Engineering Design How to build stuff that works

Engineering Design How to build stuff that works, and make it work better Dr. Tom Clarke, Second Year Electronics Laboratory Coordinator

Why learn design? o EEE courses teach theory needed to do real engineering. o Applying that theory happens later in projects, and some coursework. It helps you: n n Motivate theoretical work Become better at solving real problems Perform better in your year 3 & 4 project work Have fun on your EEE or ISE course Engineering design in Second Year Electronics Laboratory 2

What is design? o 1. Specify the problem o 2. Explore top-level decomposition o 3. Discover relevant information from diverse sources o 4. Perform system analysis (where possible) o 5. Make assumptions, prioritise problems o 6. Perform detailed prototype design o 7. Evaluate prototype n test assumptions n identify problems & areas for further work o 8. Improve design Engineering design in Second Year Electronics Laboratory 3

Design is not linear Top-down Specify Assumptions Bottom-up These activities are inter-dependent and concurrent Top-level Analysis Information Prototype Evaluate Improve Engineering design in Second Year Electronics Laboratory 4

Putting it all together o Not all elements of design are “open”. n Some problems are closed, with specific constraints that allow only one solution n You will often find these problems in EEE coursework or exam problems o Use analysis to solve closed problems, to simplify the design space n No-one tells you which bits of analysis to do! o Design requires both analysis and creative exploration Engineering design in Second Year Electronics Laboratory 5

Specify Example o Need to set bias voltage (Vb) input to alphanumeric LCD display module. o From LCD datasheet: n LCD module supply is 5 v n Vb >0. 5 v, Vb < 3 v o Circuit must adjust Vb to an unknown correct value in this range o This sets LCD contrast n Ib < 10 u. A n ILCD = 2 m. A (typical) Engineering design in Second Year Electronics Laboratory 6

Top-down Bias Adjust Circuit +5 V Bias Adjust Circuit Vb LCD Module GND Engineering design in Second Year Electronics Laboratory 7

Design ideas Multiple viewpoints o Use voltage regulator IC? n Need to read datasheets to see how to make adjustable over required range o Use Zener diode (last year’s circuit) n Does not help since not variable o Use potential divider (P. D. ) with variable resistor n Simplest solution if feasible o Could combine P. D. and Zener for slightly better stability n (probably not worth it) Engineering design in Second Year Electronics Laboratory 8

Variable resistors Information o Preset resistors have three terminals, with a fixed resistance between the two ends and a slider which can move anywhere between the two ends. PR 1 Engineering design in Second Year Electronics Laboratory 9

Detailed circuit design using variable resistor +5 v R 1 Vx 2 v PR 2 Vy Vb 0. 5 v Detailed design o This circuit will allow Vb to be adjusted between 0. 5 v & 2 v o What values R 1, R 3, PR 2? R 3 GND Engineering design in Second Year Electronics Laboratory 10

Know your resistors Information o 22 k resistor is not 22, 000 ohms! n 22 k resistor has specified tolerance (1%, 2%, 5%) o 2% tolerance: 0. 98*22, 000 < R < 1. 02*22, 000 n Variable resistors typically have tolerance 10% n Resistors have preferred values: o Fixed resistors available in E 24 series and multiples 1, 1. 2, 1. 3, 1. 5, 1. 6, 1. 8, 2. 0, 2. 2, 2. 4, 2. 7, 3. 0, 3. 3, 3. 6, 3. 9, 4. 3, 4. 7, 5. 1, 5. 6, 6. 2, 6. 8, 7. 5, 8. 2, 9. 1 o Variable resistors only available: 1, 2, 5 and multiples! o Check catalogues and datasheets n Design for available precision & values Engineering design in Second Year Electronics Laboratory 11

Analysis (ohms law) o o Analysis R 3 = 0. 5/(2 -0. 5)PR 2 R 1 = (5 -2. 5)/(2 -0. 5)PR 2 Choose PR 2 first, calculate R 3, R 1 How accurate do these ratios need to be? n If Vx>2 V, Vy<0. 5 V the adjustment range includes the required range of 0. 5 -2 V n Precision not required o. R 3, R 1 can be smaller then calculated Engineering design in Second Year Electronics Laboratory 12

Assumptions o PR 2 too low => more current used in circuit. n Assume want current as small as possible o PR 2 too high => Vb will vary too much with LCD bias current change. n Datasheet does not say how much bias current changes so assume 10 u. A is possible (worst case, since we know it is < 10 u. A) n Datasheet does not say how accurate Vb must be: assume 10%. Engineering design in Second Year Electronics Laboratory 13

Analysis More analysis + approximation o Approximate analysis n Assume Thevenin equivalent resistance at Vy = R 3 (actually slightly smaller) n Assume OK at all other voltages if OK at Vy n 50 m. V > R 3. Ib = R 3. 10 u. A => R 3 < 5 k n => total divider current = 1 m. A o Not too bad, but significant compared with ILCD o R 1=50 k, PR 2=15 k, R 3=5 k Engineering design in Second Year Electronics Laboratory 14

Are values realistic? Detailed design o Variable resistors are available 10 k, 20 k, 50 k n 15 k not possible o Could scale by 2/3 n R 1=33 k, PR 2=10 k, R 3=3 k 3 n These resistor values are all available o This is not good idea. Designing precisely to limits is dangerous. o Reduce R 1, R 3 by 20% to ensure coverage of entire range even if resistor values vary n R 1=27 k, PR 2=10 k, R 3=2 k 7 (use E 24 values) o Note that precise values don’t matter n R 1=22 k, PR 2=10 k, R 3=2 k 2 would also be fine Engineering design in Second Year Electronics Laboratory 15

Optimise circuit Optimise o Why bother with R 1, R 3? o Not really needed, but allows better adjustment n Resolution = minimum change in resistance value that a variable resistor can be adjusted to. n Typically 1%. 1. 5 V across PR 2 =>15 m. V res n R 3 missing => 2 V across PR 2 => 20 m. V res n R 1 & R 3 missing => 5 V across PR 2 => 50 m. V res o R 3 probably not needed (1. 5 V -> 2 V) o R 1 maybe also not needed (1. 5 V -> 5 V) Engineering design in Second Year Electronics Laboratory 16

Guess which one is recommended in the LCD datasheets? Possible circuits +5 v 30 k R 1 27 k 2 v 10 k PR 2 3 k 0 R 3 GND +5 v Vb PR 2 20 k 50 k PR 2 Vb Vb 0. 5 v GND Engineering design in Second Year Electronics Laboratory 17

Design in EE 2 & ISE 2 Laboratory o Design activities during 1 st half of each Term n Work through examples of design n Learn skills useful in project work o PCB design o Embedded system design o Prototyping o Measurement n Conducted in laboratory pairs n Assessed individually by demo, interview, & logbook Engineering design in Second Year Electronics Laboratory 18