6. Electronics design

I used to think this week would be tough, but it turns out not that difficult to draw a simple dev-board with some inputs and outputs after picking up my high school physics memories and learning EAGLE.

1.1 Assignments of the Week

1. Group assignment:
   • Use the test equipment in your lab to observe the operation of a microcontroller circuit board (as a minimum, you should demonstrate the use of a multimeter and oscilloscope)
   • Document your work on the group work page and reflect what you learned on your individual page
2. Individual assignments:
   • Design a development board to interact and communicate with an embedded microcontroller

1.2 Multimeter and Oscilloscope

Our test equipment is a multimeter and oscilloscope in one device.

The Test :

We connected a potentiometer to the A0 pin of Arduino, a Micro Servo to the D9 pin, and the D5 pin is used to output the mapped analog signal. When we turn the potentiometer, we can observe the rotation of the servo, and detect the corresponding waveform, voltage, current and resistance through the D5 pin.

Here is the code. I made some modifications on dfrobot's source code.

          #include 
          Servo myservo;

          int potpin = 0;
          int val_in;
          int val_servo;
          int val_out;
          int pin=5;

          void setup() {
                myservo.attach(9);
                pinMode(pin,OUTPUT);
          }

          void loop() {
                 val_in = analogRead(potpin);
                 val_servo = map(val_in, 0, 1023, 0, 179);
                 val_out=map(val_in,0,1023,0,255);
                 analogWrite(pin,val_out);
                 myservo.write(val_servo);
                 delay(15);
          }
        

1) Oscilloscope

Press the "Mode" to switch to the oscilloscope function. Connect positive to D5 pin, negative to GND. Then turn the potentiometer, you'll see the square wave on the screen.

A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions between minimum and maximum are instantaneous.
The square wave is a special case of a pulse wave which allows arbitrary durations at minimum and maximum amplitudes. The ratio of the high period to the total period of a pulse wave is called the duty cycle.
Square waves are often encountered in electronics and signal processing, particularly digital electronics and digital signal processing. Its stochastic counterpart is a two-state trajectory.

---- from Wikipedia

2) Multimeter

Press the "Mode" to switch to the multimeter function. Connect positive to D5 pin, negative to GND. Press F1~F4 to select the corresponding function.

1.3 Electronic Components

• Switch and Button: button is connected when pressed, and disconnected when you let go, which is different from switch.
• Capacitor: capacitor can store and release energy. It is used in a electronic circuit to suppress voltage spikes, clean a signal and as local energy storage.
• Diode: the main characteristic is unidirectional conductivity. It is often used to connect the DC Motor to prevent the motor from rotating in the opposite direction and generate reverse current.
• Transistor: the most important component of modernelectronics. They are used as a digital switch in a variety of function. It is often used as a voltage regulator or to record 0 and 1.

1.4 PCB Design

1) What is PCB?

A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. PCBs require additional design effort to lay out the circuit, but manufacturing and assembly can be automated. Electronic design automation software is available to do much of the work of layout.

A basic PCB consists of a flat sheet of insulating material and a layer of copper foil, laminated to the substrate. Chemical etching divides the copper into separate conducting lines called tracks or circuit traces, pads for connections, vias to pass connections between layers of copper, and features such as solid conductive areas for electromagnetic shielding or other purposes. The tracks function as wires fixed in place, and are insulated from each other by air and the board substrate material. The surface of a PCB may have a coating that protects the copper from corrosion and reduces the chances of solder shorts between traces or undesired electrical contact with stray bare wires. For its function in helping to prevent solder shorts, the coating is called solder resist or solder mask.

---- from Wikipedia

2) Software: EAGLE

EAGLE is electronic design automation (EDA) software that lets printed circuit board (PCB) designers seamlessly connect schematic diagrams, component placement, PCB routing, and comprehensive library content.

DOWNLOAD EAGLE

3) Schematic and Board Layout

• Schematic

A schematic in electronics is a drawing representing a circuit. It uses symbols to represent real-world electronic components. The most basic symbol is a simple conductor (traces), shown simply as a line. If wires connect in a diagram, they are shown with a dot at the intersection. (reference)



• Board Layout

The board layout shows how the electrical components and traces will be laid out on the board.(reference)



• Switch Views in EAGLE
Click this button below to switch between schematic and board layout views.

4) Design Steps

• Add Library

Download, Copy and Paste the Fab library to the "Users\Documents\EAGLE\libraries". Click "Add Part", there are already some default libraries. To add more, click "Open Library Manager" to load other available libraries. I added the "Fab" and "Xiao" libraries.



• Add Dev-Board and Rename the Components

I added Seeed Xiao first. The default name is "U1". Click "Name" and then click the origin of the component to rename it to "Xiao". The same way to rename other components.



• Add Supply



Right click to change direction of the arrow. The same way to change directions of other components.



• Add Pinheads

For beginners, better to use SMD header. The default "pinhead" library are pad header.



• Net

Click "Net" to connect the components. Rename the net to connect with where has the same name.



• Add Led and Button



• Route in the Board View

You can select wire bend and set the wire width on the top bar. I forgot to set the wire width at first so I got lots of warnings when doing design rule check.



There are some design rules for PCB design. Click "Tool→DRC“ to set the clearance and sizes.



mil: thounsandth of an inch

• DRC

DRC means "Design Rule Check", to make sure the circuit design can be manufactured by CNC. Traces and pads are not getting too closer and the traces width is wide enough to be cut out.

I forgot to set the wire width in the beginning, so I got all these width error. I had to "Ripup" all the wires and re-route them.

5) Pads and SMD

• Version 1 Pads

In the 1st verison, I didn't figure out the meaning of pads and SMD. I randomly chose all pads. This is more difficult for soldering.



• Version 2 SMD

So I drew version 2. I changed Xiao and supply pinhead to SMD, but still couldn't find the 1*7 and 1*4 SMD pinhead.

6) Final Work

BOM

Part	Value	Device	Package	Description
PINS1		PINHD-1X7/90	1X07/90	PIN HEADER
PINS2		PINHD-1X4/90	1X04/90	PIN HEADER
R1	499 ohm	RES-US1206FAB	R1206FAB	Resistor (US Symbol)
R2	1k ohm	RES-US1206FAB	R1206FAB	Resistor (US Symbol)
S1		6MM_SWITCH6MM_SWITCH	6MM_SWITCH	OMRON SWITCH
SUPPLY		PINHD-2X3-SMD	2X03SMD	PIN HEADER
U$1	LEDFAB1206	LEDFAB1206	LED1206FAB	LED
XIAO	10201038814PIN_600MIL-SMD	10201038814PIN_600MIL-SMD	14PIN_SMD-600MIL	Check availability

Fab_Xiao PCB Download Here