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Voltage regulator for IR Receiver plugged on serial port

Discussion in 'Electronic Components' started by Nelis, Nov 10, 2003.

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  1. Nelis

    Nelis Guest

    Hi,

    I'm trying to build an IR Receiver for PC (plugged on serial port). To
    build this, I need a voltage regulator LP2950CZ. The voltage provided by
    the serial port is between 5V and 15V and the IR receiver need 5V. I
    need then a regulator with a little difference between input voltage and
    output voltage. Badly, I can't find the LP2950CZ, I have then to replace
    it with another regulator. I'm a newbie in electronic so I don't know
    with wich regulator I can replace it. Someone told me that I can use
    LM2940-05 or LM4805 regulator, can someone confirm it ? Can I just
    replace it or should I change some capacities/resistances ?

    Here is the schema I want to build :

    http://home.t-online.de/home/tb_electronic/vdr/lirc/lirc_rx.html

    LP2950 datasheet :
    http://www.national.com/ds/LP/LP2950.pdf

    LM2940 datasheet :
    http://www.robozes.com/robot_pi/componentes/LM2940.pdf

    LM4805 datasheet :
    http://www.usmicrowaves.com/ldoreg/L4805.htm

    Thanks a lot,

    Laurent
     
  2. Digikey has them in stock in several package sizes:

    http://www.digikey.com/

    They sell in single quantities as long as you make a total
    purchase, including other items, of $25.00.
     
  3. Michael

    Michael Guest


    Schema says a 78L05 may be substituted for the LP2950. The former is a
    5v 100ma regulator in a TO99 case, if memory serves. Cheap and readily
    available.
     
  4. Nelis

    Nelis Guest

    Schema says a 78L05 may be substituted for the LP2950. The former is a
    But my laptop serial port provice only 5,6V, so I can't use 78L05 (I
    think it need 7V as input to provide 5V). Also I want to make a receiver
    that works on any PC ...
     
  5. The only thing that needs 5V is the IC1 receiver. Therefore all you
    need is a resistor between C1 and C2 (delete the regulator), and a
    5.1V zener diode across C2, cathode to positive. The resistor should
    be low enough to supply the current, plus a mA or so more. I would
    try a 3.3k to begin with. Measure the currewnt going to IC1 with a
    DMM and go from that value.

    [snip]

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  6. I think you're confused. The ground of the receiver circuit connects
    to the TXD, which should be a negative voltage on your laptop. The
    total, both positive and negative, should give you 10V or more across
    the circuit.


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  7. Guest

    The TSOP17 datasheet shows 5 ma - which brings up a question
    for you. Could he use LEDs in place of the zener? If I get
    a chance, I'll experiment tomorrow - but you may already have
    done extensive experiments with this. I'm assuming Vf of
    1.6 for garden variety red leds - am I on target?
     
  8. I'd look at the specs for the IC1 receiver. It it's okay with more or
    less than 5V, then it might be okay. If it's like the regular TTL,
    which wants +-5% then maybe a zener would be a better choice.

    Red LEDs seem to be about 2V nowadays. But they may vary
    considerably.

    I'd experiment too. The problem with today's PCs is that the MoBo
    furnishes all the serial and parallel signals. So if you foul up a
    chip, it could be a major problem. :0)

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    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  9. Guest

    Thanks for the reply.

    Today I got a chance to play around with this a little.
    I found that the Vf on several LEDs varied a bit. These
    are "grab bag" LEDs - a bag of 100 various sizes, colors
    etc. for a few bucks. I only looked at 5 of them for Vf.
    I intend to do some more selecting - but here's what I
    found, using three of them in series with a 330 ohm
    resistor:
    Supply voltage: 12** (see below)
    Load 1K
    Vf no load 5.63
    Vf full load 5.41
    I load 5.4 mA
    I LEDs ~17.? mA (forgot to write it down)

    Test Circuit:
    Supply----/\/\/\/-------+------<-Iload->-+
    330 |<----+ |
    LED | |
    | | /
    LED Vf \
    | | / 1K
    LED | \
    | | /
    ^ | |
    Ileds | |
    v | |
    | V |
    Supply------------------+-----+----------+

    (Meters are shown as Vf, Iload and Ileds)

    **The supply was set around 12 v on the above - but I did
    not use a digital meter to read it. I was more interested
    in no load to full load comparison, which comes in at a little
    better than 5%. I tried various currents through the LED
    string by varying the supply voltage, but the best I could get
    for no load to full load was a delta of about .22 (Vf) volts,
    so I don't think shunt regulation with LEDS alone can be
    improved much for this circuit.

    That said, the .2 volt variation is well within acceptable
    variation as shown on the data sheet, which shows basic
    characteristics of the supply voltage of 4.5 to 5.5 volts.
    Further, it shows typical I as .4 mA minimum to 1.5 mA max,
    which will reduce the no load to full load variation. That
    will be part of my next experiments.

    What remains to be seen is if I have any LEDS that combine to
    give a Vf of closer to 5 volts. From what you posted, and from
    my initial experiment, I doubt I'll get significantly closer.
    I may try 2 LEDS + 2 diodes to see what that does. I want to
    look at line regulation as well as load regulation, and I want
    to use a typical maximum load of 1.5 mA versus the 5.4 mA max
    I used today. I want to try some different colors, too.
    But whatever the results, I don't see the LED "regulator"
    as a general replacement for a 5.1 zener any time soon.
     
  10. Nice experiment. The big If is will the serial port put out enough
    current.

    Probably a simpler way to get 5V from the PC is to steal some of it
    from the game port, or even the USB cable.

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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  11. Guest

    wrote:



    Today's results below - 3 different supply voltages,
    new circuit - substituted small signal diode for one
    of the LEDs, changed the LEDs to green, Rload = 2.91K

    Supply voltage: 5.66 10.00 13.64
    Vf no load 4.61 5.14 5.49
    Vf full load 4.51 5.08 5.42
    I load 1.54 mA 1.73 1.87
    I LEDs full load 2.0 mA 13.3 25.0
    I LEDs no load 3.2 14.8 26.7

    Summary
    The LEDs and diode circuit will regulate to supply the
    IR chip specs over a supply range anywhere from 5.66 V
    minimum to 13.64 volts maximum, under all load
    conditions (0 to 1.50 mA, no load - full load) The
    "sweet spot" - 10 volts - matches the OP's schematic,
    and provides excellent regulation, no load to full
    load.

    The best selection turned out to be two green LEDs
    and a small signal (1N914) diode.

    While this would work for the OP's needs, a zener
    is better, if for no other reason than reduced
    parts count.




    Test Circuit:
    Supply----/\/\/\/-------+------<-Iload->-+
    ^ 330 |<----+ |
    | LED | |
    | | | /
    | LED Vf \
    | | | / 2.91K
    Vs DIODE | \
    | | | /
    | ^ | |
    | Ileds | |
    | v | |
    v | v |
    Supply------------------+-----+----------+

    (Meters are shown as Vs, Vf, Iload and Ileds)
     
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