Conjuring Trick

Over the years, I have designed several electronic conjuring tricks. The most popular was my ESP Conjuring Trick, which has gone into production in at least two versions. However, I have long had in mind a more basic version of this trick. Here it is. It informs the conjurer what is inside a box through (ow!) heat. To describe it simply, the conjurer invites someone to place two objects in a box, or one or the other or none at all -- and to close the box. The conjurer places his or her hands on the box (touching crucial studs), pauses for a while, and announces what is in the box. Here is the vital information (click on the image to enlarge). The trick is activated when two moist (licked) fingers touch both Studs A and B on the outside of the box. S1 and S2 are reed switches which are closed when items (hiding magnets) are inserted into the box. If, when the trick is activated through Studs A and B, both S1 and S2 are closed, R7 heats up after about 15 seconds. If only S1 is closed, about 20 seconds. If only S3 is closed, about 25 seconds. If neither reed switch is closed, R7 never heats up. R7 is the vital part -- a 2W 38R (38 Ohm) resistor. Since the circuit runs off 9V, R7 is slightly underrated, so heats up when TR2 conducts. R7 is closely joined to Stud C on the outside of the box, so that heat flows quickly to the stud, and this is felt by a finger. Note, however, that the characteristics of resistors vary, and the value of R3 may need to be increased or decreased to vary the heat (that is, the time that current flows through R7 may need to be varied), or another 2W resistor may need to be tried for R7 (or even 1.5W or 1W, if the value of R3 is much reduced -- R7 should not be allowed to be hot for more than a second). Some care will be needed in placing studs and in routing wires (note that hinges can be used to conduct current). OBSERVATION: The components in this design can all be replaced by equivalents, but IC1 must be a CMOS version of the 555 timer, or Studs 1 and 2 will fail to function. S3 may be a tilt switch, but it is really shown just as a reminder that the circuit needs to be switched off somehow after use. If the trick is well made, it will be inscrutable. One might think of variations, too, such as an arrow pointing any one of four directions inside the box.

NOTE: For now, I am keeping this design exclusive to this blog -- but feel free to link to it.