Weird IR receiver

Discussion in 'General Electronics Chat' started by bug13, Sep 7, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    So I am using and IR receiver to pick up some IR signal at 38kHz, but the interesting thing is, my circuit work fine when my light bulb is on, but if I turned off my light bulb, the circuit seen to pick up the IR signal all the time, why?

    My circuit is just a IR distance sensor, my IR receiver picks up the IR signal reflect from my hand, and control a mini servo motor. When there is no hand/object in front of the sensor pairs, nothing should happen.

    My IR LED is connected to a digital IO with series resistor, the IR receiver just connect to a digital IO directly.

    My light bulb is an energy saving light bulb.

    I am using this IR LED:
    http://nz.element14.com/kingbright/kp-3216f3c/led-ir-1206-940nm/dp/2290435?Ntt=2290435

    and this IR receiver:
    http://nz.element14.com/vishay/tsop6238tr/photodiode-ir-receiver-38khz/dp/4913220?Ntt=4913220
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Look on the bottom of the datasheet (PAGE 1), do you have the 4.7uF capacitor in place between output and ground as shown? You said you only have a series resistor so I assume the capacitor might be missing.

    Other than that, it should work.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The TSOP62 has an automatic gain control inside. With the light bulb off you've essentially removed any ambient light so the gain will automatically increase, and that makes it susceptible to tiny signals from anywhere, say the LED reflecting not off your hand but the far wall.

    You can prove that by turning out the lights then disabling the LED, and see if the false trigger goes away.
     
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    The false trigger goes away after disabling the LED and turning off the lights. why do they put an AGC in it anyway?

    So is there a way to fix this? Or what kind of IR receiver should I look for when I don't want this happen?

    PS:
    The good news is I have learned something:D, the bad news is I have already got the board made:(
     
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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  6. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    All IR receivers with demodulators will have false triggers, worse is they're not designed for continuous IR (the ACG circuit will be a problem). They are designed to work with a decoder such as a small MCU and bursts of PWM encoded IR.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I don't know what distance you expect to activate this but the sensors you have specify 35meters and 30 meters for the two you have posted - with the emitter specified (60 mW/sq meter at 100mA).

    The minimum intensity is 0.5 per square meter on your sensors.

    The kingBrite emitter you listed is only 3 mA / sq meter. It has a low amperage limit coupled with very broad viewing angle (120 degrees) that disperses the ir light.

    The emitter you elected will not broadcast very far.
     
  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I should have mentioned that I am using a MCU, driving the IR LED with bursts of 38kHz signal. So I can handle a short period of false triggers/ in software.
     
  9. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I want it to activate it anywhere within 75mm, so I think it should be good?

    The reason I am using a broad viewing angle IR LED is, I want it to have a broader activate area, but I am not familiar with this, should I still be better of using an narrow angle IR LED?
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Best way to tell is to try it. You can always drill out your pads and add a through-hole LED emitter later if needed. It won't be pretty but it can be corrected if it doesn't work the first time.
     
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