9. Input data from the outside world

This week I prototyped some input tags for my spice bottles, and a corresponding Multiplexer PCB board which can expand the ability of my Xiao board to read up to 16 tags.

An input device is a piece of hardware that sends data to another device or digital system, translating environmental data and human actions into signals that the machine can understand. - Wikipedia

This week’s assignments:

Group assignment:
- Probe input device(s)’s analog levels and digital signals

Individual assignment:
- Measure something (Add a sensor to a microcontroller board that I have designed, and read it)

Groupwork:

In this week’s group assignment, we probed 3 input devices’ Analog levels. When I have the chance, I would also like to look into understanding more about Digital signals, as I’m wondering how that might help me improve my final project design.
Link to Group Assignment documentation.

Measure something with my microcontroller board:

This week I made a range of input tags with different resistance values, and a board for reading these tags (which is a Multiplexer PCB that expands the Analog pins of my Xiao PCB to read up to 16 different Analog inputs).

My personal goals for this week’s assignment was to answer these questions;

• How can I use the multiplexer to expand the number of analog inputs? and what’s the time lag of this multiplexer to read multiple inputs simultaneously?
  
• How do different placements of resistor(s) translate to actual voltage readings? What range/variety of tags could potentially be provided with this approach?
  
• And how might the presence of magnets impact the resistor readings?
• What are potential designs for tags (and corresponding slots for the tags), and how could I optimise the user experience?
  

1. Grasping the mechanism

The biggest hurdle in this week’s assignment was actually getting my head around the mechanism of the circuit I was building. Unfortunately, since I needed to complete the circuits by the end of the week, I only fully understood this mechanisms after I long finished building and testing the circuit. But for the reader of this documentation, it will help to understand the whole picture before reading about the development process.

Essentially, the circuit I made is based on the concept of the Voltage divider.
Placing two resistors in series creates a lower Voltage output (Vout) from a higher Input voltage (Vin), and the Voltage out is proportional to the ratio of the resistor values (R1 and R2).

The output voltage (Vout) is calculated using the following formula:
Vout = (R2 / (R1 + R2)) * Vin
For example;

The circuit I created this week was based on this principle; using a voltage divider and variety of Input tags to create a range of Voltage out readings, that will enable me to assign unique tags for each spice bottles.

Based on this understanding, I was able to calculate the expected Vout values of each tags, by calculating the relative resistance of R2 versus the R1 of 100 Ohms. Seeing these values gave me much better idea of the range of tags I should have ideally tested (although unfortunately it was a little too late).

Once I understood this mechanism, I realised that I was essentially making a very basic type of Potentiometer. And that perhaps what I should be doing is to look into making a simple Potentiometer using some type of resistive material (instead of struggling to string together multiple resistors, which are limited in their variety).
But the assignment served the purpose of helping me understand this, and I hope to continue exploring this approach for my final project.

2. Breadboarding a Test Circuit

With great help from Nagano-san, I eventually managed to Breadboard the below test circuit, which I could in theory use to test various resistor combinations.

I didn’t manage to test a lot of tags with this circuit, as for some reason, the readings on my Serial Plotter was fluctuating at high frequency, which made reading their values a little tricky. When we plugged the same circuit and code to Nagano san’t computer, it produced relatively clean, straight lines. Nagano san initially thought the problem was the AC adapter not being plugged in (resulting in less secure ground connection) but plugging it in didn’t make a difference. Since the circuit itself was working on Nagno san’s Pc, I decided it must be functional and proceeded to the board fabrication.

For reference, this is the Multiplexer board I used;

16-channel Multiplexer	Specs
Chip used	HP4067
power supply voltage	2-6V
On-state resistance	70Ω at 4.5V supply, 60Ω for 6V
Dimensions	41x19mm
Arduino library	Link

And this is the code I used;

#include <CD74HC4067.h> // Include the multiplexer board library

#define SIG_PIN D0 // Define D0 pin as the Analog Sig_pin

CD74HC4067 my_mux(D10, D9, D8, D7);  // Xiao D10-D7 pins connect to Multiplexer S0, S1, S2, S3 pins

void setup() {
  analogReadResolution(12); //Set the resolution to 12bit (Convert the input voltage into digital values of 0~2046)
  int sensorValue = analogRead(D0); // Read the Analog inputs from pin D0 as integer values
  Serial.begin(9600); // Initialise serial communication at 9600 bits per second
}

void loop() {
    Serial.print(0); // Print the lowest value
    Serial.print(",");
    Serial.print(2046); // Print the highest value
    Serial.print(",");

    for(int i=0; i<5; i++){ // For inputs from pin# i to i<5...
    my_mux.channel(i);
    Serial.print(analogRead(SIG_PIN)); /// Read the Analog value on D0 sig pin
    Serial.print(",");     // and Serial.print that value
    delay(1);
    }
    Serial.println("");
}

3. Exploring Tag designs

Before designing the Tags, I tried to explore several potential approaches for physically implementing the tags on the spice bottles.
See the Final project section for documentation. (to be updated)
At this moment, I am leaning towards the third design, because of its simplicity and flexibility.

4. Creating the devices (Tags & Board)

For my input tags, I designed these 8 with different resistance values (some for soldering overlapping resistors - i.e., to try Parallel circuits). Later, I realised that I should have made more variety of the serial circuits (with more resistors strung together), as the objective was to see where the fluctuating lines would start to overlap (due to each resistors’ deviation/tolerance values).

I also designed a Multiplexer board for simultaneous analog input.

There was no footprint available for the Multiplexer I was using.
so I decided to modify pin headers
I used Xiao Socket footprint as reference for the distance between the headers.
xxxxx

Once happy with the design, I milled and soldered them.

After soldering, I used the multimeter to test the board for continuity and short circuits. As I learnt later, it is very important to thoroughly and systematically test the circuit and fix any bad wiring/soldering or faulty components before plugging in the board.
Because I failed to spot a short circuit during this step, I nearly burnt my PC’s USB port when I plugged the board to the PC.

5. Reading the Tags with the Board

I connected all the boards together as outlined before, and uploaded the same code from the breadboard test.

These were the readings I got xxxx

Useful links:

• Useful Multiplexer documentation
• Multiplexing Code for Arduino
• Multiplexer Hookup Guide
• Multiplexer Datasheet

https://www.instructables.com/Wire-a-Potentiometer-as-a-Variable-Resistor/ Normally, potentiometers are wired as variable voltage dividers: connect +V to one side, connect the other side to ground, and the middle pin will output a voltage between 0 and +V (fig 2).

https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer Automotive ignition wires (Graphite cores) should yield somewhere around 10000 ohms per foot, and they are nice and flexible.

Design files:

• xxx

Reflections:

This week I learnt to…demonstrate workflows used in sensing something with input device(s) and MCU board

I understood the mechanism of the Voltage Divider Helped me deepen my understanding of Electronics, particularly the relationship between Voltage, Current and Resistance

What’s the difference and benefits/disadvantages of digital and analog signals?? And how is that relevant for my final project?

Assignment Checklist:

• [ x ] Linked to the group assignment page
• [ ] Documented what I learned from interfacing an input device(s) to my microcontroller and optionally, how the physical property relates to the measured results
• [ x ] Documented my design and fabrication process or linked to the board I made in a previous assignment
• [ ] Explained the programming process(es) I used
• [ ] Explained any problems I encountered and how I fixed them
• [ ] Included original design files and source code
• [ ] Included a ‘hero shot’ of my board