TSOP4838 sensitivity?

Discussion in 'The Projects Forum' started by rougie, Feb 24, 2013.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006

    I am using the Tsop4838 ir sensor and I am
    able to sense objects at different distances.

    I then tried to detect those same objects but
    with a flash light directed right on the tsop sensor.

    To my dismay, the sensor completely died off
    producing NO signal at all!!!!

    Is this normal? I thought this part was able to
    distinguish any disturbances such as Dc lighting,
    sunlight ect... from its 38khz signal ???

    Has anyone experienced these disturbances
    with this sensor ?

    PS... For now I have not provided the details
    of the optical signal I am using.... as I was only
    interested to know if I am the only one experiencing
    these issues. However if relevant, I can provide
    all the signal details should we think that this
    sensor really can work under intense lighting conditions
    such as dc lighting or sunlight.

    thanks for all feedback
  2. SPQR


    Nov 4, 2011
    Can you show us your circuit?
    Most of the problems I've found with Google are related to missing capacitors and resistors that are required for noise-free functioning.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Did the sensor come back after you turned the flash light off? It sounds like your flash light overwhelmed the sensor. If you dump too much ambient light the sensor just runs out of headroom to distinguish one signal from another.

    Yeah it's normal.
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    Hi guys,

    Thanks for your replies.

    For my IR LED, I am providing it a 38KHZ burst for approximately 600 us (aprox. 22 cycles). Then I am allowing a gap time of approximately 4.8 ms (aprox. 182 cycles).
    According to the datasheet I *think* I am within specs (not sure though)…. You can view the signal the IR is generating in "LED_OpticalSignal.jpg" attachment.

    Please view the “Schematic.jpg” attachment for more details.

    In the "Schematic.jpg" attachment, I simply vary the Rx resistor to obtain the power desired for the infra red led. Doing so, permits me to see different distances. For example, if I require to detect an object at 3 feet, well there is a particular setting of Rx that will detect an object at 3 feet or closer. All the sensor does is output a continuous square wave when the object is within 3 feet of the sensor as shown in the “SensorOuputSignal.jpg” attachment.

    When the lighting in the room is approximately 0-800 LUX, the TSOP sees the wall which is at aprox. 36” and its signal is normal as shown in the “SensorOuputSignal.jpg” attachment.

    However, when I direct a flash light right at the TSOP4838 part the TSOP produces no signal as can be seen in the “NoSignalFromTSOP.jpg” attachment.


    1) Can you confirm if my IR LED is receiving the correct signal ?
    2) I am generating the 38KHZ burst from the PIC32. I am actually generating it from a timer interupt which is:

    -Prescalar of 64 and a duty cycle of 16. Ths give the following calculation:

    80,000,000/64 = 1,250,000 hz
    1,250,000/16 = 78,125 hz

    But since it takes two interupts to create 1 cycle, we divide by two to obtain our 38KHZ

    78,125/2 = 39062.5 Hz

    Now, I have no choice , this is the closest I can come to 38KHZ, since a prescalar value of 17 as opposed to 16 would give me 36764.7 hz which is even worst!

    So my question is, would it make a big difference if I have 39.0625 Khz as opposed to 38.0 KHZ?

    3) I am using the TSAL6100 IR emitter led. I can't remember where I read this, but apparently the 6200 IR led would be a better match for the TSOP4838 sensor in terms of wave lenght. Should I stay with the 6100 or try the 6200?

    I really wonder if this part can handle the brut intensity of the sun and still measure my distances as described above?

    All help appreciated.... Thanks

    Sincere regards
    Last edited: Feb 25, 2013
  5. rougie

    Thread Starter Active Member

    Dec 11, 2006
    Hi ErnieM,

    No, it doesn't right away, I have to sort off wave something in front of it and then it comes back!!!!

    I don't get it, these sensors are suppose to be able to handle disturbances such as sunlight!!!!

    So if someone has to do a beam breaker outside somewhere, then he can't do it ????