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Operational amplifiers and feedback

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					Year: 2008                                                                              PX2505



              A Simulation Exercise Using the ‘Multisim’ Software Package


The Multisim software

Multisim is a powerful simulation package that enables circuits to be designed and tested
before being built. You will learn how to use Multisim to draw circuits like the ones in the
lab handout and also how to activate your circuit and see how it behaves. On-screen
instruments are available, such as a multi-meter, logic probe and oscilloscope. Switches can
be operated from the keyboard. Inputs can be fixed voltage sources, like Vcc, or function
generators like the one in the lab. A basic guide to the use of the software is given below.

The software is available in the computer classrooms. To start the software, select icons
Physical Sciences – Engineering and Physical Sciences - Physics – Multisim 2001 –
Multisim 2001. The complete Multisim package will then be transferred to your PC; this
avoids problems that can occur with network use. The transfer can take a good 2 minutes.
Be patient! The workspace panel will finally appear.

Simulation exercise

In this exercise you will ‘build’ a circuit equivalent to the one shown below. This is a circuit
that you have already made in hardware (Buchla’s Digital Experiments textbook (fig 4-11, 3rd
and 4th ed'n; fig 5-1, 5th and 6th ed'n):




Circuit construction

The stages in the assembly of the above circuit are described overleaf. To ensure the wiring
is nicely aligned on a rectangular grid it is helpful to make visible the underlying drawing
grid of points using the View menu and Show Grid. This is optional.




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Year: 2008                                                                           PX2505

(a) Selecting components.




The circuit that you should build in the workspace is shown below:


Constructing the circuit 'on screen' is straightforward. Circuit components are selected from
the 'components bin' and instruments from the ‘instruments bin’. The general procedure is
as follows; the location of the required components is given on the next page.


   Open the appropriate bin and move through the expanding menus until you find what you
    need. The switch you need is called a ‘single-pole double-throw’ or SPDT for short. The
    ‘pole’ refers to the number of moving arms (1 here) and the ‘throw’ the number of
    contacts an arm can connect to. The default activation of the switch is the spacebar.

   Single click on the component or instrument, move to the desired position in the
    workspace and click again. The orientation can be changed by ‘popping up’ (right mouse
    click) on the component and choosing from the menu.




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Year: 2008                                                                                 PX2505

                              The components that you need are in the components parts bin.

                              Earth point, +VCC
                              and clock source          Sources parts bin.
                              Resistors and switch:     Basic parts bin. Choose virtual resistors
          Diodes                                        and ‘SPDT’ switch.
                              XOR gate:                 Misc Digital parts bin. Choose ‘EOR2’
                                                        in the component browser.
                              Probes:                   Indicators parts bin.

                              To change the properties of a component, double click on its icon
                              and make changes according to the options offered.

                              The Oscilloscope is in the Instruments parts bin.

                              Adjustments to an instrument may be made on its expanded
                              panel, which is obtained by double-clicking the instrument icon.
                              To change numerical values, the keyboard 'NUM LOCK' must be
                              activated.


(b) Wiring

Wiring the circuit is carried out as follows:
    (1) Point to the first terminal so that it highlights; click left mouse button.
    (2) Move the cursor to the second terminal; click again.
    (3) If a terminal is joined to any point on a wire, it will make a connection to it.
    (4) The Esc key terminates wiring.
    (5) To delete a wire, click on it and press the Delete key or pop-up on it and choose Delete.
    (6) To colour a wire, pop-up and choose ‘color’. Choosing different colours for the A and
        B oscilloscope input wires (as shown in the circuit diagram above) has advantages
        since the corresponding traces on the oscilloscope screen appear in the chosen colours.

Experiment

Wire up the circuit on-screen. The simulator is started by pressing
the activate switch shown on the right. Click on the oscilloscope
to display its traces.
Note: the simulator may behave unreliably if values are changed while it is running.
   Verify the expected performance for the circuit.

Multisim offers hundreds of virtual components, which simulate ‘real life’ behaviour. One
such is the 7486 (QUAD 2-input XOR) chip (TTL parts bin), which may be substituted for
the symbolic XOR gate in the above circuit. This choice simply provides 4 gates designated

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Year: 2008                                                                                    PX2505

sections A, B, C, D, the difference from the generic ‘ideal’ gate being that it simulates real
performance. The software can convert wiring diagrams to a PC board layout that takes
account of the pin connections on the various multi-gate chips, but don't attempt this.

Guidelines on drawing more complicated circuits

These points won’t mean much until you come to drawing circuits with more on them than
you have had in the practice circuit above. However, they are all relevant to the assessed
Multisim exercise described in the final section of these notes.

      1)     Draw the Vcc line along the top and the earth line at the bottom. Don’t sprout Vcc
             connections at points in the middle of a diagram.
      2)     Layout the components so that unnecessary wiggles in the wiring are removed.
      3)     Mirror any logical flow within the circuit in the layout. For example, put input
             devices on the left, output indicators on the right and align items at similar logical
             stages, if possible. If the circuit has identifiable subsections, then it can help to use
             colour coding for components or wiring. For example, in the exercise in which you
             will draw your circuit for a controller for several motors, to draw wiring for the
             different motors in different colours is very helpful both in tracking what is going on
             and in debugging your circuit if you have to.
      4)     Implicit in the above is the desirability of grouping similar items together. For
             example, put indicators for your input switches altogether rather than dotting them
             about in the circuit.

