Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
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There are two sets of terminal blocks. One set consists of two terminals and is for the power (on the circuit board you can see 12V + and – printed). The other set is for the relays. A word of warning about these units. First, like a lot of modern electronics they are sensitive to static electricity, so if you are working in a dry climate where static is likely to built up, make sure you are not wearing nylon clothes, or working on a synthetic carpet. Touch your hand to something metal from time to time to discharge any static that has built up. Second, you really don’t want the underside of the unit to short-circuit to anything else, so before you go any further, I recommend sticking some duct tape, or insulation tape to that exposed side.
The unit runs from 12V, and this means that you will have to use a different battery to power it from the AA batteries used for heating the muscle wire. The best thing to use are small 12V batteries with the generic name A23. If you want, you can get a battery holder for this or you can just do what I did, which is solder directly to the terminals. Because these batteries are small they do not hold a lot of charge and by my calculations, they will go flat after about 11 hours of continuous use (not including the times when the relay is on, which will take a lot more current). So we need an on off switch. Rather than have a switch somewhere on the outside of the frame I decided to use a tilt switch inside it. In the old days switches like this contained mercury and they are sometimes known as mercury-replacement switches. The way they work is that they are on when upright, and off when upside down. You will also need a holder for the 2AA batteries.
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
Construction Remove the backing hardboard that comes with the frame, and take off any flaps used for standing the frame upright (e.g. on a table). Better still, replace the hardboard altogether with a piece of 6mm plywood, which is much sturdier, although it affects the weight calculations. If you do this you’ll cut the plywood to the right size to fit snugly into the frame. From now on I’ll refer to this piece as the back.
Originally (i.e. when you buy the frame) the back fits up against the picture (so that there is a space behind the picture and back, but we’ll have the back flush with the back edges of the frame, so that the space is now hidden between the picture and back. Cut four pieces of timber approx 2cm wide to go round the outer edge of the back. You’re going to be gluing them to the back. When the glue is set, the back, together with these four pieces, will fit snugly into the frame and create a hidden cavity. You’ll have to calculate the thickness of these edge pieces so that the back will not protrude when in place. Now measure across the width of the back to find the mid point, and drill a small hole about 2cm from the top. This is where the hook will protrude from the picture. Now we will make the lever. This is simply a piece of timber, about 10cm x 2cm x 1cm. At one end, drill a 6mm diameter hole, 6mm deep, in the thin edge of the lever. Glue one of the magnets into the hole (superglue is fine). At the other end, about 5mm from the end, drill a small hole – this is where the hook will go. Place the back flat on your work surface and line the hole in the lever with the hole you drilled in the back, so that the lever rests horizontally across the back and with the magnet facing towards the top timber edge. On the back, mark the position of the magnet.
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
Now you need to attach the other magnet to the top timber edge, so that when the lever is horizontal the two magnets line up with each other. You will need to glue a piece of scrap timber to the top edge and glue the stationary magnet to that – it’s easier to look at the picture than for me to describe it! See figure 4 (which shows the lever in the position it will be in when activated).
Figure 4
Make sure you glue the magnets so that they attract rather than repel each other. So now, we have a lever with two ends – at one end is the magnet which matches the stationary magnet on the frame’s edge. At the other end, there is a hole where the hook will go. What we need now is to drill a hole for the fulcrum. Positioning the fulcrum is crucial, and although to some extent you can calculate where it should go, in the end you may have to use trial and error. If you look at my pictures (Figure 6) you will see where I drilled two wrong holes in the back before finding the right place.
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
However it’s probably worth a detour into a bit of mechanics so that you can understand what’s going on. (Don’t worry it you don’t understand this – you can just skip down a few paragraphs.) There are three forces in operation, as shown in figure 5:
Figure 5
1) The force which the hook on the wall exerts on the lever. This is an upward force and is equal to the weight of the picture. We’ll call this force a. 2) The force which the magnets exert on the lever – what we’ll call force b. It’s also upwards 3) The force which the muscle wire will exert on the lever when it’s actuated. We’ll call it force c and it’s downwards. The effect of a force on a lever depends on its distance from the fulcrum – this is known as moment, and is the force multiplied by the distance from the fulcrum.
