Week 8: Electronics Production

*First and final board of the week.

Group Assignment

Find the complete group assignment here.

Safety training

There are two main types of PCB copper blanks: FR-1 and FR-4. We use FR-1, whereas FR-4 is considered toxic.

Difference between FR-1 and FR-4:

FR-1: paper-based, less heat-resistant, safer to mill.
FR-4: fiberglass-based, highly heat-resistant, generates hazardous glass dust when cut.

Machine calibration

At our node, we have Roland SRM-20 milling machine. Randomly enough, I found this video from FabLab Aalto. Students from my node, who had graduated in 2023, had used the video to set up the same machine.

To set your X and Y lines, use the controls to move the cutting bit to your starting spot. This is usually the bottom-left corner. Press the X/Y button, and the machine will mark this exact spot as your zero point.

Whenever you change the bit, the height changes too, so you will need to reset the Z axis. Lower the bit until it is close to the board. Next, use the right hex key to loosen the bit, let it drop gently until it touches the surface, and tighten it back up. Then, just press the Z button to lock in your zero height.

Keep in mind that if you use .nc files, the machine can save a few different starting locations using codes G54 through G59. If your files come from modsproject, it will use G54 automatically.

Test 1: milling

*Prompt8.1

There are two main types of milling bits – V-bit and regular endmill. To test and see the differences between the results of these two we downloaded the ‘traces’ file, and cut it on our laser cutter.

Test 2: etching

For our group assignent, Mariam and I took a scrap piece of FR-1 PCB blank, and cut it into two parts, using the DeWALT Scroll Saw. Then we painted it on both sides with matte black spray paint, and left it to dry overnight. Again using the ‘traces’ file, we etched it on the laser cutter.

To ensure maximal removal of the paint, and distinct paths, we took a Petri dish, and poured a natural etching solvent.

The solution proportions:

1 part vinegar
1 part hydrogen peroxide
a pinch of salt

In reality, this was our second attemp of the same test. First and most important change was leaving the spray paint dry overnight. The second, we ended up changing the proportion of the ingridients. We decided to make these changes as the first etched PCB had no masked lines left, which would be a failure if it were an operational PCB.

NOTE: the initial proportions were 3:2, and a pinch of salt.

Individual Assignment

For my final project I am designing a drone rescue parachute.

The components:

• 1x XIAO RP2040
• 1x GY-521 Accelerometer and Gyroscope
• 1x Buzzer
• 1x 1206 SMD LED
• 1x 0Ω Jumper Resistor
• 1x 220Ω Resistor

NOTE FROM FUTURE: After revising, I also added a 10kΩ resistor for the ADO port on GY-521, and switched the places of SCL and SDA on the XIAO RP2040.

Step 1: designing

Yet againg we return to KiCAD.

First you open the Schematic Editor, and add the predetermined list of components. When adding these onto the sheet you need to make sure the symbols also come with footprints. If not, you either go online and download the footprints corresponding to your pieces, or create them yourself in Symbol/Footprint Editor [depending on your needs].

Once you have layed out a desired schematic, you switch to PCB Editor, and wire the board according to the pieces.

Schematic editor

In my example, I did not have a symbol or a footprints for my buzzer. So I googled the name of it online and found the files through this website.

If I were unable to find them online also, my steps would be to measure the distamce between the anode and the cathode and sketch a simple circle with two holes in it. This would serve as the soldering pinholes for the buzzer.

PCB editor

After having every component present in the schematics, I switched to PCB editor. There I placed everything as neatly and tightly as possible. For through hole components I pressed F to mirror their footprints, and flip them upside down in 3D view.

For this PCB I have decided to use the copper surface area as the groud. To mark this in the editor you need to go into blue tool with a dot, or press B, and mark the area for GND.

Additionally, in Constraints, under Design Rules I typed in the sized of the paths and through holes.

Step 2: G-code

To generate the toolpath for the milling machine go to mods, a web-based CAM.

To Generate G-code: Program > Mill 2D PCB > Select file > Invert > Choose bit type > Calculate > View > Toggle switch on > View again > Download

Step 3: milling

I began by attaching an FR-1 PCB blank with double-sided tape onto the machine bed. Then I clamp it down with for about 5 minutes to ensure good adhesion.

Once the .nc files are ready, typically two file, one called F_Cu for the ground wiring, the other called Edge_Cuts for edges and through holes. Once in a while you might also have other layers, i.e. a Silkscreen, to carve text, or logos, but not cut through the copper layer.

