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Procedure Solver

Can you think like a computer and work out what this flow diagram does?

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Adding Machine

Can you set the logic gates so that this machine can decide how many bulbs have been switched on?

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Not Another NAND!

Prove that you can make any type of logic gate using just NAND gates.

Circular Circuitry

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3


This problem involves circuits which feed back into themselves: The output of a gate can be traced through the circuit back into the input of the same gate! This can lead to all sorts of interesting behaviour, which will be explored in this problem. Please note that the gates here are not interactive: they are thought experiments .

What will happen when you switch on these circuits?

What will happen if you change the gates to different types?

The following two circuits have the same gates. Look carefully at the wires which are switched on. What do you notice? What do you think will happen when the switches are flicked on and then flicked off?

Can you create your own feedback circuits? Investigate their properties.

Click here for a poster of this problem.


A key concept in computing is that of random access memory : whilst computers logically process commands they are also able to store user inputs whilst a procedure is being followed. Memory is built using logic gate circuits which feed back into themselves. The most simple of these is the 'flip-flop' which you can read about in the Wikipedia article . The flip-flop is a device which has two different stable states, and can thus 'remember' if a switch has previously been flipped.