CNY70 photo sensor too sensitive, too many alternating readings...

[js]

Hi,

So I have connected a CNY70 photo sensor and a resistor to a LM339N comparator.

Whenever the voltage across the CNY70 exceeds the voltage across the resistor, the comparator sends out a HIGH, else a LOW.

The problem that I'm having is, that when I move VERY SLOWLY (but constantly in one and the same direction) over the threshold voltage, I get lots of alternating HIGHs and LOWs ...

Is this bouncing? How can I avoid this without using delays ???

Thanks,
Joël

[gcjr]

you can add a feedback resistor to the comparator to add hysterisis, shifting the reference voltage depending on the state of the output.

[js]

Thanks, I will read into that.

[MarkT]

You need a divider to set the reference voltage, like this:

R2/R3 set the reference point, and the much larger resistor R4 provides a bit of hysteresis.
Note that a comparator, like any other logic chip, requires 100nF ceramic decoupling capacitor right
close to the supply pin.

A stiff pullup of a few k ohms provides faster response than built-in pullups should this matter
(normally it won't and internal pullup is sufficient, but note that its only the pullup that drives
the feedback - Rpullup is a good idea so you can test the circuit without the Arduino connected)

Almost all comparators have open-collector/open-drain outputs, so they can be paralleled for
the window comparator circuit (and so they can have different supply voltage), hence the need for pullups on the outputs.

[js]

Thanks Mark. It works. Now, I'll have to figure out why

btw, why does the cap have to be ceramic, and not electrolytic for example?

Joël

[js]

Well, it doesn't seem to work when I try it on my own build.

Here's a drawing of it:

The CNY70 is represented by the photocell and the LM339N comparator is represented by the L293D IC, as I didn't find the correct parts in the Fritzing app.

So, in my project, the 2 potentiometer set 2 reference voltages (e.g. 500 Volt for potentiometer at the top, which is connected to negative input 1 and 1500 Volt for potentiometer at the bottom, which is connected to negative input 2). The CNY70 connects to pos. input 1 and pos input 2. Output 1 of the comparator then goes to digital pin 2 and output 3 goes to pin 3.

Here's the code

unsigned long start, finished=0, elapsed;
bool sema1,sema2=false;
unsigned long vitesse;
 
 
void setup() {
  Serial.begin(115200);
  pinMode(2, INPUT); // start INPUT
  pinMode(3, INPUT); // stop INPUT
}



void loop() {
  
  if (digitalRead(2)==LOW && sema1==false) {
    start=millis();
    Serial.println("start>>>>");
    sema1=true;
  }  
  
  if (digitalRead(2)==HIGH && sema1==true) {
    sema1=false; 
    
  }
  
  if (digitalRead(3)==LOW && sema2==false) {
    finished=millis();
    Serial.println("<<<<finish");  
    sema2=true;
  }
  
  if (digitalRead(3)==HIGH && sema2==true) {
    sema2=false;
  }

}

I want to measure the velocity between the 2 thresholds.

Here's the results I'm getting when I approach my finger to the CNY70:

start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
<<<<finish
<<<<finish
<<<<finish
<<<<finish
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
<<<<finish
<<<<finish
<<<<finish
<<<<finish
<<<<finish
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>
start>>>>

The problem: Too many consecutive starts and finishes.... I need only 1 of each, whenever I pass the thresholds!
And I don't want to use delays to achieve this!

Thanks,
Joël

[js]

Can it be resolved with hardware, or is it a software problem?

[gcjr]

maybe you should just put the sensor in series with a resistor and do an analogRead(). you can have separate thresholds for on and off

[js]

Can someone please check if I have done the circuit right ?