Englishالعربيةעברית

Phase 3 — Excel Brain

Build a tiny AI inside Excel. Watch it learn from pictures. Reach 90% on the test.

About 60 minutes · Excel · only formulas, no code

Starter file (Excel)Finished exampleNext: Phase 4 — Detective Map

🎬 Watch the video first

A silent video with captions builds the whole brain step by step. Pause, rewind, or jump to a chapter:

🚀 Your mission: earn 5 stars

Answer the quick checks in the guide. Each correct answer lights a star.

⭐⭐⭐⭐⭐ 0/5

What you will build

  1. A picture of 5×5 squares (pixels). A dark square is 1, an empty square is 0.
  2. A “brain” made of 25 weights (one per square) and a bias. The brain adds up weight × pixel and gets a score.
  3. After every picture the brain checks its answer and nudges the weights. That nudging is learning.
  4. At the end the brain is tested on pictures it has never seen. Goal: 90% or more.

Before you start

  1. Excel on a computer (Excel for the web also works).
  2. Download the Starter file at the top of this page and open it.
  3. Yellow cells = you type. Blue cells = formulas (you type them, or they are already there).

0The idea in one minute▶ Watch this part 0:07

An AI does not “see” an apple. It sees numbers. Our brain gives every square a weight. Squares that appear in apples get a positive weight. Squares that appear in bananas get a negative weight.

Score = bias + (weight 1 × pixel 1) + (weight 2 × pixel 2) + … + (weight 25 × pixel 25). A big score means Apple, a small score means Banana. The probability is 1/(1+EXP(-score)): a score of 0 is 50%.

Real AIs like Teachable Machine do exactly this — with millions of weights instead of 25.

1Meet the workbook▶ Watch this part 0:39

The workbook has six sheets (tabs at the bottom):

SheetWhat it is for
Pictures12 training pictures + 4 free slots for your own
Learnthe engine room: one row = the brain looks at one picture and corrects itself
Brainthe dashboard: learning rate, charts, what the brain looks at
Test10 pictures the brain has never seen
Drawdraw your own picture and ask the brain
Datahelper sheet (do not type here)
Colours: yellow = you type here, blue = a formula.

2Look at the training pictures▶ Watch this part 0:39

  1. Open the Pictures sheet. Each picture has a title (Picture 1 …) and an answer in the top corner: 1 = Apple, 0 = Banana.
  2. Look at the apples. Which squares are dark in all apples? Look at the bananas. Which squares are dark in bananas but never in apples?
  3. Slots 13–16 are empty. You will use them at the end.
The Pictures sheet: 12 training pictures (answer 1 = Apple, 0 = Banana).
The Pictures sheet: 12 training pictures (answer 1 = Apple, 0 = Banana).
Do not change the first 12 pictures yet.
✔ You can point to 3 squares an apple uses and a banana does not.
🎯 Quick check

In the Pictures sheet, what does a 1 in a square mean?

3Teach the brain: type 5 formulas▶ Watch this part 0:52

Open the Learn sheet. Row 7 is the starting brain (all zeros). In row 8 the brain looks at picture 1. Click each cell and type (or click a formula to copy it):

Tip: click a formula below to copy it, then paste it into the cell.

CellFormulaMeaning
F8=L7+SUMPRODUCT(M8:AK8,AL7:BJ7)bias + sum of weight × pixel
G8=1/(1+EXP(-F8))probability that it is an Apple
K8=E8-G8answer − probability (how wrong)
L8=L7+$E$3*K8new bias = old bias + learning rate × error
AL8=AL7+$E$3*$K8*M8new weight 1 = old weight + learning rate × error × pixel
The Learn sheet before you type: blue cells are empty.
The Learn sheet before you type: blue cells are empty.
  1. Type the formula in F8, G8, K8 and L8. Press Enter after each.
  2. Type the weight formula in AL8. Then select AL8, grab the fill handle and drag right to BJ8 (25 weights).
  3. Now select F8 and drag across to BJ199… or easier: click F8, hold Shift, click BJ199, then press Ctrl+D (Fill Down).
The weights (w1-w25) change a little after each picture.
The weights (w1-w25) change a little after each picture.
After the formulas are filled down: numbers everywhere.
After the formulas are filled down: numbers everywhere.
Look at column I (Mistake?): 1 = the brain was wrong on this picture. Early rows have mistakes. Later rows have none: the brain has learned.
✔ Column F shows numbers down to row 199 and the Brain sheet is no longer empty.
🎯 Quick check

What does the brain do after a wrong answer?

4Watch it learn▶ Watch this part 2:17

  1. Open the Brain sheet. The two charts show mistakes in each round and loss (how wrong the brain is). Both should go down.
  2. Change the learning rate (yellow cell C4) to 0.05, then 0.5, then 5. Look at the charts each time.
  3. Which learning rate reaches 0 mistakes first? Which one is slowest?
Brain sheet, learning rate 0.5: mistakes fall to 0, loss falls, the weight map shows red (apple clues) and yellow (banana clues).
Brain sheet, learning rate 0.5: mistakes fall to 0, loss falls, the weight map shows red (apple clues) and yellow (banana clues).
Learning rate 0.05: slower.
Learning rate 0.05: slower.
Learning rate 5: jumpy start.
Learning rate 5: jumpy start.
Too small = slow learning. Too big = jumpy. This is the most important setting in real AI training.
✔ You can explain what the learning rate does.
🎯 Quick check

A learning rate that is too small means…

5What did the brain learn?▶ Watch this part 2:17

On the Brain sheet the 5×5 map shows the 25 weights. Red squares are clues for Apple. Yellow squares are clues for Banana. White squares do not matter.

