Three-speaker Chladni Patterns

Team Members

• Christopher Chow (Mechanical Engineering, Class of 2010)
• Anup Tapase (Electrical Engineering, Class of 2010)
• Lingyu Xie (Electrical Engineering, Class of 2009)

Electrical Design

Circuit Diagram

Circuit schematic of circuit used for demonstration 3-speaker Chladni

TDA-2040 Audio Amp Circuit[1]

Code

Main frequency sweep code

The first thing that this code does is that it initializes the variables target_freq_reg and old_target_freq_reg to the register value of 298 Hz, which is a non-resonant frequency. See Waveform_Generation_with_AD9833,_and_SPI to find out how to convert between frequency and register value for commands sent to the AD9833 function generator chip, and sending commands to the AD9833 using SPI. After this, the analog port pin is set up and the lcd_init() function is called to set up the LCD display. The LCD displays the current frequency that the AD9833 is currently outputting and the target frequency that the chip is supposed to sweep up/down to. See C Example: Parallel Interfacing with LCDs to learn about how to interface and send information to the LCD.

Once the initial set up information is finished, the code sends the first frequency command to the AD9833, starting it at a frequency of 298 Hz by giving it the target_freq_reg register value (initialized at 298 Hz). This allows the speakers to sweep up to the first resonant frequency and oscillate the salt into the first resonant shape when the system is turned on. The register values are then converted to Hz and displayed on the LCD. From there, the code goes into a while() loop which continuously checks the input from the user interface knob which indicates the target frequency that the AD9833 should go to. After getting the target frequency from check_input(), the code compares this new frequency information to the old frequency information (old_freq_reg). If they are different, then the knob was switched to a new frequency and the AD9833 needs to sweep up or down to the new frequency. Depending on whether the new frequency is above or below the old frequency, a for-loop will keep sending new frequency register commands to the AD9833, shifting up or down by 1 Hz every 100 ms, constantly updating the LCD to display the current and target frequency, until the actual frequency is equal to the target frequency. The function check_input() is called for each iteration of the for-loop to check if the target frequency was changed. The old_target_freq_reg is then set equal to the target_freq_reg and the program then holds at this one resonant frequency until the knob sends a different target_freq_reg.

#include <18f4520.h>
#DEVICE ADC=8                   // set ADC to 8 bit accuracy
#use delay(clock=40000000)
#include "flex_lcd.c"             //must include in order to output to LCD
#use spi(DO = PIN_A3, CLK = PIN_A2, ENABLE = PIN_A1, BITS = 16, MASTER, ENABLE_ACTIVE = 0, MSB_FIRST, IDLE = 1)

int16 freq,target_freq;
int16 target_freq_reg, old_target_freq_reg;
int8 knob_val=0;

void check_input();

void main()
{
   int16 ct;
   
   target_freq_reg=2000+16384;         // initialize starting frequency of speakers to 298 Hz (non-resonant)
   old_target_freq_reg=2000+16384;     // set old target freq variable to equal target freq

   setup_adc_ports(AN0);               // Set up analog input port as pin A0
   setup_adc(ADC_CLOCK_INTERNAL);
   
   lcd_init();  // Always call this first.
   
   //INITIAL FREQUENCY FOR AD9833
   spi_xfer(0b0010000100000000); //format command, output sine wave
               
   spi_xfer(target_freq_reg);    //1st set of bits, 14 LSB, this range is good enough for our use
                                 //send the target frequency register value (freq + 16384) to AD9833 chip
   spi_xfer(0b0100000000000000); //2nd set of bits, 14 MSB
   
   spi_xfer(0b1100000000000000); //phase register: 0 phase shift (B0-B13)
   
   spi_xfer(0b0000000000000000); //unformat
   
   freq=(float).149011 * (float)(target_freq_reg - 16384);  //convert the target freq register value to actual freq value
   target_freq=freq; // initialize target freq as current freq

   printf(lcd_putc,"\fFreq: %Lu Hz\n",freq); //display current freq
   printf(lcd_putc,"Target: %Lu H\n",target_freq); //display target freq
   
   while(TRUE)
   {
      check_input(); //check the knob input to see if target frequency has changed
      
      if(old_target_freq_reg != target_freq_reg) //go in here if target freq changed after calling check_input()
      {
         if(target_freq_reg > old_target_freq_reg) //sweep up to target freq
         {
            for(ct=old_target_freq_reg; ct<=target_freq_reg; ct=ct+7) //ct+7 approximate increases frequency by 1 Hz
            {
               spi_xfer(0b0010000100000000);
               
