38kHz IR blaster

Discussion in 'The Projects Forum' started by toninph, Oct 26, 2011.

  1. toninph

    Thread Starter New Member

    Dec 10, 2010
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    I'm trying to build a 38kHz IR blaster with 10 IR LED's. With the circuits I found here, I do not get enough range. I only get about 5 meters, while with a regular remote control I can reach 20 meters.
    With the circuit below I try to maximize the current through the LED's, say 60mA at a small dutycycle of 10-15%. I am however unsure how to calculate and regulate the current through the LED's. How do I do that?
     
    Last edited: Oct 26, 2011
  2. Audioguru

    New Member

    Dec 20, 2007
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    The remote control for a TV uses a high current IR LED or two and feeds 1A pulses to them. The IR receiver IC has a lot of gain when its AGC does not reduce its gain.

    Its IR LED current is 1000/60= 16.7 times more current than yours but you use a lot of LEDs which helps.
     
  3. Butterworth

    Active Member

    May 6, 2009
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    I see by the schematic that it uses the leds in series. Would having them in parallel help in the "brightness"
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The IR LEDs are probably not rated for a high peak current. They are driven my a voltage stepup circuit. If they are in parallel then the supply and the Mosfet must be able to provide 10 times more current for the same brightness as now.

    Not a word was said about the IR receiver circuit which is very important for range.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    With a 50% duty-cycle 38kHz signal driving the inductor, the theoretical peak current in the inductor is a little over 1/2 amp. The 9V battery may have difficulty supplying that much current so you should have a large capacitor across the battery to supply the peak current (at least 100μF).

    What's the purpose of D1? It performs no useful function that I can see and it just wastes energy that should go to the LEDs.

    Since you are driving the LEDs with pulses from the inductor, which determines the LED current, then R1 is also not needed since that also reduces the current and wastes power.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If an appropriate diode is used for D1, its loss will not be huge compared to a voltage of a stack of 10 LEDs. A possible reason for retaining D1 is that in its presence a small capacitor may be added across the LED stack to obtain a less highly peaked drive waveform, which might be beneficial to LED efficiency, and may reduce peak current stress. The capacitor must of course not be made so large as to to slow the pulse decay excessively, or the 38kHz content would be significantly reduced.

    If the resistor is eliminated, which seems logical unless a current monitor is required, then care must be taken to restrict the duty cycle to values giving discontinuous conduction (i.e. the coil current falls to zero at the end of each cycle, before the transistor switches on again. If not, the current can "ladder up" to excessive levels. We have approximately 9V driving the current up with the switch on, (10Vf-9V) driving it down with the switch off.

    Thus approximately the switch on duty per unit D < (10Vf-9V)/9V. At least I think that's right
     
  7. Markd77

    Senior Member

    Sep 7, 2009
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    I'd suggest a variable resistor so you can vary the frequency. Cheap capacitors can have a tolerance of 20% of their value so the frequency could be way off.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The FET looks to be on a lot of the time with the present circuit values, unless I have made a mistake (I don't play with 555s a lot).

    If you are turning the FET on too long, the current will get too big. Depending on the LED ratings, they may burn out.

    Edit: That 220Ω resistor may save the day by wasting a lot of the output, but that is not the way to do it.
     
    Last edited: Oct 26, 2011
  9. hgmjr

    Moderator

    Jan 28, 2005
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    What is the DC resistance of your 220uH inductor?

    hgmjr
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    I would echo the suggestion of crutschow that you add a large value electrolytic or tantalum capacitor across the battery. The 9V battery is very limited in the amount of short-term energy it can deliver to the load. The cap would help provide some short term energy storage.

    Are you using a single 9V battery or six 1.5V batteries in series?

    hgmjr
     
  11. John P

    AAC Fanatic!

    Oct 14, 2008
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    That 220 ohm resistor is guaranteed to make the circuit fail. It's false to think that it's protecting the LEDs from anything--the current is set by the inductor's value and the duty cycle. I'm dubious about that way of running the LEDs though--the current will be a sawtooth, not a square wave, and that may not be best for the receiver. Think of it in terms of energy--where is it stored and where is it actually going? And just BTW, what is the average current flow in the circuit, and can the battery (if that 9V is a battery) supply it?

    And you haven't said what your receiver is like, but if it's a TV remote device, it won't work at its best if you deliver a continuous 38Khz signal. That's because those things turn down their gain if they see a lot of 38KHz coming in. What you need to do to work with the part's characteristics is to chop the 38KHz so you get a reduced duty cycle; I think about 16 cycles on then 16 cycle times off might be good, but try it. Maybe only 25% on then 75% off would be better still.

    One thing to beware of is that if the 555 fails in some way and leaves its output high, your inductor and transistor are in danger. You'd make things safer if you couple the 555 to the FET gate by way of a capacitor, so only an AC wave can get through.

    And I hate to be a pain, but have you got a scope to check that your 38KHz is really 38KHz? An LM555 oscillator isn't a high-precision setup, and unless you get lucky, you need some way to verify that it's what you need, and to adjust it till it's right.
     
  12. toninph

    Thread Starter New Member

    Dec 10, 2010
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    1
    Thanks a lot for your comments. It made me realize that I am unclear about my objectives.

    First objective:

    The IR blaster should trigger a IR receiver that in turn turns on a porch light the moment it receives any 38kHz IR signal. There is no coding or decoding required. The receiver circuit uses a regular IR receiver/amplifier, TSOP type. It's a toggle' so upon reception of the next 38kHz signal, it turns the porch light off. Yes, it means that you can turn the porch light on and off with any IR remote control too.
    The IR receiver has a passband with a peak of 940nM. The IR LED's I have are 870-880nM. Though the IR receiver "sees" my IR LED's, sensitivity is probably much lower, hence the range is shorter.

