When Does the Lamp Light?

A Switch That Flips Another Switch

Name Date
DIRECTIONS: Trace each ring with a finger. Can the current get all the way round? Write lit or dark.
cellABone road, and both gates are on it
One after the other
cellABtwo roads — the current only needs one
Side by side

Fill these in by tracing

One after the other
ABlamp
offoff
offON
ONoff
ONON
Side by side
ABlamp
offoff
offON
ONoff
ONON
A single flipper
Alamp
off
ON
The names.

You just built AND, OR and NOT. They are called gates, because each one decides whether to let something through.

The One on Your Stairs

A Switch That Flips Another Switch

DIRECTIONS: Fill in the stairs-light table, then wire up the machines below.
ABone lamptwo switcheseither one always works

A stairs light: a switch at the bottom, one at the top. Touch either and the light changes. It is not AND and not OR.

ABlamp
offoff
offON
ONoff
ONON
XOR

Lit when the two switches disagree. Next sheet, it helps a machine add.

Wire these up

Which gate, and what goes into it?

  1. A microwave that will only run when the door is shut      you have pressed start.
  2. A car alarm that goes off if the door is opened      a window is broken.
  3. A fridge light that comes on when the door is      shut.
  4. The light on your stairs, with a switch at the bottom and a switch at the top.
  5. A garden light that comes on when it is dark and something is moving — but not in the daytime, however much is moving.
  6. A door that unlocks if you have the card or you know the code, but not if the building is on fire.

The big one. Are four gates enough to build any decision? Argue it out.