IR sensor

Discussion in 'General Electronics Chat' started by bug13, Jun 13, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys

    I got this IR sensor from a local store LBT-3338A, no datasheet on Internet, it suppose to have a carrier frequency of 56KHz and peak detection wavelength of 940nm.

    I am sending the signal with a matching 940nm infrared LED.

    so if I do this, it can detect the signal:
    Code ( (Unknown Language)):
    1. while(1)
    2. {
    3.     burst_of_pulse_at_56KHz(1800us);
    4.     delay(20ms);                              //or any delay greater than 20ms
    5. }
    but if I do this, it can not detect the signal:
    Code ( (Unknown Language)):
    1. while(1)
    2. {
    3.     burst_of_pulse_at_56KHz(1800us);
    4. }
    but if I do this, it can detect the signal for the first few seconds, then the signal is gone:
    Code ( (Unknown Language)):
    1. while(1)
    2. {
    3.     burst_of_pulse_at_56KHz(1800us);
    4.     delay(10ms);                               //or any delay less than 10ms
    5. }
    if that a feature of all the IR sensor, or just me doing it wrong?
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
    397
    I think it is the nature of the sensor. ' had similar results with 38 kpps sensors.
     
    bug13 likes this.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,059
    3,823
    Post your cuircuit, you may have formed a filter that is attenuating your 56k pulse (on the transmitter or receiver side). The delay allows the caps to discharge and act like a pulse train again on re-activation.
     
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Here is my circuit, my breadboard has some permanent 100nF cap on both side of the power rail, and the resistance of my breadboard's power rail is quite high, it's about 2-5 ohm, do you think that may cause it?

    [​IMG]
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,059
    3,823
    Try decreasing the input signal frequency.

    The IR sensors start to reach their limits at 40k Hz. The filter inside the receiver module could be mis-tuned.

    Your receiver should sense the signal down to about 20k Hz (possibly lower). Try 38k Hz first. Then work down to 20kHz. Let me know if you get anything.

    Also, put a scope on your signal to the IR emitter. Make sure it is not crapping out when your pause is missing.

    I normally prefer to make my own receiver with an IR phototransistor that is capacitively coupled (high pass filter) to a transistor or opAmp. Another cap in the feedback loop serves as low pass filter if needed.
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    ha, I have tried 38KHz 20KHz and 10KHz, they all work if I send a burst of pulse with a 20ms or so delay between the burst (so much of a 56KHz as the seller told me)

    but it doesn't pick up the signal if I send continue burst of signal.

    Can you show me how you make your own receiver with a circuit diagram?

    here is the signal to the IR Emitter, the yellow is signal to the transistor, the blue is signal on the cathode of the IR emitter, the signal at the collector of the transitor is pretty clean too.
    [​IMG]
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
    397
    I think we're back to post # 2. What is your end objective? As a beam break detector it can still be used by using double modulation- ie- pulsing 56 KHz carrier, which adds a few ms of uncertainty.
     
  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Yes, it can still do what I want. (a 75mm proximity switch)

    As I test those sensors on breadboard, and I found those interesting results, I simply want to know why, because I expected that the IR receiver will pick up the signal as long as the signal present, no matter if the signal is continuous or burst of pulse with delay in between.

    There is a question mark in my mind, I simply want to get rid of it. :D
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Most of the 3-pin IR sensors won't work with a continuous signal. That is because their internal auto gain control will turn the gain right down.

    You need to pulse (modulate) the 38kHz signal to the LED, usually at about 1kHz. Then the 1kHz signal will come out the IR sensor.
     
    absf, bug13 and GopherT like this.
Loading...