# Switch Exercise

## Objective

To gain experience building circuits which use switches to compute logical expressions.

## Cat Selector Circuit

In Code, Petzold designs a circuit to decide if a cat is acceptable according to a logical expression. This is used to illustrate the link between logic and circuits. His circuit is given below:

Below is a Logisim version of the same circuit:

This circuit uses transistors controlled by pins instead of switches which would need to be opened and closed manually. A pin is an input or output of a circuit. Whether the cat is male or female is external to the circuit, so is controlled by a pin.

When the value of (M & N & (W | T)) | (F & N & ~W) | B is true, then the LED will light up. Notice that the ~W is handled with a P-type transistor which closes when a 0 is coming in. The other transistors are all N-type which close on a 1.

## Exercise: Rock, Paper, Scissors

For this lab, you'll build a circuit along similar lines which will compute who wins a game of rock, paper, scissors. In this game, two players throw hand signals representing rock, paper, and scissors. The moves are compared and a winner is decided according to the following rules:

• Rock beats scissors.
• Paper beats rock.
• Scissors beats paper.
• If the same signals are used, the game is a tie.

Your circuit should determine a winner based on the moves of each player.

You should build the circuit out of the following components:

• Pins

You should use 6 pins (found under the wiring menu). Three pins are for player 1, and three are for player 2. Each player has a rock, paper, and scissors pin.

You should change them from three-state pins to two-state pins in the properties menu. Three-state pins can also be "turned off" where they do not represent a 0 or a 1, but that's not needed for this circuit.

You should also label each pin, so it's clear what it does!

• Transistors

For each switch, you'll need a transistor connected to the appropriate pin. Be sure to select the right type between P-type and N-type.

• Power

Each transistor needs a power source on the collector (left side). Whether they each have one, or share one doesn't matter.

• LEDs

You should have two LEDs: one which represents player 1 winning, and another which represents player 2 winning. A tie game would be indicated with both LEDs off. Under no conditions should both LEDs be on!

## Testing

Be sure to test that your circuit lights the appropriate LEDs for each possibility. You should test the case where the players throw the same moves (resulting in a tie), and when no move is thrown.

You don't need to test the case where multiple throws are selected (e.g. if the pins for player 1 throwing rock and paper are both on).

## Submitting

When your circuit works, email the .circ file to ifinlay@umw.edu.