Special Switch

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Mavrisa
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Special Switch

Postby Mavrisa » Fri Nov 13, 2009 8:55 pm UTC

So for one part of a physics project (a roller coaster), I need to design a switch that will be able to do two things:

When the ball arrives at the bottom, it falls into a cart. It activates a switch that makes a connection to a motor. The motor drags the cart up a ramp, and at the top the ball is dumped out of the cart onto the track. Meanwhile, the switch is released, and it goes into its other position, which reverses the direction of current to the motor, which then pulls the cart down. Once it reaches the bottom, the current must stop flowing.

I had something working for a short while, but as we tested it, it got more and more unreliable. It has to work several times in front of our teacher without our intervention, and I'm out of ideas for making it work. I thought that someone on this forum must have a good idea to make such a system, and have it be reliable.

A few things to keep in mind:
The ball (a glass marble) weighs about 3 grams.
The more compact the better.
We cannot use prefabricated switches, it has to be home-made.

Thanks for any ideas.
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modularblues
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Re: Special Switch

Postby modularblues » Sat Nov 14, 2009 4:12 am UTC

Were you using a mechanical push-button switch? Perhaps like this?

Since the ball weighs ~3 grams, maybe it's not generating enough force when it drops to reliably operate the switch?

I was thinking of an optical switch like a photodiode, but that would be very sensitive to ambient light, hmm.

Or pressure-sensitive fabric?

Mavrisa
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Re: Special Switch

Postby Mavrisa » Sat Nov 14, 2009 5:31 am UTC

I was using a lever switch with a very weak spring underneath (made of paper actually..) that got pushed down by the marble onto the contacts. Then when the marble got dumped out, the spring was supposed to push the contacts on the other side up far enough to touch another set, in order to reverse the current. While the marble did push the switch down far enough to make a good contact USUALLY, the biggest problem came from the other position, when the ball was dropped out. As you may have guessed, paper wasn't the best choice because it started to lose its springiness.

If someone can think of a way that pressure switch could successfully reverse the current after the ball falls out, that would be really handy. Either that or another mechanism entirely, that should perform the same function. I thought about somehow using a magnetic switch, but I can't find a magnetic or steel ball large enough anywhere.

I guess my main problem is reversing the current's direction. Wouldn't be so bad if I just had to turn something on or off.

Thanks for your help so far.
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Mr. Freeman
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Re: Special Switch

Postby Mr. Freeman » Sat Nov 14, 2009 6:58 am UTC

I don't know if you can switch balls or wrap the current one in something conductive, but two wires spaced a little bit apart would do the trick. The ball bridges the two wires and a circuit is made. Without more information I really can't figure out what exactly you're trying to do or what switch you need, but this might help.

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squareroot1
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Re: Special Switch

Postby squareroot1 » Sun Nov 15, 2009 2:42 am UTC

Try to fit a clutch into the drivetrain. That way, the clutch will only engage while the motor is turning, and the bucket can fall back into place under it's own weight. Off the top of my head, I'd consider a planetary gear which sweeps upward to meet the subsequent gear. If the teeth have issues meshing, switch some wheels in instead.

You will only need one switch that way.

Even simpler method is to do away with the bucket, have the motor continually running, and fashion 'walls' to the chain with periodic 'floors'. The marble would drop onto the chain, roll down it (between the walls) until hitting a 'floor,' get carried up to the top, and drop onto the top of the coaster.

I don't know if you are required to have a bucket, but it's an idea.

Mavrisa
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Re: Special Switch

Postby Mavrisa » Sun Nov 15, 2009 6:19 am UTC

squareroot1 wrote:Try to fit a clutch into the drivetrain.

That would be an interesting method, and we might get bonus points for creativity but my teacher says we lose marks if the motor must continually run. The gears would be a good idea but I don't really have the ability to make them, so it would have to be wheels.

squareroot1 wrote:Even simpler method is to do away with the bucket, have the motor continually running, and fashion 'walls' to the chain with periodic 'floors'.

I think this would be considered taking the easy way out, which we'd lose marks for :(
That will be our last resort though.

Mr. Freeman wrote:I don't know if you can switch balls or wrap the current one in something conductive, but two wires spaced a little bit apart would do the trick.

