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There are many different components that would be needed in order to produce or construct a Random number generator or in this case the Electronic Die.

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Although knowledge of the details of the construction of components is not required by the syllabus, there are occasions when such knowledge will help with an understanding of how the components works.  When this is felt to be the case, details of construction are included below for information.

Apart from the actual value of a component, there are four other factors to consider when selecting a component for a circuit.

  1. The physical size of the component.  Some applications demand miniature components, e.g. mobile phones.  The use of Surface Mounted Devices (SMDs) is essential in such applications, while the use of leaded components is often more useful in circuits like wshing machine controllers, where space is not important.
  1. The cost of the components.  Very cheap components should not be used if it is likely to affect the reliability of the electronic system, especially in critical applications e.g. The Engine Management Units in cars!
  1. The power rating of the component.  Whenever a current passes through a component that has resistance, power is dissipated and the component will become hot.  It is important when selecting components to ensure that the component is able to withstand more
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To calculate the value of the series resistor it is first necessary to decide on the current that should flow through the LED.  The data sheet for the LED will specify a maximum current Imax.  The LED current should be less than this.  If the supply voltage is Vs, then

Vs – 1.8

            R =           Imax

The minimum value used for R should be the next largest preferred value.

LEDs are used as indicator lamps and in seven-segment displays.  They have the advantages of small size, long life, a small operating current and high operating speed.  There is now a large range of different LEDs available with outputs ranging from the infrared to blue wavelights.

Seven segment array

Electronic calculators, clocks, cash registers and measuring instruments often have seven-segment LED displays as numerical indicators.  Each segment is an LED and by lighting up different segments all numbers from 0 to 9 can be displayed.  Each segment needs a separate current limiting resistor to prevent damage to the segment by excess power dissipation.

All the cathodes (common cathode type) or all the anodes (common anode type) are joined to form a common connection.  If the driving circuit is made from transistors, so that the seven-segment display segments are connected in the collector circuits, then a common anode Figure ____ display will be required.

Seven segment Display

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The RESET terminal can be used to set the output to 0V at any time by being connected momentarily to 0V.  Normally, the RESET terminal is connected to Vs to prevent any spurious resetting of the output.The CONTROL terminal is connected to the 2/3 Vs point of the voltage divider and can be used to alter the voltage switching levels of the comparators.  Normally it is decoupled by a 10nF capacitor connected to 0V.image00.png




                    The 555 astableimage05.pngimage03.png

The common circuit for a 555 astable is shown in figure

When first switched on the capacitor, C, is discharged and so the voltage across this capacitor is less than the TRIGGER voltage and so the output goes to Vs.  The capacitor, C, charges through R1 and R2 until the voltage across C is greater than the THRESHOLD switching level, at which point the output voltage becomes 0V and the DISCHARGE terminal becomes connected to 0V.

The capacitor now discharges through R2 until the voltage across C becomes less than the TRIGGER switching voltage.  When this happens, the output voltage becomes Vs and the process repeats.  It should be noted that the first pulse is longer than the remainder, since C has to charge from 0V and not 1/3 Vs.  The same restrictions apply to the values of C and R (R1 and R2) as for the monostable.

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