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Technologies K–10 · Years 5–6

Text as numbers: writing secret messages in ASCII

Digital Technologies (NSW Science and Technology K–6, 2017); Digital Technologies: Knowledge and understanding, Data representation (ACARA v9)

Practical, model not builtLow risk

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The idea

Digital systems store every character as a number, so the word HELLO is stored as 72 69 76 76 79 and each of those numbers as 8 binary digits.

What you need

  • Printed ASCII chart for codes 32 to 126 (space, punctuation, digits, uppercase and lowercase letters)
  • 8 binary cards (1 to 128) per pair
  • Computer or tablet with a spreadsheet (CODE and CHAR functions)
  • Recording sheet

How to do it

  1. Find H, E, L and O on the ASCII chart and write the code for each letter of HELLO.
  2. Turn each code into 8 binary digits using the binary cards.
  3. In a spreadsheet, type =CODE("H") and =CHAR(72) to check the chart in both directions.
  4. Compare the codes for A and a, and for B and b. Describe the pattern.
  5. Write a short message as codes for a partner to decode.
  6. Work out how many bits a 10-letter message needs.

What you should see

HELLO is 72 69 76 76 79, stored as 01001000 01000101 01001100 01001100 01001111. A is 65 and a is 97: every lowercase letter is 32 more than its uppercase letter, a difference of a single binary digit. Each character here takes 8 bits, so a 10-letter message needs 80 bits. The learner knows it worked when the partner decodes the message exactly and the spreadsheet CODE and CHAR results match the chart.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Computers store letters as tiny pictures of letters (they store numbers).
  • Uppercase and lowercase letters have the same code (they differ by 32).
  • A space takes no storage (a space is character 32 and takes 8 bits like any letter).

Safety card

Low riskLearners carry it out

Hazards

  • None beyond normal classroom activity

Controls

  • Not applicable

Note

No chemicals or heat.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Science and Technology K–6 Syllabus (2017), NESA. The syllabus taught in 2026; the 2024 syllabus replaces it from 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.ST3-11DI-T
  • Science and Technology K–6 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.ST3-DDT-02
  • Australian Curriculum v9AC9TDI6K03AC9TDI6K04

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.digitaltechnologieshub.edu.au/teach-and-assess/classroom-resources/lesson-ideas/keeping-secrets
  4. www.digitaltechnologieshub.edu.au/teach-and-assess/classroom-resources/topics/binary
  5. classic.csunplugged.org/activities/binary-numbers

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