Assessment exercise

Armed with this experience of Multisim, you should construct a simulation of your solution
for the ‘Fillmore Furniture Factory’ problem. All the standard 74 series TTL chips are in the
TTL parts bin. The circuit diagram will be in ‘symbolic gate’ form, not ‘TTL chip’ form.

So that your circuit may be tested easily, you MUST observe the following requirements.
(1)        The input switches 1, 2, 3 and 4 must be operated by the number keys 1, 2, 3 and 4 on
           the keyboard. You have to set these from the default of ‘space’.
(2)        Logic probes S1, S2, S3, S4 must be wired to the switches so that they ‘light up’ when
           the switch is activated with the intention of operating the motor.
(3)        Logic probes M1, M2, M3, M4 must be wired to the motor outputs so that they ‘light
           up’ when the motor is operating. It is not necessary in this exercise to wire LEDs to
           the motor outputs in addition to the probes.
      (4) The probes must be grouped closely together on the circuit diagram as shown below.
          This mimics the table in Buchla’s book.

                        S1     S2       S3   S4         M1   M2       M3   M4




                                    input                         output
      (5) It is your responsibility to make sure that your simulation works. If you can’t get it to
          work, first check the logic and if that is correct then check the details of your circuit
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Year: 2008                                                                                                                                                             PX2505

           and, in particular, that objects that seem to be connected actually are. The simplest
           method is to move a suspect object and see if the attached wires are dragged with it.

     This assignment is worth 10% of the course assessment. Your submission should have
      two components, as follows:

1. A floppy disk containing your working simulation, for assessment.
2. A print-out of your circuit. The printout can be made by framing the desired area on your
   circuit diagram by dragging the cursor, copying it (edit/copy), and pasting it into a Word
   document. You'll want to copy the circuit this way if you're writing up a report of the
   Fillmore Furniture Factory.

Examples

It doesn’t matter what the circuit below was intended for. It has some good points (it works!)
but is hard to follow and is difficult to check through for errors. Aim to make your circuit
much clearer than this one.


                                                                                                       M1         M2        M3         M4
 S1       S2       S3       S4                            5V VCC                                              2.5 V     2.5 V      2.5 V         2.5 V
      2.5 V    2.5 V  2.5 V      2.5 V

                                                          R1                                                                                              VCC       5V
                                                          1.0kohm

                                          VCC        5V                                         U4
                                                                                                                                     LED1
                                                                                                                                     LED_blue             R3
                                                                                                                                                              330ohm
                                                R2                                             NAND2               U7
                                                1.0kohm
                                                    U1
                                                                                                U6                                                            VCC
                                                                                                              NOT                                                      5V
                                                 NOT
                   J1                    VCC    5V                       U5                    NAND2
                 J2
                Key = 1                                                                                                     LED2                         R6
                                                  U2                                                                        LED_blue
                                          R5                                                                                                             330ohm
                                                                                 U8
                                                                    NAND3
               Key = 2                     1.0kohm
                 J3                           NOT                                                U14              U15
                                                                                NAND3
               Key = 3                                                                                                                                           VCC        5V
                                          VCC                       U9
                 J4                                                                            NOT           NOT
                                  R4            5V

               Key = 4            1.0kohm
                                         U3
                                                                 NAND3
                                                                U11                                          U12                                                    R7
                                                                                                                                             LED3
                                                                                         U10                                                                        330ohm
                                                                                                                                             LED_blue
                                         NOT
                                                                                                            NOT
                                                               NAND3
                                                                                      NAND2                                                  VCC         5V

                                                                          U13            U16                              LED4
                                                                                                            U17                             R8
                                                                                                                                            330ohm
                                                                         NAND3          NOT
                                                                                                       NOT
                                                                                                                        LED_blue




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Year: 2008                                                                                                                                 PX2505


   VCC   5V


                                                                                                            1 M1NAND1A
              R1       Lube                                                                                 2           12
                                                                                                           13
                                                                                                                              1 M1NAND3A
          1kohm                                                                                             7410N             2            12
                   Key = 1                                                                                                   13
                                                                                                            1 M1NAND2A        7410N
              R2     Conveyor                                                                               2           12
                                                                                                 N2A       13
          1kohm                                                                              1         2
                                                                                                            7410N
                   Key = 2                                                                   7404N

              R3                                                                                            3    M2NAND1B
                     BandSaw                                                                                                      M2N1B
                                                                                                            4           6    3         4
                                                                                                 N3A
          1kohm                                                                              1         2    5
                                                                                                                             7404N
                   Key = 3                                                                                      7410N
                                                                                             7404N


              R4    CrossCutSaw
                                                                                                 N4A        9 M3NAND1C
          1kohm                                                                              1         2
                                                                                                           10           8
                   Key = 4                                                                   7404N         11
                                                                                                                              9 M3NAND3C
                                                                                                            7410N            10            8
                                        S1              S2              S3              S4                                   11
                                                                                                            9 M3NAND2C
                                                                                                           10                 7410N
                                2.5 V           2.5 V           2.5 V           2.5 V                                   8
                                                                                                           11
                                M1              M2              M3              M4
                                        2.5 V           2.5 V           2.5 V           2.5 V               7410N



                                                                                                            1 M4NAND1A            M2N1D
                                                                                                            2           12   9         8
                                                                                                           13
                                                                                                                             7404N
                                                                                                            7410N




The circuit above does the same as the previous circuit. It has a good layout and useful
colour coding that makes it clear to follow and helps in tracing potential errors.


ISM/JSR




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