  1. Compare the map with the pictures. Do the red squares match where apples are dark?
  2. Which square has the biggest weight? Why do you think so?
This is the “brain scan” of your AI.

6Test the brain▶ Watch this part 2:52

Open the Test sheet. The 10 pictures are new. Type these formulas in row 4 and fill down to row 13:

CellFormulaMeaning
C4='Learn'!$L$199+SUMPRODUCT('Data'!D22:AB22,'Learn'!$AL$199:$BJ$199)score of test picture 1
D4=1/(1+EXP(-C4))probability
E4=IF(D4>=0.5,"Apple","Banana")the brain's answer
F4=IF(B4=E4,1,0)1 if it is right
C16=COUNT(F4:F13)how many pictures
C17=SUM(F4:F13)how many right
C18=C17/C16accuracy (format as %)
The Test sheet: 10 pictures the brain has never seen.
The Test sheet: 10 pictures the brain has never seen.
  1. Type the formulas in C4, D4, E4 and F4. Select C4:F4, drag the fill handle down to row 13.
  2. In C16, C17 and C18 type the three summary formulas. In C18 press % on the Home tab.
  3. Look at the pictures that are wrong (red in column F). What do they have in common?
After the formulas: accuracy 70%. The goal line says not yet.
After the formulas: accuracy 70%. The goal line says not yet.
With the 12 training pictures the brain gets about 7 of 10 (70%). Not enough for the goal of 90%.
✔ C18 shows an accuracy. The goal line says “Not yet”.
🎯 Quick check

Why must the Test pictures NOT be in the training set?

7Draw your own picture▶ Watch this part 3:24

  1. Open the Draw sheet. Type 1 in squares to draw. Try an apple, a banana — and then a nonsense pattern like a checkerboard.
  2. Read what the brain says. Does it ever say “I don't know”?
Draw sheet: a banana.
Draw sheet: a banana.
A nonsense pattern. The brain still answers.
A nonsense pattern. The brain still answers.
The brain always answers, even for nonsense. It only knows what its 25 weights say.

8The goal: reach 90%▶ Watch this part 3:38

Add training pictures so the brain meets more situations. Use the empty slots 13–16 on the Pictures sheet: type 1 in squares and put the answer (1 or 0) in the top corner.

  1. Draw your own variations: an apple moved down or left, a small apple, a banana turned upright, a banana moved.
  2. Do not copy the test pictures. If you copy them the brain has seen the answers and the test means nothing.
  3. Everything recalculates by itself. Watch the Test accuracy and the goal line.

Stuck? Try these 4 pictures (1 = dark square). Two apples and two bananas:

apple, moved — 1
banana, upright — 0
apple, lower left — 1
banana, moved up — 0
Slots 13-16 now hold 4 extra pictures.
Slots 13-16 now hold 4 extra pictures.
With the extra pictures: accuracy 100%, goal reached.
With the extra pictures: accuracy 100%, goal reached.
✔ The goal line says GOAL REACHED! 🏆
🎯 Quick check

Which new training pictures help most?

9Think about it

  1. Why did the brain fail on some test pictures when it knew all training pictures?
  2. Why did adding differently-drawn pictures help? How is that like the Fix Pack in Phase 1?
  3. What happens with learning rate 5 and the 4 extra pictures? Why?
  4. How is this brain like Teachable Machine? How is it different?
  5. A banana drawn upright fools the brain. Real image AIs look for shapes anywhere in the picture. Why is that better?

Next stop: Phase 4 — Detective Map: how an AI decides, shown on a map.

⭐ Challenge

  1. Reach 100% on the test with only the 4 extra pictures.
  2. Draw a picture that makes the brain say Apple with more than 90% although it is a banana.
  3. Find the learning rate that reaches 0 mistakes in the fewest rounds.

Words to know

Englishالعربيةעברית
pixelبكسلפיקסל
weightوزنמשקל
biasانحيازהטיה
scoreنتيجةציון
probabilityاحتمالהסתברות
learning rateمعدل التعلّمקצב למידה
round (epoch)جولةסיבוב
lossخسارةהפסד
trainتدريبאימון
testاختبارמבחן

Questions

The Brain sheet says “fill the Learn sheet first”.

You have not typed the formulas on the Learn sheet yet (or not filled them down to row 199).

I see #NAME? or #REF!.

A sheet name in a formula is wrong. Click the cell on the sheet instead of typing the sheet name.

My numbers differ a little from the guide.

If you changed a picture or the learning rate, the numbers change. That is correct.

Can I use Google Sheets?

Yes for the formulas, but the colours and charts may look different. Excel is recommended.