Trouble with IR emitting LED circuit

Discussion in 'The Projects Forum' started by The Humanzee, Feb 25, 2010.

  1. The Humanzee

    Thread Starter New Member

    Feb 25, 2010
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    I suppose I should start by introducing myself. My name is Mark, I am going for my Master's degree in digital design right now, and I am creating an Infrared emitting LED circuit, powered by 4.5v, into a 1/8" stereo phone jack. Pretty much I am making an infrared emitter that plugs into a headphone jack. Unfortunately, I am a bit stuck right now, and I was wondering if anyone could help me diagnose the problem. The LED is not lighting at all, and I tried it on my computer's camera, (which can detect IR because I tested it with my cable remote).

    I am using a NPN transistor. I am using 3 silver oxide, 1.5v hearing aid batteries stacked together.

    Here's the schematic:

    [​IMG]

    Here's the circuit:

    [​IMG]

    Any thoughts? Should I be using an NPN transistor? Can you not stack batteries together like that? (I superglued the outside..)

    Thanks in advance,

    -Mark
     
    Last edited: Feb 25, 2010
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
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    4.5 volts. measure it. Yes it should be npn. 22 ohms is low, internal resistance of batteries is probably higher. What are the specs on the led. What is the measured input to the circuit. A scope would be good, by can probably make due with a multi-meter.
     
  3. The Humanzee

    Thread Starter New Member

    Feb 25, 2010
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    Thanks for the response. I switched to 220 ohm, still no luck. I am using the NPN2222 transistor. The LED is the emitter from the Radio Shack emitter/detector package. (276-0142). I am using the tinted one. That's the emitter, isn't it?

    This:

    http://www.radioshack.com/product/index.jsp?productId=2049723

    I should also mention, I have the Base of the transistor going into a 1k resistor into the TX (which I believe is the +4.5v, right?)

    Thanks!!!
     
    Last edited: Feb 25, 2010
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Are you going to modulate the IR by switching TX?

    If TX is always tied to 4.5V, why even use the transistor.

    Use a multimeter to measure the voltage drop across the 22 (ne 220)Ω resistor. Divide the voltage by the resistance. That will tell you the current flowing through the resistor is consistent with the IR LED emitting, and even being wired the right way around.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Did you read the reviews of this garbage from RadioShack?
    One guy says "Don't buy It".
    It has no datasheet so you don't know which is the emitter and which is the detector. You don't know if the detector is a diode or is a transistor.
    You don't know if the detector is sensitive or not.
     
  6. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
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    an ohm meter properly used will tell you which pin is which on a led.
    Why not power up the led from a battery with a current limiting resistor and make sure it lights. If everything is ok go back to the circuit.
     
  7. The Humanzee

    Thread Starter New Member

    Feb 25, 2010
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    Hey guys,

    Well I tested the circuit on a breadboard, and the infrared LED stays lit when i put the 4.5v battery up to it, and when I take the battery off, it goes off, obviously. My only concern is, the LED is on the whole time that it is connected to the battery. The IR LED for my project needs to be in an off state until it receives data from the TX port.

    Any ideas?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The transistor is turned off when the TX signal goes low (0.4V or less) which should turn off the IR LED. The transistor is turned on (and also the IR LED is turned on) when the TX signal goes high.
     
  9. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Get a digital camera, and by viewing through the display, you can see whether the IRLED is lit up or not. The human eye can't detect IR, but cameras sure can.

    Austin
     
  10. The Humanzee

    Thread Starter New Member

    Feb 25, 2010
    17
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    Ok cool. I was unsure exactly how the "TX" port worked. So am I correct in wiring the TX to +? The schematic said to go to a 1k resistor to TX.. my teacher said +..

    What I don't understand.. is that the circuit, in its current state, when attached to the battery, the IR LED is on..

    What I need, is for the LED to be off until, like you said guru, the transistor's TX signal is high. So I guess I have it backwards as of now.. the TX signal is high as soon as the circuit is connected to the battery..

    Do I need a larger resistor than 1k from Base of transistor to TX?

    Am I wired correctly? (Wired it exactly to the schematic)

    I have been using camera to detect infrared. I didn't know that either until my teacher told me. When I press buttons on my cable remote, I see nothing. When I press it up to my iMac camera, digital camera, iphone camera, etc.. I see the infrared flashes. So I've just been using the digital camera to test my circuit for now.

