原创 Can you solve this 49-year-old conundrum?

I happened to mention one project that I created when I was 10 years old. This project made my dad so proud that he showed it to all his friends. When Max heard about it, he asked me to write it up to share it with other members of the EETimes community, so here we go...

This all happened back in 1965. Perhaps the best way to start is to describe what my project did and how it appeared to outside observers. First, there was a small cardboard box with holes in it that looked like a microphone. This was mounted on the wall next to the closed door of my bedroom. When I walked up to this box and said "open" into it, the door quickly opened and I stepped into the room. I then turned around and faced the people outside the room as the door closed slowly behind me.

Now, given that this was 49 years ago when there were no microcontrollers and such, and given the simple and inexpensive components that were available to the average 10 year old at that time, can you figure out how this was achieved? When you've pondered this for a while, continue to the next page for a detailed explanation.

Take a look at the drawing below. The system involved nails, screw-eyes, both heavy and thin rubber bands, string, thumb tacks, paperclips, wire, paper, cardboard, D-size batteries, a permanent magnet, and tape.

 

Opening the door
For the part that opens the door, heavy rubber bands were strung together with one end tied to a paper clip and the other end tied to the doorknob inside the room.

The bedroom had an interior wall that was against the hinged side of the door and also perpendicular to it, and this is where a headless nail went. This nail was placed into the wall near the floor at an angle that pointed up to where the knob would be when the door was fully open.

The paperclip at the end of the heavy rubber bands was slipped over this nail. With the door closed, the paperclip griped the nail due to the steep angle of the bands. As the door opened, however, the angle of the bands increased to the point where the paperclip slipped off the nail, thereby stopping the bands from pulling.

Closing the door
The next step was to make the door close. At the top of the door was another string of very thin rubber bands. Using thumb tacks, one end of this string was attached to the top of the door, the other end was attached to the door jam. This band was much weaker than the opening-band, thereby giving the door a much slower closing speed.

Latching the door
Now onto the door latch. Two screw eyes were installed adjacent to the door striker plate. These held a nail that was used to keep the door closed. Having a string tied around its head, the nail slid inside the screw eyes such that when the string was tugged, the nail moved so as to release the door.

The other end of the string was tied to a permanent magnet, which served to tug the string whenever the magnet fell. In order to get the magnet to fall when I wanted it to, I needed it to stick to something and then release as required.

The solution was an electromagnet, which I constructed using wire wrapped around a nail. In the absence of any current flowing through the electromagnet's coil, the magnet stuck to the nail inside the electromagnet. The electromagnet's orientation was such that, when it was activated, its north pole faced the north pole of the permanent magnet.

Thus, activating the electromagnet repelled the permanent magnet causing it to fall, thereby pulling the nail out of the two screw eyes. Once the nail had been pulled out of the two screw eyes, the heavy rubber bands would cause the door to open as described earlier.

The master control switch
Powering the electromagnet was two D batteries taped in series, with wires also taped on to the ends. To finish out the design, there was a rather clever switch to turn it all on. This switch was made out of cardboard, fine wire, a thumb tack, a paperclip, some paper, and some tape.

A very fine wire was made by stripping down one strand of a bundle, which was then connected to a paperclip and taped to a small piece of paper, thereby forming one contact of a switch. The use of fine wire was required to keep the weight of the wire low and to keep the paper flexible.

The other contact of the switch was just a thumb tack, which was pushed into the wall where the paperclip could bang up against it. A piece of cardboard was used as a spacer to hold the paper away from the wall, thereby providing an air gap for the open contacts.

To finish the switch, a cardboard box with air holes was placed over it for looks. The switch worked because the letter 'P' in the word "Open" provided a puff of air that blew the paper such that the paperclip and thumb tack contacted each other.

So there you have it. Saying "Open" into the box caused the contacts to close, which caused the permanent magnet to fall, which pulled the string, which slid the nail out of the screw eyes to release the door. At this point the heavy rubber bands caused the door to open quickly. When the door was fully open, the heavy bands were released, thereby allowing the thin bands to slowly pull the door closed.

The final challenge
One pretty bad operating gotcha to all this is that my brother would have to be inside the room to get it set up for the demo. I never got around to designing a way to get it set up from outside the room, partly because my dad didn't want me to mess the room up too much and partly because it would have gotten way more complicated.

Would you care to take up the challenge? Remembering that I was only 10 at the time, and keeping in mind the limitations in technology available to me, do you have any ideas as to how I could have arranged things so this system could be set up from outside the room?

Ivan Cowie

Chief Engineer

MaxVision