(Electronics) How would one build... The Mystery Box?

Factual Questions
1

I was sent a link to a page set up by a self-proclaimed “crackpot inventor,” featuring an amazing piece of furniture called “The Mystery Box.”

See for yourself.
http://www.bmumford.com/art/box.html

In a nutshell, it’s a box on a pedestal that closes whenever someone approaches, then slooowly opens when they walk away. Very nifty, but unfortunately the creator charges $1,600 for one of them.

So, I’d like to take a shot at building one by myself. Considering that my sum knowledge of electronics comes from a single college class… this is gonna be an effort. I hope that you all might be able to answer some of the questions that came up right at the start of this project.

Does this seem like something that could be built with off-the-shelf parts? Obviously, there is a motion detector and a motor to raise and lower the lid, but what about the timer to make it gradually open?

And in regards to the timer, is there some sort of… here’s where my knowledge fails… some sort of timer chip one could buy that would do this, or would one need to write a computer program to control it?

How could you keep the box tightly shut when triggered to keep people from simply prying it open, or just unplugging it?

Thanks for your input on this. It seems like a great, fun project to start off on, but I hope it’s not too complex.

2

I don’t know if it’s a motion detector or something more like sonar. With a motion detector, you could concievably trick the thing just by moving slowly enough or periodically standing still as you get closer.

It could be very easily done with off the shelf parts. If I were doing it, I would use a PIC micro or something similar to control the thing. This would require a bit of programming, but I’m a programming weenie so that’s no big deal for me. For someone not so experienced, programming it would be a much bigger effort. Something like a BASIC Stamp might help ease the programming effort required.

When the box shuts, trigger a solenoid to lock the lid. Then in order to open the box, they would have to break the lock mechanism.

So, a simple microcontroller, a driver capable of powering a solenoid, a driver capable of driving a motor (to open and shut the lid), and some sensors of some sort, and you’re in business.

Note that there are other ways of doing this, including doing it all without the use of a microcontroller (e.g. feed the sonar output into a motor driver, and have the solenoid latch if the sonar signal gets large enough).

3

By the way, do a google search on “ultrasonic” and “passive infra-red” sensors and you should be able to find quite a few things to suit your needs.

4
      • I would bet --from the dark panels of the pedestal-- that he used infa-red sensors.
        ~
5

I’d make the lid so that it clicks shut to the locked position simply by the action of the lid falling, then make it necessary for the device to withdraw a solenoid to unlock it on reopening, I’d also consider using something like a rack and pinion to open it and rather than reversing the action to close, have another solenoid that lifts the motor and pinion off the rack, causing it to shut quickly Or you could do the same sort of thing with a winch and cable, releasing a clutch to ensure a quick slam shut.

There are a number of different ways of sensing a person approaching, PIR is reasonably good, but it can be tricked - more reliable would be some kind of proximity detector, these may be ultrasonic (i.e. sonar), infra red (triangulation), electromagnetic, electrostatic etc.

There’s an exhibit in the Science Museum in London, in the kids section - a large circular table with a hole in the middle, through which is protruding the top half of a large metal sphere, try to touch the sphere and it drops down out of reach.

6

Solenoids tend to be pretty noisy, I think the effect would work best silently.

I think a small hydraulic pump running an accumulator would be smooth in operation and could be extremely quiet.

After deciding upon hydraulics as the prime mover, you could then use hydraulic rams with restricters to set the required speed of operation.

Electro/hydraulic valves are easy enough to do, this lot probably would not be too cheap, you’d have to look around junk equipment places probably.

7

Thanks for your advice, guys! The solenoid sounds like a great idea, and you’re probably exactly right about the motion detector not working - I had a cheap “burglar alarm” that used one and yep, you could fool it by creeping slowly towards it.

One thing that came up after looking closely at the pictures - how in the world is this guy lifting the lid? The only connections between the top and bottom are the hinges, and they seem to be on the outside of the box.

8

I saw a documentary on Lev Terman, inventor of the theremin, and he invented something like that for Clara Rockmore as a birthday present–only, when she approached it (whatever it was), it started turning, I believe, instead of a box that opened or closed.

9

In terms of detection, I was thinking of something that detects a change in capacitance. That’s how those touch lamps works; a circuit could be rigged up to detect a change in capacitance as you approach said circuit. The other option is an electrostatic detector. I have a very simple version of such a circuit. Its just an LED, a FET and an antenna. Walk towards it and it the LED lights up. As you back away it dims off again.