goes to the foil of the holes. The minus side goes to the wire going to the tweezers. Inside the head of the tweezers, the wire is connected to each prong through its own resistor. Before touching the foil, both prongs have the same potential. When you touch one prong to the foil, that prong, because the resistance to the foil falls to zero, becomes the same potential as the foil. The other prong, however, remains at the opposite potential, resulting in a little less (because of the voltage drop between the prong's internal resistor and the skin resistance between your two fingers) than 100 volts appearing between the two prongs and a resulting shock.

However, since you say there is a residual shock between the prongs when the foil is not touched, static electricity must be involved.

Implementation #2:

Connect the wire to the prongs through a pair of capacitors and supply the circuit with static electricity (I think 100 volts DC may be enough). When one prong touches the foil, the capacitor between that prong and the wire allows current to flow until is charges up to 100 volts (a capacitor is like a quick, fraction of a second charging and discharging battery). During this charging, a difference in potential occurs between the two prongs and between your fingers, causing current to flow and the shock sensation. Because of this current flow, the two prongs and their associated capacitors quickly reach the same potential. Now both capacitors are charged. Now if one or the other capacitors becomes discharged (perhaps to ground after you touch only one prong) a potential will appear again across the prongs and a second shock, without touching the foil, can be felt if you complete the current again between the prongs with your fingers.

So, I believe there is a very low current DC power source in the base unit of perhaps 100 volts, with one side connected to the foil, the other side connected to the wire, and the wire connected to prongs that are insulated from each other except for a capacitor (perhaps 0.1 microfarads) running from each prong to the wire. It's also possible the wire has two conductors, allowing the capacitors (or resistors in #1) to be hidden inside the base. Some kind of drawing or photo of the junction between the tweezers and the wire would help indicate where the capacitors are.

A wide range of voltages/capacitances would work, precise values resulting in differing intensity (voltage) or duration (capacitance) of the shock.

Oh, by the way, #1 could also work by connecting a DC voltage source through a resistor, then across a capacitor which supplies the foil and tongs through a pair of resistors. I think that's the way I'd design it and my money goes on that as the actual design.

Does the base plug in or use batteries? What kind of batteries and how many? Is there an on/off switch? Does it make a whining sound like a flash camera the descends and rises in volume when triggered? There are many ways to make this work, and putting various test instruments to the foil and prongs could come close to nailing the exact circuit.

Something like this is probably close to what's really in there, or at least the way I'd design it, although the 1K resistors are more likely the capacitors mentioned in #2 (to account for the secondary shock):

 

[dogjones]

Thanks for replying! Each prong leads to a separate little wire. Base unit takes 3 AAA batteries. We dismantled it and exposed the circuit board but we don't really understand it - it's basically a green square thing with a couple cylinders sticking out - we surmise these are capacitors - but again, we are ignorami...

 

[Mr. Scott]

Thanks for replying! Each prong leads to a separate little wire. Base unit takes 3 AAA batteries. We dismantled it and exposed the circuit board but we don't really understand it - it's basically a green square thing with a couple cylinders sticking out - we surmise these are capacitors - but again, we are ignorami...

Then the circuit board has a voltage inverter -- a transistor oscillator to convert DC to AC, a coil to boost the 4.5 volts combined of the 3 batteries to around 100 volts, one or more doides to convert the AC back to DC, then the capacitors and resistors for the tong and foil circuit. Any combination of these components may be integrated into the green square thing (potted with epoxy). If I were still at the lab I used to work at I could build a working model in a couple of hours.

The circuit below might be exactly what's in there:

 

[This is The End]

So someone has rigged up an Operation game to where, instead of a sound buzzer and a lighted nose, the electricity is used to shock the player?

http://en.wikipedia.org/wiki/Operation_(game)

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

 

[MRC_Hans]

Combining parts from the answers above, I come up with this circuit idea:

The circuit has a generator which steps up the voltage from the batteries to some, probably quite high, voltage, I'd guess 500 volts or so. The current is very low, so the shock is not dangerous. However, the voltage from such a generator will build up too slowly to produce a convincing shock, so a capacitor is needed. This still keeps the shock harmless, because the dangerous part of a shock is the energy, and you can control this by the size of the capacitor. So we have each side of the tweezers connected to its own capacitor, and a fairly large resistor to the generator. Now the capacitors charge to an equal voltage, which means there is no potential between the sides of the tweezers,so you dont get any shock touching it. The metal linings are connected to the common ("ground") potential of the circuit.

Now, when you touch the metal with one side of the tweezers, that capacitor gets discharged, and the other one gives you a shock. As the discharge through the metal lining is very quick, this design ensures the even if you only touch them metal very briefly, you still get a full shock.

However, if you let go of the tweezers before that capacitor is entirely discharged (through your fingers), and touch it again before the other capacitor is recharged, there is another shock for you.

Hans

 

[CurtC]

Are you sure you're being shocked, and it's not just that you're startled by the loud buzzing? I can't imagine a commercial kid's game would actually shock you, and I'm familiar with the Operation game 'cause it's been around since I was a kid.

