solenoid spectacular

Part III in The World’s Best Mechanical Engineer Explains It All for You series:

Joe-Henry,

Q: How does the driver of the max train control the doors?

A. This is probably the most complicated question you asked, so tell your Mom to get the car keys.

The pneumatic cylinder in the door is controlled by a solenoid valve. A solenoid is a magnet that you can turn on and off using electricity. This magnet moves a valve that allows air to flow to the pneumatic cylinder. A valve is something that turns flow on and off, or redirects it. When you turn on the water in the kitchen sink, you are turning on a valve.

Why don't they just put a bunch of valves like your kitchen sink in the driver’s compartment you ask? Because there are so many things for the driver control. Every car in the train has lights, brakes, doors (probably on both sides) and maybe a bunch of stuff we don't even know about. The driver would need a huge room of controls and there would be hundreds of air hoses between the cars of the train. If you look between the cars, there is probably one or two electrical cables connecting them.

Now it is time for you to make a solenoid. Go to the Radio Shack and get a spool of wire. Preferably "Motor Wire"- thin wire with thin clear insulation. If you can't get motor wire, get the smallest diameter regular wire that you can find. Sometimes radio shack has magnets. If they have any magnets shaped like small rods, buy at least one. Buy a D size battery too.

Now go to a good hobby shop. You need to get a piece of tubing that is just a little bigger around than your rod shaped magnet. Brass is ok, aluminum is ok, plastic is ok, even cardboard is ok. Don't buy a steel tube. If you couldn't get a rod shaped magnet at Radio Shack, the hobby shop should have something. The rod shaped magnet should slide easily in the tube. If all else fails you can make a tube by curling up a piece of cardboard, but it will be kind of hard to work with.

Have your Mom stop at 7-11 and pick up a six pack of Mountain Dew on the way home. This is going to take a little while.

When you get home, wrap a piece of tape around and around and around the outside of the tube at the end. Do this until the tape is about 1/4" thick. Now do the same thing again about an inch down the tube.

You have made a spool to wrap your wire around. Now wrap your wire around the spool as tightly as possible. Leave about 6" of wire outside of the spool at the beginning. Start at one end of the spool, and wind to the other. When you get to the end, bring the wire back to the beginning of the spool and start winding again. The more layers you make, the more powerful your solenoid. So wrap until you're out of wire or until your Mom starts talking about bedtime. Leave about 6" of wire hanging off the end also. Wrap a little tape around the wire so it doesn't come unwound. Scrape the insulation off of both wire ends.


When you get to the end of the spool, bring the wire back to the beginning and start winding again.


Pop open one of those Mountain Dews. Enjoy the anticipation of firing up your creation. Share one with your Mom.

You're ready. Put the magnet inside the tube. Touch one wire to one end of the battery, the other wire to the other end of the battery. One of two things will happen: either the magnet will get sucked into the tube, or the magnet will get shot out of the tube. Reverse the wires, and the opposite will happen.

This is how a valve gets shifted by a solenoid. Most pneumatic cylinders are controlled this way.

— The World’s Best Mechanical Engineer

Also in this series:
• holy hydraulics, batman!
• springs & things
• the world's best mechanical engineer explains it all for you