IR emitter/detector with 555 timer

Discussion in 'General Electronics Chat' started by cvrider, Dec 4, 2007.

  1. cvrider

    Thread Starter New Member

    Dec 3, 2007
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    I have a 555 timer circuit attached to an IR emitter as shown in the attached diagram. I have also built the IR sensor as seen in the other diagram. The only difference is, I used a Radio Shack 276-640 IR sensor.

    When I check the frequency across the IR emitter I see 38.3 kHz which should work with the detector. However, the detector circuit always stays high (+5V). I can point my remote control at the sensor, and when i push a button, the circuit goes low (0V). I assume this sensor from radio shack is reverse logic therefore.

    For some reason I cannot get the sensor to work with my emitter. How sensitive is this dectector. Do I have to be right on at 38kHz. I seem to only be able to do slightly above or slightly below, which both don't work. I also tried different distances (up to 6 inches away) with no luck.

    Any suggestions? Is there something here I am not seeing?
     
  2. cvrider

    Thread Starter New Member

    Dec 3, 2007
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    Could this have something to do with duty cycle. From my calculations the duty cycle is about 76%. Does the detector only work close to 50%?
     
  3. nemaroller

    New Member

    Dec 1, 2008
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    I ran into the same issue. I assumed my duty cycle was close to 50% since the circuit I followed had the C1, r1, r2 values specified to be close to 50%.

    However, I wonder if the IR detector from RadioShack actually filters out the IR carrier signal (it won't send low when simply detecting the 38khz square wave) - and only detects the purported IR code sent (the modulated pulse). I noticed a few other IR detectors with the same properties.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Most IR receiving ICs require bursts of data to work properly. They have an automatic gain control inside that reduces the gain when continuous IR occurs from compact fluorescent light bulbs.
    The datasheet from Panasonic has no details.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Just for grins, try running it at 19.15kHz at a 25% duty cycle.

    You'll need to change your 555's R1/R2 configuration or (easier) flip the IR LED around, use the 555 to sink current from the cathode, and connect the anode to +5v via a resistor.

    BJT 555's work better when they're used to sink current vs sourcing current. When sinking current, the output can get within a few mV of ground; when sourcing current they drop a volt or so due to the Darlington voltage follower configuration.

    As Bill Marsden mentioned below, re-calculate the resistor needed for your IR LED.
    Rlimit = (Vsupply - VfLED) / DesiredCurrent

    For example, if your IR LED is rated for a typical Vf of 1.2 @ 20 mA, then:
    Rlimit = (5v - 1.2v) / 20mA = 3.8V / 0.02A = 190 Ohms. In this example, you could use a single 200 Ohm resistor, or a 100 Ohm and a 91 Ohm resistor in series.
     
    Last edited: Dec 1, 2008
  6. Wendy

    Moderator

    Mar 24, 2008
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    My calculations say it is around 6-7ma current. How much is it rated for?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Just for clarification, you're talking about the IR LED, right?

    I get about that same current, too.
    I figure about a 1v drop from the Darlington output of a standard bipolar 555, a Vf of the IR LED of about 1.2 (wild guess), which leaves 2.8v/470 Ohms, or about 6mA.

    It sure won't be very bright.

    My thought about running it at 19.15kHz 25% duty cycle is that would be the same as running 38.3kHz 50% duty cycle and leaving it off every other pulse. That might get around the AGC situation.
     
  8. Von

    Active Member

    Oct 29, 2008
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    You can use the camera in your cellphone or a late model video camera to see if the IR LED is working.
     
  9. nemaroller

    New Member

    Dec 1, 2008
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    The Vishay TSOP21 IR IC datasheet describes the gain control as you mentioned, I can only assume RadioShack's vendor manufacturer and Panasonic's IC does the same.

    "Some examples for such disturbance signals which
    are suppressed by the TSOP21.. are:
    • DC light (e.g. from tungsten bulb or sunlight)
    Continuous signal at 38 kHz or at any other fre-
    quency
    • Signals from fluorescent lamps with electronic bal-
    last (an example of the signal modulation is in the fig-
    ure below)."

    http://www.allelectronics.com/mas_assets/spec/PCM-1.pdf
     
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