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Starting task

Posted: January 31st, 2010, 12:44 pm
by cypressfalls Robert
This is a discussion for the first task of dropping a golfball into the machine and triggering a mousetrap which will do the next step.

- mousetrap completes a circuit
- mousetrap pops balloon
- mousetrap turns on switch
- mousetrap turns off switch(to electromagnet)
- mousetrap lifts shade allowing light to shine on phtocell/photoresistor
- mousetrap hits lever that hits something else

This just a small list of things we thought of to use, any other ideas welcome. :)

we had a problem with our machine because of the first task. Whenever the mousetrap would go off the mass on our electromagnet would fall from the vibration. We tried using some hard sponges to absorb some of the vibrations but it doesn't work all the time, does someone have any other way to slove this problem?

Re: Starting task

Posted: January 31st, 2010, 1:24 pm
by Flavorflav
1. Make the electromagnet stronger
2. make the mass it holds smaller
3. Put the mousetrap platform on some kind of shock absorber
4. Have the mousetrap snap onto tempurpedic or something else really squishy

I don't know if any of these would be practical with your design, they are just thoughts.

Re: Starting task

Posted: January 31st, 2010, 1:35 pm
by cypressfalls Robert
how can I make the electromagnet stronger, right now im using a 9v battery---

Re: Starting task

Posted: January 31st, 2010, 1:53 pm
by Flavorflav
cypressfalls_Robert wrote:how can I make the electromagnet stronger, right now im using a 9v battery---
More wraps. I would consider switching batteries, too, or else plan to check them frequently and be sure you have backups. I don't know what kind of current it's pulling, but your battery only hold .5 amp hours or so. For about the same price you can get a 6 volt that holds 11 amp hours.

Re: Starting task

Posted: February 7th, 2010, 9:52 am
by Uncle Fester
Seriously, bag the rectangular 9v battery. I can't count the HUNDREDS of step/transfer failures that centered around one of those things. Conversely, you can start a forest fire with a D cell.

More than You Ever Wanted To Know About Electromagnets

Posted: February 7th, 2010, 11:57 am
by fleet130
It's not volts that generate the magnetic field, it's the amps. Every battery has internal resistance that limits the current it can put out.

Rule of thumb is that a 9v Transistor Radio Battery will put out about 100ma, a "D" cell will put out about 1 A (1000ma). Internal resistance (and therefore maximum current) varies widely with cell chemistry, temperature, state of charge and other factors.

Switching from alkaline to NiCad cells ups the max current to about 1 amp for the nine volt and 8 amps for the "D" cell. NiCad cells have a negative temperature coefficient, so as they heat up when current is drawn from them, they can deliver even more current. When connected to a dead short, this can lead to a condition called "thermal runaway". The usual end game for thermal runaway is fire and/or explosion. If you happen to be holding on to the wire creating the short, you can get a nasty burn in a matter of 1-2 seconds. BE CAREFUL!

Back to the Electromagnet. There are several ways to increase the strength of the magnetic field:

1. Add more turns to the coil. - The strength of an electromagnet is approximately proportional to the number of turns in the coil. Doubling the number or turns doubles the strength of the magnetic field. Of course, adding more turns increases the coil resistance, which, in turn decreases the current. The decrease in current is negligible until the coil resistance is several times the internal resistance of the battery.

2. Increase the amperage through the coil. This is simply a matter of Ohm's Law. (E = I x R or I = E/R) Current can be increases by using higher voltage or by lowering the resistance of the coil and/or battery. Increasing battery voltage has little effect if the coil resistance is less than several times the internal resistance of the battery. Internal battery resistance can usually be lowered by switching to a larger form factor cell. Size of the individual cells is what's important, not overall battery size(Those 9-volt batteries have 6 individual cells inside connected in series). Coil resistance can be lowered by using fewer turns (which will decrease the strength of the electromagnet), by reducing the diameter of the coil (this decreases the length of the wire needed for the same number of turns), by using a larger gauge wire (this lowers the resistance of the wire itself), or by using wire made from low resistance metal alloys (not a practical solution - silver is best, but cost prohibitive).

3. Wrap the turns closer together The idea is to concentrate the magnetic field into the smallest possible volume. Air/space between the wraps is your enemy! Wrap the wire in even layers with no space between turns, not scramble wound. It sometimes helps to put a layer of thin tape in between layers of the coil to keep the surface even for overlying turns. Besides requiring less wire, smaller diameter increases the concentration of the magnetic field. To get the same number of turns into a small diameter coil, the coil must be longer. Lengthening the coil makes the turns at each end further apart, decreasing the concentration of the magnetic field. As a rule of thumb, the length of the coil should be between 1 and 2 times its diameter. Long skinny coils and short fat coils produce weaker electromagnets.

4. Use a different material for the core. The material in the center of the coil (core) captures the magnetic field and concentrates it at each. Some materials are more permeable to the magnetic field and therefore work better than others. Soft Iron is a good material while steel is not. Common nails and bolts (not stainless steel, brass or aluminum) work fairly well.

One problem you may run across is residual magnetism. The core material retains it's magnetism after the current is turned off and the object being held is not released. Using a different material for the core can solve the problem, but it may be difficult to find the right material in the size/shape you need. Another solution might be to immediately apply an instantaneous pulse of current to the coil in the opposite direction when it is turned off. Exactly how this might be done if left for you to figure out.

Re: Starting task

Posted: February 15th, 2010, 7:13 pm
by Primate
Great post, fleet130. Wish it had been around when I started work on my device. Here's what I discovered: as long as you use a 6-volt lantern battery (or 4 D-cells in series) and magnet wire, there's really no way to screw this up. Sure, you can definitely play with it and make a stronger electromagnet, but basically anything you create will be more than powerful enough to do something useful.

Links: Magnet wire, latern battery