Embedded Files
Intent and Purpose
The goal of OBATS (On-Board Analog Temperature Sensor) is to develop a practical breakout board for the Raspberry Pi Zero Computer that contains an analog temperature sensor and MPU6050 Gyroscope/Accelerator embedded within the required circuitry for the interpretation of a digital system. The resulting output will be received by the necessary pins throughout the 40-Pin GPIO on the Pi Computer. The final product will be connected in the cabin of a car where we can gather desired information.
From this project we are determined to extensively understand the operation of each component. This includes the ADC, Temperature Sensor, Operational Amplifier, and the H/L Pass Filters. With this approach, proficiency in professional software like Altium Designer and LTspice will be refined.
Method
Discuss various ideas and approaches
Sketch a Block Diagram to define operation
Characterize design requirements and conditions
Source components
Create a Schematic
Review Schematic and convert to PCB
Size PCB to fit as a Pi Zero Breakout Board
Review and place order
Testing and Assembly
(OBATS VER_C)
Operational Amplifier Block
Using LTspice, the Analog Temperature Sensor VOUT is simulated as "tmp235". The output was acquired from the datasheet voltage output values from -40 degC to 150 degC and fed into an Excel Spreadsheet to obtain a function representing the unaltered output (Green). The Resulting Output (Blue) is the result of the Low Pass Filter and Operational Amplifier. The final range of Analog VOUT is 0.2v to 3.36v. The intent of this block is to achieve a wide spread of analog values that can be fed into any channel of the ADC for much more precise conversion that can be represented across more digital values.
Our Operational Amplifier has a Gain of 1.68 determined by R2 and R3. The Gain can be multiplied by each Initial voltage value to determine the Resulting.
Analog to Digital Converter
In Altium Designer, the Resulting Output is connected to one of our eight channels of the ADC (MCP3008) along with other pins coming from the GPIO header.
The Low pass filter operates in the following way. Capacitors are an active device they will offer resistance for different levels of frequencies, offering very high resistance for low frequencies and very low resistance for high frequencies. This is due to their "charging" operation. In this situation, our low pass filter favors low frequency signals so our capacitor has a very high resistance. The current will take the path of least resistance and low frequencies will pass with ease while high frequencies run to ground.
ADC Calculations
The LSB or Least Significant Bit is the smallest level that an ADC can convert. The ADC needs a reference voltage to convert an analog signal, Varying by the number of bits it has. The ADC will divide the reference into small levels called counts. In our 10-Bit ADC we have 1024 counts(2^10). Our reference is 3.3V making our LSB equal to 3.2mV according to the provided equation from this value we can work backwards and see how with each 3.2mV increment which we have 1024 of, they add up to 3.3V.
The Digital Output Code is how the ADC represents each incoming Analog value as a Digital Code. For example, a temperature of 30 degC may provide us with a 800mV (.8V) which is plugged into this equation to get 248 and when converted to a binary digital output 11111000.
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