 

[Jimbo07]

Are you sure you're being shocked, and it's not just that you're startled by the loud buzzing? I can't imagine a commercial kid's game would actually shock you, and I'm familiar with the Operation game 'cause it's been around since I was a kid.

Yes, from what I understand, Operation probably has a very simple circuit. I think if you make the circuit, the board buzzes, vibrates and a light goes off. It would need modification to be... shocking!

 

[Mr. Scott]

The original game had a simple circuit: Two batteries in series with the light bulb and buzzer, with the tweezers and metal holes forming a switch that completes the circuit when they touch.

Googling reveals the game came out first in 1965. The hair style appears to be a dorky retro style for that time.

BTW: A buzzer can be the active part of a shock circuit, since when the buzzer contacts open, the magnetic field across the coil collapses very quickly, creating a very high voltage. This is the principle of pre-solid state auto ignition systems, which convert 12 volts DC to thousands of volts for the spark plugs.

 

[Fnord]

Simpler explanation:

In the original Operation game, a series loop was formed by the tweezers, the playing surface, the battery, and the buzzer/light combination.

The buzzer is essentially a solenoid coil in series with a set of normally-closed contacts. When the coil is energized, the contacts open and break the current through the coil. A lack of current through the coil allows the contacts to close again, and the cycle repeats itself. As the contacts open and closed repeatedly, they make the characteristic buzzing sound.

When current is interrupted to a solenoid, a "flyback" pulse is generated as the magnetic field collapses. If the coil has sufficient windings, a pulse of over 100 volts can be generated from a 3-volt battery. If this pulse can be routed back to the tweezers, the person holding them will feel a shock.

Note that the flyback pulse is not "free energy" as it is produced from the energy stored in the solenoid's magnetic field as the field collapses.

If the light bulb is in parallel with the solenoid only, then it will absorb the excess energy. If it is, instead, connected in parallel with the entire buzzer, then the pulses can be routed to the tweezers.

I'll try to produce a diagram later today.

 

[Pidge]

Go get yourselves a 1.5 battery, and "decent sized" inductor (e.g. use the primary winding of a low current 110V to 12V AC transformer), and three lengths of wire.

strip the ends of the wire
attach one wire between one terminal of the battery and one end of the inductor windings
attach one end of one remaining wire to the other terminal of the battery
attach one end of the last wire to the other end of the inductor windings

now, in each hand, hold the bared ends of each of the free ends of wire (one in each hand)

Touch the bare ends together.

Then take them apart.

Let me just say this - I won't willingly do those last two steps. Nasty trick to pull on physics students so they really get a "grip" on V=L.dI/dt.

The cirucit diagram shows my guess of what the circuit diagram will be.

Hopefully the lack of labels won't be too confusing.

When one part of the tweezers touches the foil lining of the holes, it completes a circuit, allowing current to begin flow through one half of the inductor.

Once the circuit is broken, the only path available for the current to continue flowing in the inductor is the circuit completed by a hand in contact with both parts of the tweezers. And since there will have been a very sudden drop in current, the inductor will develop a very large voltage across it (V=L.dI/dt, L = inductance of inductor in Henries), generating the shock felt.

(And I started this post 9 hours ago but got interrupted, so I'm repeating what Fnord and Mr Scott wrote...)

 

[Fnord]

Here is my version of the "Operation" game.



B = Battery (12V, to power the circuit)
C = Capacitors (to bypass the current around the battery and contacts)
D = Diode, Rectifier (to protect the battery)
L = Lamp, Incandescent (indicator, remove for shock effect)
P = Platten (conductive game surface)
S = Solenoid (12Vdc relay wired for "buzzer" effect)
T = Tweezers (metal, non-insulated)

There may be enough "body capacitance" to allow a student to feel a mild shock without requiring them to physically complete the circuit.

This is for educational and/or entertainment purposes only. The person(s) constructing and/or using this device are solely responsible for the safety of the users. No guarantee or warranty is expressed or implied.

 

[TheDoLittle]

And remember to buy the T-shirt when you get there!

http://www.threadless.com/product//Operation_Needed

 

[Modified]

Are you sure you're being shocked, and it's not just that you're startled by the loud buzzing? I can't imagine a commercial kid's game would actually shock you, and I'm familiar with the Operation game 'cause it's been around since I was a kid.

Even the buzzer could fray your nerves after a while.

 

[Mr. Scott]

If the light bulb is in parallel with the solenoid only, then it will absorb the excess energy. If it is, instead, connected in parallel with the entire buzzer, then the pulses can be routed to the tweezers.

I think you meant to say: If the light bulb is in parallel with the solenoid only, then it will absorb the excess energy. If it is, instead, connected in series with the entire buzzer, then the pulses can be routed to the tweezers.

Also, the simple flyback circuit you propose would create a continuous shock. I believe the OP mentions only a momentary shock. Your circuit would also not produce the residual after-shock.