Presentation: Binary Code Bracelets

Slide 1 of 7 · Settling starter · Computer Science · Year 7

5 minutes

Binary code bracelets

A computer stores a letter as a pattern of on and off. Today you use the same idea with beads: a dark bead is a 1 and a light bead is a 0. 5 beads per letter is enough for the whole alphabet.

While the register is called: write your name, class and date on your worksheet, then read “How the code works” and look at the first line of the practice table, letter M.

Starter question: The 5 beads are worth 16, 8, 4, 2 and 1. Why is 01101 the letter M?

DCF: Data and computational thinking → Problem-solving and modelling No computers needed · a dark and a light coloured pencil

Slide 2 of 7 · Today’s brief and key words · Computer Science · Year 7

2 minutes

Your brief: decode a bracelet, then design your own

You will practise turning letters into 5-bit codes, decode a mystery bracelet somebody else made, design your own bracelet for a 5-letter Welsh word and check it works, then answer 4 questions about how many patterns are possible.

Bit

One binary digit, a 1 or a 0. On the bracelet a dark bead is a 1 and a light bead is a 0.

Place value

What each bead is worth, left to right: 16, 8, 4, 2, 1. Add up the dark beads to get the number.

Decode

Go backwards: read the beads, add up the number, then count that far into the alphabet (A = 1, Z = 26).

DCF: Data and computational thinking → Problem-solving and modelling The working column is where the marks are

Slide 3 of 7 · Step 2: conversion practice · Computer Science · Year 7

9 minutes

Step 2: complete the conversion practice table

Fill in every empty cell. For each letter, find its number, then keep asking “does 16 fit? does 8 fit?” and write 1 or 0 for each bead. The last 2 rows go the other way: start from the code and find the letter.

Letter	Number	5-bit code	Working
M	13	01101	8 + 4 + 1 = 13, and 16 and 2 are not needed
A	1	00001	1 = 1

Conversion practice on your worksheet · 9 minutes Show the place-value working for every row

Slide 4 of 7 · Step 3: decode the mystery bracelet · Computer Science · Year 7

9 minutes

Step 3: decode the mystery bracelet

5 letters. It is somewhere you can see from a school window in north Wales. For each group of 5 beads: write the bits on the top line, then the number and the letter on the bottom line. A red divider marks the end of each letter.

Letter	Beads	Adding the dark beads	Number	Letter
1st	00101	4 + 1 = 5	5	E

Decode the mystery bracelet on your worksheet · 9 minutes Same bead pattern means the same letter

Slide 5 of 7 · Steps 4 and 5: design your bracelet, then count the patterns · Computer Science · Year 7

20 minutes

Step 4: design your own bracelet. Step 5: how many patterns?

Step 4 (13 min): choose a 5-letter Welsh word, for example CYMRU, DRAIG, CALON, GWLAD, TEULU, or a place such as CONWY or TYWYN. Work out the 5 codes with working, then colour the beads: dark for 1, light for 0. Swap with a partner and decode theirs without being told the word.

Step 5 (7 min): answer the 4 “How many patterns?” questions.

Letter	Number	5-bit code	Working
C	3	00011	2 + 1 = 3. 16, 8 and 4 are all bigger than 3, so those beads are light.

Design your own bracelet and How many patterns? · 20 minutes Your bracelet is only correct if your partner decodes the right word

Supply teacher script (no subject knowledge needed)

Say aloud: “Pick a 5-letter Welsh word from the list or one of your own. Fill in the table, one row per letter, with the working, then colour the beads in the strip. The first letter of CYMRU is on the board as a model. When you have finished, swap with a partner and decode theirs. After about 13 minutes, move on to the 4 questions about how many patterns.”

Walk the room: to check any bracelet, read the dark beads left to right as 16, 8, 4, 2, 1, add them up, and count that far into the alphabet. CYMRU is C 00011, Y 11001, M 01101, R 10010, U 10101. The full 26-letter key is at the bottom of the worked example sheet.

Quick check, how many patterns: 1. 32, because 2 × 2 × 2 × 2 × 2 = 32. 2. Largest number is 31, pattern 11111, 16 + 8 + 4 + 2 + 1 (one of the 32 patterns, 00000, stands for 0). 3. 4 beads only give 16 patterns and the alphabet needs 26, so 10 letters would have no code or 2 letters would have to share one. 4. 32 minus 26 leaves 6 spare, for a space, a full stop, a comma, a question mark or a capital-letter marker. A space is the important one: without it the bracelet is one long word.

Slide 6 of 7 · Self and partner check · Computer Science · Year 7

2 minutes

Did your partner get the word you meant?

Tick each success criterion on your sheet that your work meets. Then correct 1 thing: if your partner decoded a different word, find the letter that went wrong and fix its working.

I can turn a letter into a number and then into 5 binary digits, and show the place-value working.
I can go the other way and decode a pattern of beads back into a letter.
My own bracelet is correct: a partner decoded it and got the word I intended.
I worked out how many patterns a given number of beads can make, and used that to explain why 5 beads are needed.
I noticed that a code needs a way of telling where one letter ends and the next begins.

2 minutes · self and partner check Finished early? Try the 8-bit code at the bottom of your sheet

Supply teacher script (no subject knowledge needed)

Say aloud: “Tick the success criteria you have met. If your partner got a different word from the one you meant, find the letter that went wrong and correct it. The last criterion is the one most people miss: without the red dividers, how would anyone know where one letter stops? Real computers fix this by making every character exactly 8 bits long.”

Extension answers: 8-bit place values are 128, 64, 32, 16, 8, 4, 2, 1. Capital C is 67 = 64 + 2 + 1 = 01000011. Capital Y is 89 = 64 + 16 + 8 + 1 = 01011001. 8 bits give 256 patterns, needed for lower-case letters, the digits 0 to 9, punctuation, the space, symbols such as £ and @, and invisible control characters such as “new line”. For the 6-bit version, all light beads is a bad choice for space because it looks like nothing is there and it already stands for 0.

Slide 7 of 7 · Plenary review · Computer Science · Year 7

3 minutes

Quick-check answers before handing in

Check your main answers before your worksheets are collected.

Question	Correct answer
What is the 5-bit code for S?	S is 19, so 10011: 16 + 2 + 1.
What word was on the mystery bracelet?	ERYRI: E 00101, R 10010, Y 11001, R 10010, I 01001.
How many patterns can 5 beads make, and how many are left over after the alphabet?	32 patterns, 6 spare after the 26 letters.

Check your name and class are on your worksheet Put coloured pencils back before you leave