Take me to the Steps
dishonestfortuneteller-2jb
Photo by Hep Svadja
m55_cover_hirez
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Probably you’ve seen those little “executive decision makers” that illuminate an LED to assist you in making a yes-or-no choice. Personally, I don’t want to reach decisions on a random basis — but what if the output could be faked? That suggests some interesting possibilities.

Suppose your friends want to eat at a cheap diner where the greasy, spicy food gives you heartburn. “I suppose we could go there,” you say, “but let’s see what my decision maker thinks.” You pull out your handy gadget, slide the secret switch to the “no” option, press the button, and it gives you exactly the output that you wanted.

A toy of this type has never been marketed, so far as I can tell. But now, with a handful of parts, you can make your own. I call it the Dishonest Decider.

Basic Principle

Figure A. Block diagram showing the principle of the thing. Diagrams by Charles Platt
Figure A. Block diagram showing the principle of the thing. Diagrams by Charles Platt

In Figure A, a 7555 timer (using less power than a 555) is wired in bistable mode. When the Run button is pressed, the timer’s Reset pin is pulled low, forcing its output low, which grounds three LEDs representing Yes, Maybe, and No. The LEDs are flashed in sequence by a decade counter driven by a free-running 7555 at the bottom of the circuit. The LEDs keep blinking so long as you hold down the Run button.

Now for the interesting part. When you release the Run button, the LEDs stop flashing — but not immediately. The Disable pin of the counter is controlled by the output from an AND gate. When the Run button is released, a pull-up resistor makes one input to the AND gate go high. The other input only goes high depending on the position of a concealed rotary switch, which has selected the LED that you want. The counter only stops when that LED is lit.

To use the Dishonest Decider, you secretly set the switch, then press and release the Run button — or let someone else press and release it. Either way, the Decider doesn’t stop until your choice is selected.

A Reset button pulls the input of the bistable timer low, so its output goes high. Because the LEDs are grounded into this output, it stops them from glowing. You can now readjust the position of the rotary switch, if you wish, while the LEDs remain dark. Note that the circuit still uses power in this mode, and a separate on-off switch is necessary to avoid draining the battery.

What if someone suspects that the Decider is — well, somewhat dishonest? No problem! Simply turn the rotary switch to its fourth position, which holds the left-hand input of the AND gate constantly high. Now the counter will stop as soon as the Run button is released, and anyone can test the Decider repeatedly, because its output is genuinely random in this mode.

Figure B. Schematic layout suitable for breadboarding.
Figure B. Schematic layout suitable for breadboarding.

A schematic is shown in Figure B. This includes everything except the power supply, which can be a 5V AC/DC adapter or a 9-volt battery with an LM7805 voltage regulator.

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Project Steps

The Desing

Figure C
Figure C. Step shots by Charles Platt

To hide the rotary switch while allowing you to turn it surreptitiously, I mounted it inside a 3″ PVC coupling — a piece of plumbing that you should find in any large hardware store. The Dishonest Decider is shown completed in Figure C, with its base visible in Figure D. When you rotate the base, this turns the internal switch. Three wooden feet make the base easier to grip.

Figure D
Figure D

Tracing the top

Figure E
Figure E

Fabricating the Decider must be done accurately. First check the internal diameter of your coupling, as in Figure E. Punch 2 small holes spaced half of this distance apart in a piece of cardboard, and use it to draw a circle on a piece of ¼” plywood, as in Figure F. To cut around the circle, the cheapest tool is a coping saw, as shown in Figure G.

Figure F
Figure F
Figure G
Figure G

The Knob

At this point I can’t resist mentioning my new book, Make: Tools. This is a comprehensive guide to workshop tools, with a lot of information about saws and plywood — and plastics, too. It’s especially useful when you want to build enclosures for electronics projects.

The Dishonest Decider needs a 1-pole, 4-position rotary switch, but you can use a 2-pole or 3-pole switch and ignore the extra poles and contacts. You also need a push-on knob that matches the switch. I found these parts cheaply by searching for “rotary switch” on eBay.

Figure H
Figure I

A typical plastic knob is shown in Figure H. Drill 2 holes in it, as in Figure I, then invert the knob and drive 2 screws up into it through matching holes in the wooden disc, as in Figure J. The holes in the disc must be slightly larger than the thread diameter of the screws, while the holes in the knob must be smaller, so the screws will bite into it. The knob must be precisely centered.

Figure J
Figure J

Make the feet

Figure K
Figure K

You can make feet for the Decider by cutting slices of 5/8″ round dowel, applying epoxy, and clamping them as in Figure K. It’s easiest to round the end of the dowel with sandpaper before you cut a slice.

Mount electronics

Figure L
Figure L

Your rotary switch is mounted in a second disc of wood with a hole in the center, as in Figure L. Switch threads are not usually long enough for material ¼” thick, so you’ll probably have to glue it into place.

Figure M
Figure M

Figure M shows the pieces I’ve described so far, with a battery secured to the center piece using double-sided velcro tape. On the left, the electronic components are mounted on perforated board glued to a semicircle of plywood.

Figure N
Figure N

Figure N is a cross-section showing how the parts of the Decider are assembled, and Figure O shows them seen from above, before I added the top panel.

figure-o

I suggest you start by breadboarding the circuit, using the layout in Figure B, and read about the pinouts of the chips in datasheets online, to help with debugging the circuit if it doesn’t work initially. Note that the actual part number of the 74HC4017 may be something like CD74HC4017E, but so long as the sequence 74HC4017 is in there, the chip will do what you want. (You’ll find more information about this chip in my book Make: More Electronics.)

Figure P
Figure P

To make a permanent version of the circuit, small enough to fit the PVC tube, I soldered the parts with point-to-point wiring. The top side and underside of the board are shown in Figure P. Note that the underside is shown flipped left-to-right. You may want to use a hacksaw to cut the board, as its glass-fiber content can blunt saws made for wood.

I put a couple of headers on the board so that I could plug pieces of ribbon cable into it, but you can solder the wires connecting the LEDs, the pushbuttons, and the rotary switch directly into the circuit if you prefer.

Because the LEDs light up one-at-a-time, a single series resistor is sufficient to protect all 3 of them. This is the 330Ω resistor shown in Figures B and P. However, if you use 3 colors of LEDs, each requires a different forward current for optimal performance. To get around this, use LEDs with their own series resistors built in, such as the Chicago Miniature series rated for 5VDC. Now you can ground the LEDs directly, eliminating the 330Ω series resistor, and all 3 will be equal in brightness.

USING AND ABUSING IT

When you have assembled your Dishonest Decider, the base should turn freely, so long as the switch and the knob are properly centered. With some practice, you can manipulate it without anyone noticing.

There’s an obvious temptation, here: to use the Decider for rigged gambling purposes. This bothers me, not just because fraud is illegal, but because I don’t want to be responsible if you get beaten up when someone figures out that you’ve been cheating.

I prefer harmless applications. For instance, use the Decider as a magic trick. In your hands, it always lights the green LED, but when you give it to someone else (after quietly turning the base), it always lights red. Take it back, and it goes back to green. Very mysterious!

You can also use it for fortunetelling. After all, fortunetellers traditionally tell people what they want to hear. The same is true in business, where an insecure manager may want independent validation to reassure him that he’s doing the right thing.

In situations like these, the Dishonest Decider can keep everyone happy — so long as no one guesses its clever little secret.