Here is an example of how to use system blocks. We will construct an electronic dice circuit. We start with the power block – using a 9v PP3 battery, but we could use 6v or 4.5v using AA or even AAA batteries. With an alteration to the final display circuit you could even use 3 volts. Most of the blocks are standard system blocks, just a few were specially created. Your GCSE project should use standard blocks as far as possible. Any extra ones will need to be built and tested separately before putting the whole system together. The arrangement of blocks used here is just one possibility – there other ways of doing things.

A stab at the push switch starts the circuit going. The lights will all flash, fairly fast, for a time and then stop. This block triggers the monostable.  If you were doing this for your coursework, you would need to research switches and justify your final choice.

The monostable latch is a timer block.  It is triggered by a low input and its output goes high for a time given by 1.1xRxC.  With the values shown that works out at a maximum of 11seconds.  You will probably want to use a shorter time than that.  If you were doing this for your project you would need to examine different monostable circuits (perhaps using logic gates) and justify your chosen one.

A 555 timer chip is used in astable configuration.  The frequency of this block is given by 0.7/(RxC) or about 7Hz.  You will probably want it to go faster than that.  It needs to be fast enough so the outcome is random, but no0t too fast otherwise it doesn’t look like a dice display.  Again, for your project you would need to examine different astable circuits and justify your chosen one.

To convert the pulsing output of the astable to 6 outputs for the dice we use a 4017 decade counter.  This sequences through ten outputs normally, but we reset it after the sixth output so that it counts in sixes.  Two PCB layouts are shown, first with outputs in the correct order and then second not – since it doesn’t matter what order the numbers for the dice go in.  Using the logic block means that we actually only need five outputs – you may need to think about that!

The simplest method of seeing if the circuit is working so far is to use this six led block.  You will need to use the 4017 in-order block or else it will look odd.  You could use a single current limiting resistor but that could lead to excessive reverse voltages which could destroy the LEDs.

There are lots of ways you can turn the outputs from the 4017 into a traditional dice display.  Perhaps the simplest is to use three NOR gates (available in one chip the 4025).  In fact we need two two-input Nor gates and one three input gate.  Using this logic (complicated in Crocodile Technology) lights all seven LEDs using only five outputs from the 4017.  If you were doing this for your project you would need to draw truth tables and design various logic systems and justify your chosen one.

Finally, here is the traditional 7 LED dice display.  If you want to run this on 3 volts then you will need to change the circuit a little so that the pairs of leds are in parallel instead of in series.  There are lots of variations on this set of blocks for electronic dice – this is just one possible method.  The beauty of system blocks is that it is so easy to try other arrangements.  One more thing: how about using a PICAXE chip?