    The short pulses and low duty cycle were meant to get the maximum output from the LED's at a maximum current but this may not be optimal for the IR receiver. I think it is better to use a 50% duty cycle and maximize the current through 940nM IR LED's.
    Thus optimized I need fewer IR LED's and don't need to step up the voltage with an inductor. I suspect this was messing with the 38kHz signal anyway.
    In the attached schematic the LED's are directly fed by the LM555. With 47 Ohms in series the LED current would be about 200mA, the maximum current the LM555 can handle in continuous mode. Is this advisable? If I want a higher current, e.g. upto 1A as Audioguru mentioned, a transistor or FET would have to added.

    The power supply is 12V (car battery) but I bring it down to 9V to ensure stable supply and current for the LED's. D1 (a Schottky diode) in the original schematic was indeed to be combined with a capacitor parallel to the LEDs. I think that that messes up the 38kHz signal so I took it out.
    I took the suggestion to make the oscillator frequency adjustable by adding a potentiometer. I added a switch to turn the oscillator on or off using pin 4 of the LM555.

    Second objective:
    An IR illuminator. That is where the 10 870nM IR LED's come in. My thinking was that I could combine both objectives into one. But that's not optimal.
    The illuminator could be designed along the lines of the original schematic with the difference that I am not stuck to 38kHz. What could be the optimal frequency and duty cycle to get the maximum out of 20mA IR LED's?

    ...in the mean time...
    I read John P's comments only after I wrote the above. Thanks John P.

    From the attached schematics you will see that a continuous 38kHz signal is now used. I should have realized that the IR receiver turns down amplification when is sees such a continuous signal and that low frequency chopping should be applied. This requires another oscillator. I could do that with another LM555 or an LM556. The low frequency oscillator turns the 38kHz oscillator on and off. Is that the way to do it?

    Thanks for the capacitor/FET suggestion. I did not know that there is a chance of 555 failure, leaving it's output high. I will have to add in a FET anyway in the next design.

    Unfortunately I do not have a scope. I will have to optimize by testing.
     
    Last edited: Oct 27, 2011
  13. 2N3055

    New Member

    Oct 18, 2011
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    0
    This is a kind of flyback inverter supply that provides current for the leds. - The 9V supply is not enough to make this number of leds in series conduct on their own.
    Q1 turns on and the current through L1 ramps up - (as the bottom of L1 is pulled low). When Q1 turns off, the collapsing magnetic field in L1 produces a positive going pulse at this same point that flows through D1 and the led string.
    You might substitute the I/R leds for some visible types and observe the affects obtained when changes such as the value of L1, number of leds, on/off ratio of drive to Q1 etc are made. Keep in mind that the frequency (for good remote control function) may have to be set with some precision. Tweaking the resistors R3 & R4 will change the frequency and set the drive pulse width to Q1.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    The datasheets for TSOP IR receivers and nearly all other IR receivers show that their AGC turns down the sensitivity when it receives continuous 38kHz IR pulses to avoid interference from compact fluorescent bulbs. This reduces your range a lot.

    They say to send "bursts" of 38kHz pulses (which a remote control does naturally as it sends data). A second 555 can reset the oscillator at the recommended intervals to create the bursts.
     
  15. toninph

    Thread Starter New Member

    Dec 10, 2010
    14
    1
    Following the comments of John P and Audioguru and others, I'm trying the circuit as attached. The original design is the transmitter part of a "door minder" of of KITSRUS, kit no.130v2, based on a CD4093.
    I think I change the transistor to a more powerful one and replaced the IR LED with multiple 940nM LED's. As a matter of fact, I already built the original circuit with one 870nM LED but the range is far from satisfactory, probably because of reasons mentioned before.
    The circuit below chops at a frequency of 250Hz. I do not know how to change that into 16 time cycles or less, as suggested by John P. What would be the optimal chop frequency in this case, considering a TSOP type IR receiver/amplifier?

    P.S. I will put the "IR illuminator" discussion (my second objective) in a separate thread.
     
    Last edited: Oct 28, 2011
  16. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for each TSOP IR receiver IC states the recommended burst rate. 250Hz is too for the ones I looked at.

    Your latest schematic has no supply voltage.
    The LED current is about 180mA which might burn out your IR LEDs. Then the base current for the transistor is far too low since it must be 18mA. Changing the transistor will not help unless it is a very high gain darlington (two transistors together).
    The circuit must be re-designed to use more IR LEDs.
     
  17. toninph

    Thread Starter New Member

    Dec 10, 2010
    14
    1
    Thanks Audioguru.
    The supply voltage of my latest schematic is to be 9V, as before. My mistake to have omitted that in the schematic.
    The transistor is not driven by DC but with a 250Hz/38Khz burst. This should not burn out the LED's, although at may be at the limit. To be on the safe side I would increase the limiting resistor R6.
    Nevertheless, a transistor or darlington might not be the optimal solution due to base current required and the output current limitation of the CD4093. In stead, could a low power FET like 2N7000 be used (I have them in stock)? My understanding is that it only needs the supply voltage on its gate with hardly any or no current but it is able to handle up to 200mA on its drain-source and maybe more when being pulsed.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    The kitsrus circuit is designed for a PNP transistor. It could use a P-channel Mosfet but a 2N7000 is N-channel and a re-design is needed to use it.
     
  19. toninph

    Thread Starter New Member

    Dec 10, 2010
    14
    1
    I see...thanks Audioguru. What P-channel Mosfet would you advise for this application? I have little experience with Mosfets.
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    A constant current source or sink made with an ordinary Mosfet needs a supply that is more than 9V. It might work with a logic-level Mosfet.
     
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