For the latest IgniteGuelph contest, we needed a way to measure the speed of homebuilt electric motors. Simple RPM, not torque was the goal, so we opted to go with a light sensor on an arduino to count the light\/dark cycles.<\/p>\n
First, we need to figure out the threshold value of the light sensor. \u00a0We started with a CdS photocell, but the response time was too slow to count anything faster than 150 rpm. \u00a0Working with what we had on hand, we built a photo sensor with an LED. This tested out fine up to 600 RPM, or so. \u00a0Ideal would be a proper photodiode or phototransistor, but we were limited to what we had on hand.<\/p>\n
#define LED_N_SIDE 3
\n#define LED_P_SIDE 2<\/p>\n
void setup() {
\nSerial.begin (115200);
\nSerial.println(“Start”);
\n}<\/p>\n
void loop() {
\nSerial.print(“Sensor : “);
\nSerial.println(readLight());
\ndelay(100);
\n}<\/p>\n
int readLight() {
\nunsigned int j;<\/p>\n
\/\/ Apply reverse voltage, charge up the pin and led capacitance
\npinMode(LED_N_SIDE,OUTPUT);
\npinMode(LED_P_SIDE,OUTPUT);
\ndigitalWrite(LED_N_SIDE,HIGH);
\ndigitalWrite(LED_P_SIDE,LOW);<\/p>\n
\/\/ Isolate the pin 2 end of the diode
\npinMode(LED_N_SIDE,INPUT);
\ndigitalWrite(LED_N_SIDE,LOW); \/\/ turn off internal pull-up resistor<\/p>\n
\/\/ Count how long it takes the diode to bleed back down to a logic zero
\nfor ( j = 10000; j > 1; j–) {
\nif ( digitalRead(LED_N_SIDE)==0) break;
\n}
\nreturn j;
\n}<\/p>\n
Once we have a threshold value for the light sensor, we use the code below to spit out the RPM to the Serial<\/p>\n
#define PHOTOCELL 5
\n#define LED_N_SIDE 2
\n#define LED_P_SIDE 3<\/p>\n
int photocellThreshold = 2000;
\nint currentState = 0;
\nint leeway = 4;
\nunsigned long lastOnTime = 0;
\nunsigned long times[5] = {0,0,0,0,0};
\nint timesPos = 0;<\/p>\n
void setup() {
\nSerial.begin (115200);
\nSerial.println(“Start”);
\n}<\/p>\n
void loop() {
\n\/\/Serial.print(“Photocell Value: “);
\nint currentVal = readLight();
\nif (currentState == 1 && currentVal < photocellThreshold – leeway) {
\nlightOff();
\n} else if (currentState == 0 && currentVal > photocellThreshold + leeway) {
\nlightOn();
\n}
\n\/\/Serial.println(analogRead(PHOTOCELL));
\ndelay(5);
\n}<\/p>\n
void lightOff() {
\ncurrentState = 0;
\n}<\/p>\n
void lightOn() {
\ncurrentState = 1;
\nunsigned long now = millis();
\nif (lastOnTime) {
\nunsigned long thisTime = now – lastOnTime;
\ntimes[timesPos] = thisTime;
\naverageTimes();
\ntimesPos = (timesPos + 1) % 5;
\n}
\nlastOnTime = now;
\n}<\/p>\n
void averageTimes() {
\nunsigned long sum = times[0] + times[1] + times[2] + times[3] + times[4];
\nfloat avg = 300000 \/ float(sum);
\nSerial.print(“RPM : “);
\nSerial.println(avg);
\n}<\/p>\n
int readLight() {
\nunsigned int j;<\/p>\n
\/\/ Apply reverse voltage, charge up the pin and led capacitance
\npinMode(LED_N_SIDE,OUTPUT);
\npinMode(LED_P_SIDE,OUTPUT);
\ndigitalWrite(LED_N_SIDE,HIGH);
\ndigitalWrite(LED_P_SIDE,LOW);<\/p>\n
\/\/ Isolate the pin 2 end of the diode
\npinMode(LED_N_SIDE,INPUT);
\ndigitalWrite(LED_N_SIDE,LOW); \/\/ turn off internal pull-up resistor<\/p>\n
\/\/ Count how long it takes the diode to bleed back down to a logic zero
\nfor ( j = 10000; j > 1; j–) {
\nif ( digitalRead(LED_N_SIDE)==0) break;
\n}
\nreturn j;
\n}<\/p>\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
For the latest IgniteGuelph contest, we needed a way to measure the speed of homebuilt electric motors. Simple RPM, not torque was the goal, so we opted to go with a light sensor on an arduino to count the light\/dark cycles. First, we need to figure out the threshold value of the light sensor. \u00a0We […]<\/p>\n","protected":false},"author":2,"featured_media":3844,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ngg_post_thumbnail":0,"footnotes":""},"categories":[101],"tags":[],"class_list":["post-2281","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blogposts"],"featured_image_src":"https:\/\/diyode.com\/blog\/wp-content\/uploads\/2014\/03\/Arduino-for-Beginners-1.jpg","author_info":{"display_name":"Simon Clark","author_link":"https:\/\/diyode.com\/blog\/author\/simon-clark\/"},"yoast_head":"\n