I then sent the .nc files to the computer which has the VPanel app connected to the Roland SRM-20. In the interface I level out the X/Y axes, then finally the Z axis. The Z-axis requires one to untie the milling bit, set the zero, and the tighten the milling bit back.

Only if all the axes are leveled, press Cut > Delete All > Add, select the file to be cut and finally press Output.

As it did not go according to plan, I went back to the scroll saw, and taped a piece of plywood onto the cutting bed to ensure a straign cut. I wired the steel wire though the copper plate, and cut the 4th edge off.

Step 4: soldering

After milling the PCB, I fetched the electronics components to begin soldering.

The code

To make sure I have life in this PCB, I wrote a simple code to make the SMD LED blink, and the buzzer tweet. But then after, I asked Claude.ai to generate code based on the MPU6050 library, which would also take in account the gyroscope.

I then found this website with some documentation on how to wire the gyroscope with the RP2040. The code did not use the library I had, which was MPU5060.

I asked chatGPT to take the minimal code I had, and connect it to the reference code above, yet using the functions found in my downloaded library. Here was the prompt I gave to it:

C++

#include <Wire.h>
#include <MPU6050.h>

#define LED_PIN  D0
#define BUZZ_PIN D2
#define THRESHOLD 5000

MPU6050 mpu;

void setup() {
  Serial.begin(115200);
  while (!Serial);

  pinMode(LED_PIN,  OUTPUT);
  pinMode(BUZZ_PIN, OUTPUT);

  // Startup blink/beep
  digitalWrite(LED_PIN,  HIGH);
  digitalWrite(BUZZ_PIN, HIGH);
  delay(500);
  digitalWrite(LED_PIN,  LOW);
  digitalWrite(BUZZ_PIN, LOW);

  Wire.begin();
  mpu.initialize();

  if (!mpu.testConnection()) {
    Serial.println("MPU6050 not found! Check wiring.");
    // Rapid blink = error
    while (1) {
      digitalWrite(LED_PIN, HIGH);
      delay(100);
      digitalWrite(LED_PIN, LOW);
      delay(100);
    }
  }

  Serial.println("MPU6050 Found!");
}

void loop() {
  int16_t ax, ay, az;
  int16_t gx, gy, gz;

  mpu.getMotion6(&ax, &ay, &az, &gx, &gy, &gz);

  Serial.print("Accel  X: "); Serial.print(ax);
  Serial.print("  Y: ");      Serial.print(ay);
  Serial.print("  Z: ");      Serial.println(az);

  Serial.print("Gyro   X: "); Serial.print(gx);
  Serial.print("  Y: ");      Serial.print(gy);
  Serial.print("  Z: ");      Serial.println(gz);
  Serial.println("---");

  if (abs(gx) > THRESHOLD ||
      abs(gy) > THRESHOLD ||
      abs(gz) > THRESHOLD) {
    Serial.println(">> MOVING!");
    digitalWrite(LED_PIN,  HIGH);
    digitalWrite(BUZZ_PIN, HIGH);
    delay(300);
  } else {
    digitalWrite(LED_PIN,  LOW);
    digitalWrite(BUZZ_PIN, LOW);
  }

  delay(200);
}

*Prompt8.2

Problems

The first problem was the incorrect of the toolpath generation, please scroll up to Step 3: milling to read more.

The second problem: things seemed to be worse than I thought they’d be. I quite liked my fisrt-time soldering reults, but soon after found out that some pins were placed incorrectly, to be precise SDL and SCL.

Also, on my GY-521 [gyroscope and accelerometer], I disregarded the ADO pin, which was supposed to be connected to GND for this purpose. Therefore after I had already milled the FR-1, I was advized by my local instructoir Onik, to bridge the ADO pin with a 10kΩ resistor.

Conclusion

Resources

Source files:

• PCB Schematics

Prompts

Prompt8.1 Please visualize an FR-1 with a copper blank, and a PCB milling machine. It should take away the copper layer, leaving the backing to created isolated paths.

Prompt8.2 Write a simple C++ script for an RP2040 that blinks an LED on pin D0 and beeps a buzzer on D2 when the MPU6050 gyro senses a lot of movement. Please make sure the code won’t freeze if the board is powered by a battery instead of plugged into a computer. If the gyro chip isn’t wired right or can’t be found, make the LED blink super fast continuously so I know there is an error. Finally, add placeholders where I can easily type in my specific I2C pins, and make the LED and buzzer trigger whenever the gyro readings pass a threshold of 5000.