The design uses tactile sense (or rather, neural sense) to reveal the contents of a box. The box contains 4 shapes, as in a child's shape sorter. The conjurer asks a person to secretly remove from the box any shape or shapes of his or her choosing, then to close the box, and to place a silk over it. The conjurer then rests a hand on the silk, and through "extra-sensory powers" reveals the contents of the box. The trick works as follows: each of the 4 shapes hides within it a magnet or magnets. The circuit sends a series of low-current high-voltage pulses to the surface of the box (by way of metal hinges, screws, or studs). When a shape is removed from its receptacle in the box, a reed switch (one of S1 to S4) under the receptacle opens, causing the pulse for that receptacle to drop out, so that one may identify which shapes are missing from inside the box. There is also a longer "marker" pulse (D1 and D2) so that one may identify the first shape in the sequence (the sequence repeats indefinitely). Figure 2 shows the pulses when all 4 shapes are inside the box:
The trickiest part of the trick is to learn whether the circle shape is missing or not, since its corresponding pulse is tacked on to the "marker" pulse. Figure 4 shows how the magnet is positioned in the square shape, and the reed switch beneath it. An additional magnet may be positioned in the shape in case it is inserted into its receptacle upside down.
The high-voltage pulses are imperceptible to a dry hand (eg. others handling the box), yet if the conjurer moistens a finger or fingers with the tongue, and touches the electrodes (hinges, etc.) through the silk, these pulses will be easily felt. The best result is obtained when separate fingers of the same hand touch the electrodes. Though a high voltage is present, the circuit is safe, since the current flowing through the electrodes is very low. Especially if separate fingers of the same hand are used to detect the pulses, there should be no risk to life or health whatsoever. Nevertheless,any persons using a pacemaker, or having any history of epilepsy, would be advised to avoid this circuit.
Ideally, the shapes will follow the usual conjuring sequence as follows: 1 side (circle) - 2 sides (rectangle) - 3 sides (triangle) - 4 sides (square). Since no switch should be visible, a mercury switch (S5) was used to switch off the circuit when the box was laid on its side. The circuit ought to operate for 4 hours off a small PP3 9V battery. IC1 is a slow astable multivibrator, which sequences IC2 through 6 pulses, the first 2 of which form the "marker" pulse. VR1 controls the speed of the pulses. S1 to S4 determine whether any of the pulses will drop out, by altering the potential at TR1 emitter. NAND Schmitt triggers IC3a and IC3b convert TR1's emitter potential to a logical high or low, which is used to switch IC4. IC4 is an astable multivibrator, wired to obtain a duty ratio of 10%. IC3c inverts the logic of IC4, while TR2 amplifies the output current from IC3c. Note that VR2 should initially be turned to 33K, then slowly turned back to obtain the desirable pulse strength across electrodes E1 and E2 - otherwise one may receive a small shock. A perceptible sensation is all one needs!
T1 is a 230V to 3V step-down transformer with its secondary (3V) coil in circuit. The primary coil serves as the electrodes. Note that the 555 IC's both have the CN suffix - they are a low power version of the 555, with slightly different characteristics (TS555CN). One might wish to have a visual read-out of the pulses in order to practice the trick more easily. For this, a LED may be connected temporarily across the electrodes (observing the correct polarity). In fact a high-brightness LED could be mounted behind a pinhole in the box to offer alternative visual pulses to the conjurer instead of the "neural" pulses.
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