That is a good idea, and yes we can use any ball we want (within reason.. A soccer ball might be pushing it :P ). But doing the double switch might not work with that setup. If I can get the cart to slide back down on its own, then I'll probably go with something like this. But the cart is pretty low mass so friction might cause a few problems. I guess I could make another cart but that might require a better motor.. (Related to this: If you add a second battery, to give the motor more "torque" you would add the batteries in parallel, right? Or can you not change the torque..? It's a little motor I got out of a cereal box toothbrush.)

Anyway, thanks for all the help so far. We've got a while still so if anyone else has suggestions, they'd be quite welcome.
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scarecrovv
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Re: Special Switch

Postby scarecrovv » Sun Nov 15, 2009 6:56 am UTC

See the attached picture for my idea. orange things are conductors, if it wasn't obvious.
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TescoPeeledPlums
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Re: Special Switch

Postby TescoPeeledPlums » Sun Nov 15, 2009 4:45 pm UTC

I'm not sure whether this is entirely within the rules of this competition, the plausibility of it and whether it would actually work are all questionable, but can't you have a steel or iron ball, something magnetic.

Well, actually, I have 2 theories now:

1. You place electromagnets at intervals beneath the tracks, activated by foil wrapped around the tracks just below the electromagnets, i.e. as the ball comes up to the electromagnet, one further up is activated, and the one it is at turned off, in effect pulling it magnetically up the ramp. A railgun, in fact, just not as powerful.

2. You use the motor instead to pull a magnet up the track, which would pull the ball up in turn. The motor is activated by, again, foil wrapped around the tracks, and when the ball gets to the top the magnet carries on on its own set of tracks, while the tracks for the ball end, the ball drops and the circuit powering the motor breaks, and the magnet falls back down to the bottom.

I would have drawn them out in paint, but the lack of a mouse kind of hinders me.

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scarecrovv
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Re: Special Switch

Postby scarecrovv » Sun Nov 15, 2009 5:46 pm UTC

TescoPeeledPlums wrote:1. You place electromagnets at intervals beneath the tracks, activated by foil wrapped around the tracks just below the electromagnets, i.e. as the ball comes up to the electromagnet, one further up is activated, and the one it is at turned off, in effect pulling it magnetically up the ramp. A railgun, in fact, just not as powerful.


Actually that would be a coil gun. A rail gun would be even simpler. You just make sure the ball is conductive, and have two parallel conductive wires for it to roll up (obviously spaced less than the radius of the ball. Then hook up the bottom ends of the wires to an appropriate voltage source capable of giving enough current to drive the ball up the wires via electromagnetic forces. You could determine the appropriate current via math (hard), simulation (easier), or just plain old experimentation (which could possibly be dangerous, or result in melted copper flowing all over the floor). This would even turn off the current automatically when the ball isn't there, because when it rolls off the top of the lifting rails, the circuit breaks, and only forms again when it returns to the bottom. Remember, when trying to build this, know what you're doing and be safe (the explanation I've given here is NOT a substitute for taking a physics course in electricity and magnetism, and thinking through your specific design very carefully).

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Re: Special Switch

Postby other » Sun Nov 15, 2009 10:05 pm UTC

squareroot1 wrote:
Even simpler method is to do away with the bucket, have the motor continually running, and fashion 'walls' to the chain with periodic 'floors'. The marble would drop onto the chain, roll down it (between the walls) until hitting a 'floor,' get carried up to the top, and drop onto the top of the coaster.

I don't know if you are required to have a bucket, but it's an idea.


More complicated than this in construction, but allowing the motor to run only as needed, would be the same "endless chain", with switches on the bottom of each compartment. If any switch is depressed, the motor runs, so it starts when the ball falls in, and stops when it falls out at the top. More switches means more fiddly construction though, and more spots things could go wrong. This variant also requires you to include a pair of moving electrical contacts, since you can't rely on a wire up to the cart at the top that then folds up as the cart comes back down.

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Goemon
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Re: Special Switch

Postby Goemon » Mon Nov 16, 2009 2:46 am UTC

Couldn't you do it more reliably with two switches?

At the start, a switch at the bottom is open and a switch at the top is closed. When the ball enters the bottom, it bumps the bottom switch, which stays closed permanently. For example, two metal plates: one is balanced so it's about to fall onto the other - the ball hits it and it topples down and remains there. Then do the same thing in reverse at the top: two metal plates that are touching, but one is balanced so it's about to tip over and fall off (permanently).

