Engineering Skills 1 – Design Build & Test - 2024

Design Build & Test Assignment

From OrCAD 1 and Advanced OrCAD, you should now have all the necessary skills for the ‘Design Build & Test’ project. This constitutes the main part of your Engineering Skills 1 grade (60 % - subject to change) . You will now have had feedback on your first project, and have had practise at creating schematics, PCBs,a bill of materials (BoM), assembly drawings, and photomasks. You should be familiar with adding libraries to schematics, setting up PCB   boards and including the correct footprints before exporting the netlist. You have also had the opportunity to learn soldering skills.

For Design Build & Test, you will first build the board you designed in Advanced OrCAD. This will give you a chance to see how your design impacts the practicalities of populating a PCB.

You might find there are somethings you would do differently after the experience. After this you will then design, build, and test the final circuit; ‘Electronic Dice’ .

The Design Build & Test course will be graded on:

•   Your completed, populated XMAS lights board – 5 % of grade

•   Your electronic dice schematic, PCB design and associated files (e.g. BoM, assembly drawing, photomasks) – 25 %

•   Your completed, populated Electronic Dice board – 10 %

•   A verbal communication test – 10 % (Details will be provided on Moodle)

•   A critical review document  – 10 % (Details will be provided on Moodle)

Note: These contributions percentages are subject to change.

Electronic Dice PCB

Here you are building an ‘Electronic Dice’ .  This is a PCB board containing LEDs in the pattern that mimics the face of a dice.  When you hold a button, this is equivalent to ‘rolling’ the dice i.e. it is constantly changing value at a speed you wont be able to see.  When you release the button it will stop on a ‘random’ value.

From the above figure, you can a see that you would need 7 LEDs (a – f), to be able to capture all possible numbers of the dice. By inspection, you can also see that certain LEDs are always lit in pairs, for example, when f is on, c is also always on.

This allows us to simplify the design, and use less signals to drive the LEDs. The below Figure (Fig. 2) shows the circuit diagram, and the associated ‘truth table’ (Table 1) which indicates which LED pairs are lit for a given dice number.  Here, the number ‘ 1’ means the LED is lit, and a ‘0’ means it is off.

Electronic Dice Schematic

Below is the schematic you should use for your electronic dice design. There is a circuit description in the following section. NOTE: The 74HC175 schematic part has pins in a different order to what is shown below, but it is the same component. Take care here so that you make the correct connections in the schematic, i.e. pay attention to which pins are connected in the below circuit, not just the way the wires are drawn.

Basic Circuit Overview

The design contains various circuits which will be briefly discussed here.  At the top left of the schematic, you can see a power switch for the board, which comprises a battery (in the schematic this is the J1 connector, but you’ll use the footprint for a small lithium battery cell holder) and a switch, with power and ground connections setup through the labels so you can connect the rest of the circuit appropriately.

There two different logic chips used in this schematic. The first is the 74HC175, which is a chip containing 4 clocked, positive-edge triggered D-type flip-flops (you’re not expected to know what this means yet!) .  Simplistically, each of these flip-flops are devices that sample the voltage at their input (i.e. if it’s ‘high’ or ‘low), and they evaluates this input on the rising edge of a clock cycle signal (CLK) i.e. when the clock signal voltage goes from low to high. At  this point in time, the output of the flip-flop, Q, then assumes the same state as the input (i.e. either high or low). Q, outputs the opposite state of Q.  You can chain these flip-flops together in a certain way using feedback, that forces them to toggle through different combinations, or ‘states’ overtime as the clock ‘ticks’ (i.e. the outputs of different flip-flops  change in a particular way as they continuously receive rising edges from the clock). You can see the chip and the feedback between flip-flops in the centre of the schematic.

This circuit has been designed, so that you can use these particular flip-flop output states to drive the LEDs, and cycle through all combinations relating to the face of a dice (see Table 1). However, you can’t use the outputs of the flip-flops directly, you require some logic gates so that the LEDs turn on at the correct time. In essence, you’re using logic gates to convert the signals from the flip-flops to the correct signals to drive the LED combinations. This might be confusing for now, but you will learn much more about these ‘state machines’ in Digital Electronics courses.

The logic devices that are used here (i.e. to translate states of the flip-flops to certain LED outputs) are ‘NAND’ gates.  This is a (NOT – AND). An ‘AND gate’ outputs a high voltage when both its inputs are high, a NAND gate does the inverse, i.e. it outputs a high voltage when both its inputs are low. In this design, the 74HC132 chip is used, which contains 4 independent NAND gates. You’ll notice that (just like in the XMAS lights design) you have 4 gates that appear separately on the schematic but relate to the 1 physical chip. The NAND gates have been used to generate the correct logic to drive the LEDs, and also as part of the  clock generation circuit (see U1B in schematic above) and in the reset circuit (see U1A in the schematic above).

Notice that there is a push button switch that connects the clock circuitry to the state machine. When you press this, you’re causing this ‘state machine’ to rapidly toggle through all its possible states (this is like rolling the dice), and when you let go of the switch it appears to stop randomly on a value, which is displayed on the LEDs.  It only appears random because it’s changing through the states so quickly that you couldn’t possibly control which result it lands on!