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
In theory, you can work out where the fulcrum should be through maths: If a is the weight of the picture (about 1kg in this case) b is the separation force of the magnets (about 850g) c is the force exerted by the wire (about 120g) and: x is the distance from the fulcrum to the hook y is the distance from the fulcrum to the magnet Then for the picture to stay in place when unactivated, b y > a x (i.e. the moment of the magnets must be greater than the moment of the weight) and for it to fall when the wire is activated a x > (b-c) y (i.e. the moment of the weight must be greater than the moment of the magnets minus the moment of the force exerted by the wire.) These two equations can be expressed in terms of the ratio of y to x as follows:
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
a/(b-c) > y/x> a/b
You can measure a, b, and c using weights (e.g. work out what weight is needed to separate the magnets), and then use the equation to work out a possible ratio of y to x (and x + y is of course the length of the lever). This is all fine in theory but doesn’t take into account the friction in the system, which tends to hold the picture in place. This means that you will have to use some trial and error. Basically with the components I’ve specified you’ll find that the fulcrum should be almost in the centre but slightly towards the hook. If the picture doesn’t hold in the “unactivated” mode, then you need the fulcrum nearer the hook. If it doesn’t fall in the “activated” mode then you need the fulcrum nearer the magnets. When you have made your best estimate as to where the fulcrum is going, drill a hole through the lever and the back, and secure the lever in place with a bolt and nut. You should also put a washer between the lever and back so that the lever can move without rubbing on the back. There mustn’t be too much play in the lever or the magnets will move relative to each other, which will compromise their holding power. The hook which goes through the back will need to swing in an arc when the lever moves, and so you will need to cut a curved slot for this through the back. You can trace the arc by putting a small tack through the hook hole and swinging the lever so that the tack scratches the path that you’ll need to cut. Cut it by drilling a series of holes along the path, which you can then file to a slot; or use a coping saw if you have one. Get this slot smooth so that the hook doesn’t catch as it moves. See figure 6, which also shows the hook and the fulcrum holes I originally drilled in the wrong place.
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
Figure 6 – the back of the unit
Now for the Flexinol wire. Refer back to Figure 2 showing the inside of the mechanism. Cut about 12 cm of the Flexinol – the exact length is determined by the distance from the lever to the bottom timber edge, plus a bit more to make the electrical connection. It will need to be secured at both ends, both electrically and mechanically. You can’t solder this wire without potentially damaging its properties, so you need another method, and the one we use here is crimping. Crimps are electrical connectors that accept a wire into a hollow metal tube, which is then squashed flat so that the wire is trapped inside. The crimp connectors used in this case have a ring on them. Fold the end of the Flexinol back on itself and insert it into the tube. Now insert a piece of ordinary thin construction wire into the tube as well. Squash the tube flat either using a crimping tool, or a pair of pliers, or a mole grip. The Flexinol wire is very thin so you need to exert quite a bit of force to make sure it’s trapped in the tube. Do this for both ends. You should now have about 10 cm of Flexinol with a metal ring at both ends and a piece of wire running from both ends too. Page 12
Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
To test that you’ve got a good electrical connection, connect the two free ends of the wires to a pair of AA batteries. The Flexinol wire should visibly contract (this is real magic!) while the current flows. The Flexinol needs to be secured at one end to the bottom timber edge, and at the other to the lever, near the magnet. It’s important to get the tension in the Flexinol right, and it will need some fine tuning. The best way to do this is to have one end adjustable. You can do this by having one of the crimp rings loop over a small screw hook that can be screwed up or down. The other Flexinol crimp ring can be firmly anchored to the end of the lever with a small screw, or a tack, or a staple. When the lever is in the off position, the Flexinol should have no slack in it. (Don’t worry if the Flexinol doesn’t yet separate the magnets when it’s activated – it’s not strong enough to do this on its