Then just connect the motor through both switches in series. The motor starts running when the bottom switch is closed (since the top switch is already closed), and will stop when the top switch is triggered.
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Velifer
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Re: Special Switch

Postby Velifer » Mon Nov 16, 2009 2:16 pm UTC

Just build an analog timer circuit with an n-type MOSFET a capacitor and a few resistors. A momentary switch would charge the cap and allow the MOSFET to conduct current to the motor. As the ball goes up, the cap drains charge into the MOSFET, keeping it open and keeping the motor running until the cap is discharged (so size the cap for the amount of time you'll need to get the ball to the top).

Here's an example:
http://www.instructables.com/id/Tap-Light-LED-Auto-Off-Hack/
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Mavrisa
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Re: Special Switch

Postby Mavrisa » Wed Nov 18, 2009 11:45 pm UTC

Okay we'll go in order:

scarecrovv: That's a good idea. The wedging of the grey bit might be a bit tricky. I'll suggest it to my group and try out a few things.

TescoPeeledPlums wrote:I'm not sure whether this is entirely within the rules of this competition, the plausibility of it and whether it would actually work are all questionable, but can't you have a steel or iron ball, something magnetic.

Yep, that's allowed.

TescoPeeledPlums wrote:1. You place electromagnets at intervals beneath the tracks, activated by foil wrapped around the tracks just below the electromagnets, i.e. as the ball comes up to the electromagnet, one further up is activated, and the one it is at turned off, in effect pulling it magnetically up the ramp. A railgun, in fact, just not as powerful.

2. You use the motor instead to pull a magnet up the track, which would pull the ball up in turn. The motor is activated by, again, foil wrapped around the tracks, and when the ball gets to the top the magnet carries on on its own set of tracks, while the tracks for the ball end, the ball drops and the circuit powering the motor breaks, and the magnet falls back down to the bottom.

Number 1 might be tricky because of the precision with which we'd have to lay down the contacts and it may be a bit unreliable, but it is a possibility. I like your second idea, but getting the magnet to fall back down on its own could be tricky. I'm thinking of a certain modification though, that might make it work.

scarecrovv wrote:A rail gun would be even simpler. You just make sure the ball is conductive, and have two parallel conductive wires for it to roll up (obviously spaced less than the radius of the ball. Then hook up the bottom ends of the wires to an appropriate voltage source capable of giving enough current to drive the ball up the wires via electromagnetic forces.

Well this sounds dangerous :P
I have to say that would be such a cool way to do it, though.

other wrote:More complicated than this in construction, but allowing the motor to run only as needed, would be the same "endless chain", with switches on the bottom of each compartment. If any switch is depressed, the motor runs, so it starts when the ball falls in, and stops when it falls out at the top. More switches means more fiddly construction though, and more spots things could go wrong. This variant also requires you to include a pair of moving electrical contacts, since you can't rely on a wire up to the cart at the top that then folds up as the cart comes back down.

Well this would work perfectly, but as you said, that would definitely be quite a complicated bit of equipment. Perhaps two rings around the line (one positive one negative) with contacts sticking out of the side of each of the carts, which are always touching the two rings. Then the ball can fall on two contacts and connect the circuit? We'll look at this possibility for sure.

Goemon wrote:At the start, a switch at the bottom is open and a switch at the top is closed. When the ball enters the bottom, it bumps the bottom switch, which stays closed permanently. For example, two metal plates: one is balanced so it's about to fall onto the other - the ball hits it and it topples down and remains there. Then do the same thing in reverse at the top: two metal plates that are touching, but one is balanced so it's about to tip over and fall off (permanently).

I may have misunderstood your idea, but the ball must run through the whole track 5 times (not sure about 5, but its 3 or more) without us intervening.

Velifer wrote:Just build an analog timer circuit with an n-type MOSFET a capacitor and a few resistors.

I'm thinking the time might vary too much for this to work. I also don't know where to get transistors or how I would calculate the drain time of the capacitor.

Anyway, thanks everyone for your input, it has been very helpful. Ideas are still welcome.
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Re: Special Switch

Postby thoughtfully » Thu Nov 19, 2009 12:27 am UTC

If you've got a magnetic object you are trying to detect the passage of, you could use a reed switch. You'd need to pass fairly close, but touching would not be required. The definition of close depending, of course, on the strength of the magnet, the particulars of the reed switch, and how fast it's moving. You will probably want some debouncing in your circuit, but the standard low pass filter with a cap to ground would be fine, I expect. At least I needed some when I used them, but my magnet was moving slowly.
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