    Thanks so much guys!!!
     
  11. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    Why not debug the circuit with a visible LED? Stick in a series resistor to adjust for different diode voltage drops (I can't see what the forward drop of the IR diode is, as the typically-horrible Radio Shack web page won't display any information). They'd both probably run around 10-20 mA.

    Also, yank out the LED and stick in a resistor as a load. Then you can measure that you're getting the proper collector current (i.e., that the transistor as a switch is actually working).
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    RadioShack spec's for electronic parts show the weight.
     
  13. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
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    Looking at your circuit board zoomed in,

    It looks like your LED is connected directly across the battery with the resistor, but your collector terminal is connected directly to the battery positive as well.

    Your collector should not be connected to the positive bus line.

    Rewire your circuit.

    emitter to neg. collectoer to LED flat side.

    1K to other LED leg.
    Resitor back to pos. term.

    Base to resitor, for input to [pos. term.
     
    Last edited: Mar 9, 2010
  14. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    The Humanzee,

    Would it be possible for you to take a picture of the bottom of your PCB? Then we can see how everything is connected.

    Austin
     
  15. The Humanzee

    Thread Starter New Member

    Feb 25, 2010
    17
    0
    Hey,

    Ok so I re-wired using my solderless breadboard, but the end of your instructions are a bit unclear.

    I have the transistor's emitter to negative, and its collector to the flat side of the LED. I have a 1k resistor going from the other leg of the LED to positive? Then base of the transistor to a 1k resistor to positive..
     
  16. hobbyist

    Distinguished Member

    Aug 10, 2008
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    I made a mistake saying the 1K resistor for the LED.

    I quickly glanced at the photo, and didn't even check to look at the value of the LED resistor. When I seen the collector on the pos. side I started typing without thouroghly looking over the component values.


    change out the 1K for a 22 ohm as your original circuit.

    The 1K at the base, is used as a input terminal, according to your schematic.

    By placing a positive potential at this resistor, should cause the LED to activate.

    If you have a regular LED, temporarily put that in place of the IR led, to see if it lights up, and how bright it is, that way you can determine if your transistor is working properly.



    If your still having problems with the LED not putting out enough signal,

    then using your detector for what this is being made for, carefully hook up your IR led with a high value series dropping resistor, across a good supply voltage. (NOT your original batteries), and begin substituting lesser values of resistors until your detector is getting a signal from this LED.

    Then check the Vdrop across both the LED, and the series dropping resistor, to determine the thresholds of voltage and current required for this particular LED.

    From there you can then bias your transistor to provide the amount of current needed to get this output.

    Another tip:

    with your original circuit using the original batteries, using a voltmeter, check the batteries voltage with the circuit UNhooked from it, then hook up the circuit and check the batteries voltage, see if it has dropped significantly.
     
    Last edited: Mar 10, 2010
  17. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    here are the specs for those "no good" devices you are referring to, I have successfully used these in plenty of devices requiring an IR LED/Photodetector pair... The blueish colored "LED" is the emitter, the darker colored one (typical of photodetectors with ambient, or daylight filters) is the photodetector.... anyone with a "cheap Chinese" multimeter would be able to tell the difference....

    B. Morse
     
  18. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    If I were you, I would not have connected this to my phone..... you are shorting the connector right to the + and - of those batteries!!! (Iknow someone pointed this out already, but you could have damaged the phones output.)

    and to top it off all your components are also ALL connected to the + of the batteries!!!!

    DOOMED PHONE.jpg

    B. Morse
     
    Last edited: Mar 10, 2010
  19. The Humanzee

    Thread Starter New Member

    Feb 25, 2010
    17
    0
    Hey man,

    Well I wired it up this way, and am now using a regular LED per your suggestion. The LED lights up and stays on when connected to the 4.5v batteries and is off when the batteries are removed from the circuit. So, I now have:

    LED + --> 22 Ω --> +
    LED - --> C of Transistor --> -
    B of transistor --> 1k Ω --> +
    E of transistor --> -

    I need the IR LED to come on only when given a signal from TX. Am I missing something?
     
  20. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The LED will turn on when the TX voltage is 0.8V and more.
    The LED will turn off when the TX voltage is 0.6V and less.
    With an input between 0.6V and 0.8V then some transistors will be turned on and other transistors will be turned off.
     
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