               spi_xfer(ct); //set AD9833 frequency to ct, which is freq register that is sweeping up by 7 each time loop is run
               spi_xfer(0b0100000000000000);
               
               spi_xfer(0b1100000000000000);
               
               spi_xfer(0b0000000000000000);
               
               freq=(float).149011 * (float)(ct - 16384);
               target_freq=(float).149011 * (float)(target_freq_reg - 16384);
         
               printf(lcd_putc,"\fFreq: %Lu Hz\n",freq); //display current freq
               printf(lcd_putc,"Target: %Lu Hz\n",target_freq); //display target freq
               
               check_input(); //see if knob position has selected different target freq
               delay_ms(90);  //delay to allow speakers to play freq for 90 ms + 10 ms (in check_input() function)
            }
               
            old_target_freq_reg = target_freq_reg; //reached target freq, set both variables equal until knob position changed
               
         }
         else if(target_freq_reg < old_target_freq_reg) //sweep down to target freq
         {
            for(ct=old_target_freq_reg; ct>=target_freq_reg; ct=ct-7) //ct+7 approximate decreases frequency by 1 Hz
            {
               spi_xfer(0b0010000100000000);
               
               spi_xfer(ct); //set AD9833 frequency to ct, which is freq register that is sweeping down by 7 each time loop is run
               spi_xfer(0b0100000000000000);
               
               spi_xfer(0b1100000000000000);
               
               spi_xfer(0b0000000000000000);
               
               
               freq=(float).149011 * (float)(ct - 16384); //convert current freq register value (ct) to frequency value
               target_freq=(float).149011 * (float)(target_freq_reg - 16384); //convert target freq register to target freq value
         
               printf(lcd_putc,"\fFreq: %Lu Hz\n",freq); //display current freq
               printf(lcd_putc,"Target: %Lu Hz\n",target_freq); //display target freq
               
               check_input(); //see if knob position has selected different target freq
               delay_ms(90);  //delay to allow speakers to play freq for 90 ms + 10 ms (in check_input() function)
            }
   
            old_target_freq_reg = target_freq_reg; //reached target freq, set both variables equal until knob position changed
         }
      }
   }
}

Checking user input

This function is used to check the knob/potentiometer position at certain points during the changing of frequencies in the main function. It sets the ADC channel to 0 and takes the analog input from pin A0 on the PIC using read_adc(). This input, set to the variable knob_val, is an integer between 0-255 which corresponds to the voltage coming out of the knob/potentiometer, which is 0 if the output is at 0V and 255 if the output is at 5V. By checking if knob_val is between a certain integer range corresponding to the different numbers on the knob (numbered 0-10), the target frequency can be set by the user. Nine target resonant frequencies were programmed in this function based on the good clarity of the shapes they produced on the plate, but more resonant frequencies exist and can be added to the if-statement in this function.

void check_input() //check knob position to see if target freq has changed
{      
      set_adc_channel(0);     // Set the analog input channel to 0
      delay_us(10);           // wait 10uS for ADC to settle to a newly selected input
      knob_val = read_adc();  // Read in knob user input to tell speakers what resonant freq to sweep up to
      
      delay_ms(10);           // delay 10 ms to allow reading of analog input
      
      //check and set target_freq_reg according to knob position (0-255 values split into 9 discrete levels)
      if (knob_val >= 0 && knob_val< 22 )
         target_freq_reg = 2281+16384; //339 Hz
      else if(knob_val >= 22 && knob_val< 54)
         target_freq_reg = 3308+16384; //493 Hz 
      else if(knob_val >= 54 && knob_val< 86)
         target_freq_reg = 3778+16384; //563 Hz
      else if(knob_val >= 86 && knob_val< 117)
         target_freq_reg = 3986+16384; //594 Hz
      else if(knob_val >= 117 && knob_val< 147)
         target_freq_reg = 4207+16384; //627 Hz 
      else if(knob_val >= 147 && knob_val< 178)
         target_freq_reg = 4362+16384; //650 Hz 
      else if(knob_val >= 178 && knob_val< 209)
         target_freq_reg = 5006+16384; //746 Hz 
      else if(knob_val >= 209 && knob_val< 239)
         target_freq_reg = 5308+16384; //791 Hz 
      else if(knob_val >= 239 && knob_val< 255)
         target_freq_reg = 5872+16384; //875 Hz 
}