Create the schematic

You will need to find the appropriate libraries. In the Advanced OrCAD assignment pdf, details were given about a trick for searching for parts. Initially, add the common libraries (i.e. as in XMAS lights) when starting the project, but then use this search tip to find parts   you’re missing. Be careful that the ‘push button’ in the schematic has four pins.

When laying out the four logic gates of the 74HC132 you must lay all the gates out at the sametime otherwise OrCAD tends to lay down multiple chips – see the OrCAD guide. Again, this was explained in the XMAS lights submission. Make sure the components have the correct values.

Check the pins of the LEDs against the pins of the LED footprints and ensure the anode and cathode correspond appropriately. You’ll also notice that not all the LEDs face the same way in the schematic!

Check the pin numbers on the switches – both the push button and the slide on-off switch and how they match with their footprints. If you’re confused about this – aska lab demonstrator!

Check that you have got the proper power and ground names so that the chips are connected correctly as they have hidden power and ground links (this is explained in the OrCAD tutorial videos for editing a part).

Use all the tips you’ve been given during feedback for OrCAD 1 and Advanced OrCAD, make the schematic neat and easy to read, add your name, GUID anda name for your design.

Footprints

For the footprints, you will have to decide which to use in some instances, for example, the logic chips are different sizes.  Google the parts and see which GU footprints they correspond to.  You can find the footprints pdf on the OrCAD training moodle page.  The pdf files required are called Page 1, Page 2 and Page 3.

Logic chips	GU-dip14, GU-dip16 etc (component dependent)
Resistors	GU-RC400
Capacitors	GU-RC200
LEDs	GU-LED5mm
Slide switch	GU-sw-slide-spdt
Push switch	GU-PUSH52 (Check PIN assignment carefully)
Battery holder	GU-cr2032-clip

Table  1:  Suggested footprints for circuit components, double check and select the correct footprints for the logic chips.

PCB Board

Once the schematic is properly checked and annotated, proceed to start PCB Edit and create an empty board. Setup the padpaths and padstacks as in Advanced OrCAD. If you’ve done this properly in the previous assignment, it should still point to the correct directories. If you had problems selecting these paths, speak to a demonstrator.

Make the board 4” x 2” (100 mmx 50 mm).

REMEMBER, IT IS USELESS TO CONTINUE WITH PCB EDIT IF THE NETLISTER SHOWS ERRORS.

You will need to use both sides of the board for tracks, called ‘double-sided routing’ . Follow the guides in Moodle. You should be able to route this board manually.

•    Remember to lay out the pairs of LEDs in their proper pattern (i.e. to look like the face of a dice!

•    Place a ground plane on top and bottom layers (to reduce overall etching time)

•    Use the correct ‘Gu-via80’ vias and not ‘through hole plating’ – minimise the amount of vias you can use! The fewer the better.

•   When using vias, make sure you terminate tracks in away that allows components to be soldered (see Introduction to Advanced OrCAD assignment).

•    Have an identifier on the board (I.e. your name or initials, or GUID for example) with text size of at least 5.

•    Make sure you don’t have design rule check (DRC) errors.

•    Use the correct track widths as recommended in the video tutorials, and use wider tracks for power and ground than for signals. Details of the constraints to use are found on the OrCAD PCBedit and PSPICE training page.

•    Use sensible ‘ keepin’ distances.

•    Design the board so that components can be easily mounted and soldered.

•    Minimise the number of vias used in the design.

Deliverables & dates

In the remaining lab sessions you will need to build and test your XMAS board, and Design, Build & test your Electronic Dice board. Use your time carefully and work on the Dice board in your sparetime, to maximise the time to build and test PCBs in the lab.

The deadline for the Dice board submission is specified on the Moodle page (where you downloaded this document from!).  It is strongly recommended that you complete the assignment prior to this cut-off date, to maximise the time you have to build and test the PCB.

You need to submit your design files for assessment. You can do this on the Engineering Skills 1 Moodle page, going to the EEE Design Build & Test page.

Submit the following in a zipped folder:

o PDF of the schematic using the guidance described above and in tutorials.

o PDF of separate PCB layers, which have been laid out in accordance with advice in this document and in tutorials

o PDF of the assembly drawing

Use landscape orientation and scale to page for the above documents

o PDF of the BoM – neatly presented

o Separate PDFs for the top and bottom photomasks (properly mirrored) . Note: please print to pdf at 600 dpi resolution as otherwise ground planes may

appear to be segmented.

These need to match the board size on the page (1:1 scaling)

o The .brd file

Other assessments

Following the lab sessions, there will be short (10 minute) verbal assessment, where you will be asked to talk about the process of designing, testing and debugging PCBs.

You will also need to provide a 1 page critical review document – more details will be provided on Moodle.

These will be required next semester but speak to the lecturer or a GTA if you’ve finished your lab book early.

Picture of the board

Below is a picture of the completed Electronic Dice Design (note that the ground plane is missing!)