What it shows
A flowchart with decision diamonds and a loop describes the steps of an ATM withdrawal, including three PIN attempts and a balance check. It shows how to plan an algorithm before coding it.
The science
An algorithm is a step by step method to solve a problem. A flowchart draws it with standard symbols: an oval for start and end, a parallelogram for input and output, a rectangle for a calculation or action, and a diamond for a yes or no decision with two arrows out. A loop is an arrow that goes back to an earlier step, here to let the user try the PIN again until the tries are used up.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A4 sheets, a pencil, a ruler and an eraser (household)
- A flowchart template of symbols drawn by you: oval, rectangle, parallelogram and diamond
- Coloured pens for yes and no arrows (optional)
- A free flowchart tool such as diagrams.net, only if you want a digital copy
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
List the symbols on a sheet with their meaning: oval, parallelogram, rectangle, diamond and arrows.
Write the steps of an ATM withdrawal in words: insert card, enter PIN, check PIN, choose amount, check balance, give cash, print slip.
Draw an oval Start, then a parallelogram Enter PIN and a rectangle that sets tries to 1 using the words tries = 1.
Draw a diamond Is PIN correct? with a Yes arrow going down and a No arrow going to a rectangle that adds 1 to tries.
After that draw a diamond Tries more than 3? with Yes going to Card blocked, End, and No going back by an arrow to Enter PIN.
On the Yes path of the PIN check draw a parallelogram Enter amount, then a diamond Amount less than or equal to balance?
On Yes draw rectangles Give cash and balance = balance - amount, then Print slip and End; on No draw Show not enough balance and End.
Trace three cases with a coin or pencil: right PIN first time, right PIN on the third try and three wrong PINs, and fix any arrow that leads nowhere.
What you should see
A neat flowchart in which every diamond has exactly two arrows, every path ends in an End oval, and the three traced cases all give the right outcome.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What shape is used for a decision?
A diamond, with a yes arrow and a no arrow.
2How is a loop shown?
An arrow that goes back to an earlier step.
3Why limit the PIN tries to three?
So someone cannot keep guessing the PIN.
4Why trace the flowchart with test cases?
To find mistakes before writing any code.
Take it further
Translate the flowchart into a Python program using a while loop and an if for each diamond.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Computer & office
Computer Fundamentals Study Book
The complete study notes, practice questions and glossary for Computer Fundamentals