I have the following photodiode (this one) connected to an Arduino Uno, and I would like to be able to record at least the waveform of an incoming light signal whether it is off a simple blinking LED or off a PC monitor directly. For now I am running my tests on an LED.
so far I am able to record the following signal off a blinking LED (on/off) after few filtering like exponential filtering followed by moving average. However, I noticed that the waveform looks reasonable only for very low frequency like 5 Hz where the waveform looks like square (first figure) but not when the frequency starts to get higher like 30 Hz or higher where the waveform starts to be less regular and more messy (second figure).
Any advice or tips for how to filter the signal better and be able to capture the waveform more clearly without being limited by the frequency (It would be great if I can measure the waveform at a high frequency like up to 300 Hz. However for now I would like at least to be able to measure it at 60, 120 Hz if possible).
Arduino Code:
//#include <TimerOne.h>
#include <Arduino.h>
const unsigned long ONESEC = 1000000; // microseconds
const int size = 600; // number of samples in 1 second
int sensorValue[size];
int dly = ONESEC / size; // delay so to fill N samples during 1 complete second (1,000,000 microseconds)
int maxValue = 0;
int minValue = 1000000;
// the following parameters are used for exponential filtering
double Alpha = 0.3; // [0.0 - 1.0] weight parameter
double filteredValue = 0.0;
double prevValue = 0.0;
double avgsum = 0.0;
// the following parameters are used for moving average filtering
int total = 0;
int avgtotal = 0;
const int s = 10;
int rounds = size / s;
int tmparr[s];
const int btnPin = 7;
const int sensorPin = A0; // sensor pin
void setup() {
Serial.begin(57600);
// init tmparr
for(int i=0;i<s;i++){
tmparr[i] = 0;
}
}
void waitForButton(int btnPin){
Serial.println("Waiting...");
while(digitalRead(btnPin) != 1){
continue;
}
Serial.println("Button has been pressed!");
}
void loop() { // code that loops forever
// waitForButton(btnPin);
// for(int i=0;i< size;i++){
// // Read sensor value
// sensorValue[i] = analogRead(sensorPin);
// // wait between readings for N microseconds
// delayMicroseconds(dly);
// }
int i = 0;
unsigned long curTime = micros();
unsigned long prevTime, startTime;
prevTime = curTime;
startTime = curTime;
while(curTime - startTime <= ONESEC){ // sofar the time between the current and the start time is less than 1 second, keep recording data
if(curTime - prevTime < dly){ // sofar the current and the previous time stamp has a differene of less than N seconds, do not record anything
curTime = micros();
continue;
}
sensorValue[i++] = analogRead(sensorPin);
if(i > size){
break;
}
prevTime = curTime;
curTime = micros();
}
///////////////////////
///////////////////////
// Data processing ////
///////////////////////
///////////////////////
// filter the signal and find maximum/minimum and avg
for(int i=0;i<size;i++){
int v = sensorValue[i];
filteredValue = (v * Alpha) + (prevValue * (1-Alpha)); // exponential filtering
sensorValue[i] = filteredValue; // re-writing the sensor data !! RAW DATA ARE BEING OVERWRITTEN !!
avgsum = avgsum + filteredValue;
prevValue = filteredValue;
if(maxValue < filteredValue){
maxValue = filteredValue;
}
if(minValue > filteredValue){
minValue = filteredValue;
}
}
avgsum = avgsum / size;
// plotting and segregating the signal
int peaks = 0;
int prevState = -1;
// moving average filter
for(int i=0;i<rounds;i++){
for(int j=0;j<s;j++){
int v = sensorValue[(i*s)+j];
total = total - tmparr[j];
total = total + v;
avgtotal = total / s;
tmparr[j] = v;
Serial.print(avgtotal); // new filtered value
Serial.print(",");
Serial.print(avgsum);
Serial.print(",");
Serial.println(v); // filtered value
if(avgtotal > avgsum + (avgsum*0.01)){
// Serial.println(1);
if(prevState != 1){
peaks++;
prevState = 1;
}
}
else{
// Serial.println(0);
prevState = 0;
}
}
}
Serial.print("peaks:");
Serial.println(peaks);
///////// process the cycles in the signal
// we have N samples (size) every (dly) second
}
- yes, sometimes this true, but you will not be able to distinguish the frequency N + x from the frequency N - x, where N is the frequency determined by the theorem.