ABOUT ROTARY SWITCHES
A rotor attached to a shaft
connects with contacts around it,
one at a time, when the shaft turns.
The rotor is the pole of the switch.
If there are 6 contacts, the switch
has 6 positions. Switches are usually available with 2 to 12 contacts,
although some have more.
Often a second set of contacts
is arrayed opposite the first, and a
second rotor makes contact with
them. This type of switch has 2 poles
Rotary switches aren’t
used as often as they once
were. Introductory books
often don’t explain how
to use them, so here are
a few fundamentals.
on one deck, and you
have to check the manufacturer’s data sheet (or use your
multimeter) to figure out how the
solder tabs on the outside connect
with the contacts inside.
Many rotary switches have multiple
poles and/or multiple decks. If you
can’t find a switch with the number
of positions you want, you can limit
the positions with a movable pin or
lug. So, an 8-position switch can be
set to allow only 7 positions.
If a rotor makes a connection with
the next contact a moment before
breaking the connection with the
previous contact, this is known as
a shorting switch. In a non-shorting
switch, there’s a tiny gap between
one connection and the next.
Rotary switches have been
displaced by cheaper, smaller
rotational encoders in many applications,
especially in audio equipment, where
a knob turns freely to control volume
or choose modes. The encoder contains 2 pairs of contacts that open
and close slightly out of sync. You
need a microcontroller to interpret
the pulses and figure out which way
the knob is turning.
In hobby electronics, we may
not want the hassle of programming
a microcontroller, and a plain old
rotary switch will work fine. Just
make sure you choose one that’s
capable of switching the voltage and
current in your circuit, so you don’t
degrade the contacts by allowing
sparks to occur.
position, and you reach 792 as the number
of combinations. If only one of them wins the
game, the chances of someone hitting it are
1 in 792. Personally, I’d be reluctant to play a
game with such bad odds. If I was running my
own little casino, I might prefer a game with,
say, 2 coins among 5 slots, giving odds of 1 in
10. Taking in 50 cents a game, I could give a
$4 prize and still make a 20% profit.
Getting back to the triangle — you’ll have
noticed that the number 12 begins the
bottom row. What if you want to figure the
odds in a game with more than 12 slots? No
problem! Each new row is created by using a
very simple rule. Every number in the triangle
is established by adding together the pair of
numbers above it. So, you can extend the
triangle downward as far as you want.
Now you can design your own game with
any number of slots and coins, to create
any level of difficulty. Your only additional
problem is how to add a rotary switch to set
your secret winning combinations. Your exact
schematic will depend on your slot/coin configuration. I’m betting you can figure that out.
I have just one more suggestion for an
enhancement. How about changing it to a
2-player game, using on-off toggle switches
160 Make: makezine.com/29
instead of coins? This would suit people who
don’t approve of gambling. And to make it
more interesting, you could revise the rules so
that the 2 players take turns to flip switches,
continuing for an unlimited number of turns
until one person finally wins by hitting the
This really requires a new secret combination for every game. In fact, your game should
create its own combination randomly from
all those that are possible. A microcontroller
would be the way to do this. The switches
would be its inputs, and it would scan them
until it “sees” the winning combination and
activates an LED or a beeper. If you’ve read
my previous columns in which I used microcontrollers to choose random numbers, you
should be able to create this circuit. You’ll also
find an introduction to microcontrollers in my
book Make: Electronics. ;
Make: Electronics book at the Maker Shed:
Charles Platt is the author of Make: Electronics, an introductory guide for all ages. A contributing editor of MAKE, he
designs and builds medical equipment prototypes in Arizona.