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Portable Low Cost Spectrometer

Portable Low Cost Spectrometer. Design Review March 22, 2007. Brett Hamilton Kodin Taylor Mateusz Wyganowski. Outline. Background High Level Design Overview Texas Instruments TSL1401-LR Sensor Array Atmel Atmega8 Microcontroller Master Graphical User Interface Electrical Schematic

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Portable Low Cost Spectrometer

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  1. Portable Low Cost Spectrometer Design Review March 22, 2007 Brett Hamilton Kodin Taylor Mateusz Wyganowski

  2. Outline • Background • High Level Design Overview • Texas Instruments TSL1401-LR Sensor Array • Atmel Atmega8 Microcontroller • Master Graphical User Interface • Electrical Schematic • Demo

  3. Background • The purpose of this project is to create a portable optical spectrometer with inexpensive components for use in environmental monitoring • Current optical spectrometers are often expensive and generally provide more power and functionality than is necessary for our desired use • Inexpensive light sensors have become available that when coupled with a prism will be able to detect levels of light at various wavelength ranges in the visible spectrum.

  4. Cost Estimation • Microcontroller: $3-4 • PCB Manufacture: $100 • Sensor/Prism and Assembly: $20-30 • RS-232 Interface chip: $10 • Power Regulator $10 • Total: < $170

  5. High Level Design Overview

  6. Texas Instruments TSL1401R Sensor Array • 128 x 1 Sensor-Element Organization • 3-5 Vcc • 2 Digital Signal Inputs • SI • CLK • Analog Output • AO

  7. Sensor Array Cont.

  8. Sensor Array Cont.

  9. Sensor Array Cont.

  10. Atmel Atmega8 Microcontroller • Features • 2.7 – 5.5V, ~7-15 mA • 0-16 Mhz CPU Clock • 8K Bytes of Flash • 512 Bytes of EEPROM • 1K Byte of SRAM • One 16-bit Timer/Counter with Separate Prescaler • Sensor Interface • Digital I/O (output) • 10-bit Accuracy ADC Input (input) • Master Interface • Programmable Serial USART with Separate Prescaler • External Clock source • Power-down modes • C Programming Language Environment

  11. Microcontroller Software Internals • Interface with Sensor • Reduced Power Mode • Raw Sensor Value Adjustment • Communication Protocol

  12. Sensor-Microcontroller Interface (Control Output) • Control Signals • Pins 0 and 1 of PortB • 16-bit timer with Periodic ISR • One large c switch statement • Cases • LO_HI, HI_LO, AD_SAMPLE

  13. Example ISR Case Statement case SA_LO_HI: PORTB |= _BV(SSIG_CLK); sensor_state.tick++; if( sensor_state.reading == TRUE ) { sensor_state.next_action = SA_AD_START; next_time = T_S; } else { sensor_state.next_action = SA_HI_LO; next_time = T_PER/2; } break;

  14. Sensor-Microcontroller Interface (Analog Input) • Reading Analog Values • 10-bit Analog-to-Digital Converter • AD Conversion Done Interrupt • Fires automatically when ATC is done after every AD_SAMPLE case in the timer ISR

  15. Sensor-Microcontroller Interface

  16. Reduced Power Mode • The ADC is turned on just before starting the read cycle and back off when finished. • Idle Mode is used when waiting for instructions. • ADC Noise Reduction Mode is used each time starting an ADC sampling. The ADC module generates an interrupt when value is ready.

  17. Value Normalization • Sensor Elements are not equally sensitive to entire supported wavelength range. • Normalization value for each element (128 total) • Possible values 1-65535 (1024 => 1) • Algorithm • output_val : uint16_t // Final normalized value • temp32_val : uint32_t // 32 bit value for calculations • raw_val : uint16_t // raw value from the sensor • norm_val : uint16_t // normalization “multiplier” • for each raw_val, norm_val value pair in memory • temp32_val = raw_val * norm_val // 32 bit result • temp32_val = temp_val >> 10 // Normalize • temp32_val = MIN( temp_val, ADC_MAX_VAL ) // Bound value • output_val = (uint16_t) ((0x00FF) | temp_val) // Lower 16-bits

  18. Master-Microcontroller Interface (Comm. Protocol) • ASCII Protocol Messages Consist of • One Start Delimiter • $:Command, Always Master  Microcontroller • @: Notification of success, Microcontroller  Master • !: Notification of error, Microcontroller  Master • %: Reserved for sensor values • *: Miscellaneous Information shown when debugging and log files. • Two character command identifier • Optional colon and comma separated parameters • Two End delimiters (CR, LF)

  19. Command Handlers • Each command is paired with a handler function. • On incoming message, correct handler function is found based on 2 character command “key”. • Handler function is responsible for parsing parameters, performing the requested operation, and returning a success or failure message.

  20. Master Graphical Interface

  21. Program Operation • Static “comm controller” class maintains serial connection • Read thread continuously listening to serial port for incoming data • All incoming comm traffic is sent to static main class for processing

  22. Sending Commands • All commands sent with $ as prefix • $sc<CR><LF>

  23. Programming Mode • Sample Programming Command String • $sv:VAR_BAUD_RATE,115200<CR><LF>

  24. Get / Set Variables • Variables that are accessible using SET_VAR and GET_VAR are as follows: • VAR_SREAD_INT: The rate at which to send continuous sensor updates to the master in milliseconds. The minimum value allowable is 100 ms. • VAR_RD12_INT: The time between the flush read and actual read (described later). • VAR_BAUD_RATE: serial baud rate. Valid values are all possible baud rates between 1200 and 115200. The value will take effect on reset. • VAR_WV_LOW: The wavelength value detected by the photodiode at the low extreme. • VAR_WV_HIGH: The wavelength value detected by the photodiode at the high extreme.

  25. Command Confirmations • Command confirmations prefixed with @ • Data requests returned with values appended to the confirmation string • Example: • Sent: • “$rf<CR><LF>” • Received: • “@rf:450,800<CR><LF>”

  26. Error Messages • Errors prefixed with ! • Returns the command that caused the error as well as possible parameters or descriptions • Example • Sent: • “$rf<CR><LF>” • Received: • “!rf:WV_LOW and WV_HIGH not set.<CR><LF>”

  27. Receiving Data • Incoming data prefixed with % • String is tokenized and parsed into integers • Parsed data is used to update GUI’s graph and tables • Example: • “%10,25,0,13,26,0,0,1,115…<CR><LF>”

  28. Data Handling • GUI • Data is plotted on a large graph • Actual data values displayed in a table with associated wavelength • Log File • Appended data is saved in a CSV file for archival purposes as well as possibly generating reports

  29. Other Messages • Other Messages prefixed with * • Anything else the microcontroller wishes to convey is ignored and sent to a debug file

  30. Electrical Schematic

  31. Linear Regulator vs. + Simple + Cheap - Potentially Inefficient (heat) Switching Regulator - Complex - Noisier output voltage + Efficient Power Supply Design

  32. Power Supply Design

  33. Power Supply Design

  34. Power Supply Design

  35. TSL1401-LR Sensor Array

  36. TSL1401-LR Sensor Array

  37. MAX232 Level Shifter

  38. Noise Reduction • Power Supply • Placement of decoupling capacitors at each chip’s Vcc (as close as possible) to provide sudden spikes in current. • ADC • Separate ADC reference point filtered with an inductor and decoupling capacitor. • Crystal Oscillator • Placement as close to microcontroller as possible. • Surround with ground plane.

  39. Demo • Master and Microcontroller Communication

  40. Feedback?

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