own: it needs the help of gravity, which only operates when the unit is hanging on the hook). Now we need to connect up the remote control unit. This has its own power supply – the 12V 23A battery – and is turned on and off with the tilt switch. The switch can be soldered directly onto the battery, or the battery holder if you are using one. The power input to the receiver is clearly marked on the underneath of the circuit board as 12V + and -. It’s important not to connect it the wrong way round, or you run a real risk of destroying it. When you press the A button on the key fob you should hear the relay click. Secure the receiver unit to the frame, with glue, double sided tape, or screws. There are two relays on the unit (because there are two channels), but we are only using one: the terminals you need are marked COM1 and NO1 (NO = “normally open). The circuit goes from the AA batteries to the one end of the Flexinol, to the other end of the Flexinol, to COM1, to NO1, and back to the battery pack. The battery pack also
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Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
needs securing. Since the picture will be falling and there will be jolts, I recommend you secure everything firmly once you know it’s all working. With the magnets disengaged, press the key fob button A and you should see the wire contract again. Finally, the hook. We use a miniature cup hook, bent through 90 degrees (easy to do with pliers) and screwed through the curved slot that you cut into the back of the lever. If you want the picture to fall to the ground, then you need to use a straight hook rather than a curved one (as in Figure 6). If you want it to just shift on the wall, but remain up, use a curved hook, which will remain on the wall hook. Figure 7 shows this.
Figure 7
When you are sure everything is working, I recommend also gluing it in place with epoxy so that it can’t unscrew itself. Warning: if you allow the picture to fall to the ground without anything to cushion its fall, it’s very likely to be damaged. Another option is to have the picture also secured to the wall hook by a slack piece of string, so that it can fall some way but not actually hit the ground. If you are going to do this, make sure the string is very firmly attached to the frame, because there are some considerable jolts in play and little screw hooks or tacks can easily be ripped out when string becomes taut – and the string can even be broken. The same Page 14
Magi-tronics Book01-02 Automatic Falling Wall Frame by Jol yon Jenkins
goes for the wall hook – it needs to be firmly anchored if it’s to hold a picture that has already started falling. Put everything back together, with a suitable picture in the frame. For the moment, just tape the back assembly to the frame with duct tape, because you will probably need to make a few adjustments before you’re done. Assuming that the electronics is working OK, the two variables are the tension in the Flexinol, and the position of the fulcrum. And make sure the lever can move freely but without being wobbly. When everything is working OK you can secure the back with tacks. Operating the unit: you can either use an assistant to press the button, or else do it yourself secretly. It’s possible to open up the key fob and attach a toe switch to the relevant terminals, but the key fob is actually small enough to fit inside your shoe on its own, unless you have very small feet. Cut out an insole from some thin durable plastic, for example a plastic bottle, or a foam foot cushion sold in many pharmacy or shoe stores, and glue the key fob to the insole in such a place that you can easily press button A with your toe. I will be working on another version of the frame in a future e-Book. The new frame will be a Desk Frame and it will topple over onto a desk or table on command. © 2011, www.magicnook.com and Jolyon Jenkins All manufacturing rights reserved. The purchase of this e-book entitles you to construct one item for your own personal use. If you are interested in acquiring a manufacturing licence please contact the author via the Magic Nook (
[email protected] ). Copying and resale of this e-book is prohibited by International Copyright law. Jolyon Jenkins is a journalist who published his first article in “Everyday Electronics” at the age of 16 and went on to write for a number of electronics magazines before drifting into a job as a radio producer and presenter. As an amateur magician, he performs as Montague The Mysterious, and has made a number of radio documentaries about magic including one about the centenary of the Magic Circle, and another about magic busking. He has two children who are bored with all his tricks but who are occasionally infuriated by new ones.
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