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.
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.
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.