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change voltage for driving an led

Discussion in 'Electronic Design' started by CampinGazz, Mar 6, 2004.

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  1. CampinGazz

    CampinGazz Guest

    Simple one here..

    I have a car alarm, which has a led that it flashes to show it's armed, been
    set off etc,

    i measured it at 2.1 volts, it had a bog standard red led in there, not that
    bright,

    I put in an ultraviolet led in it's place, which should run on 3.6 volts,
    but does ok on 2.1 volts, just not as bright as it could be,

    The alarm is in a motorhome, and i'd like another couple of led's near the
    other entry doors to show it's alarmed, obviousely i can't run more led's
    off the alarm led output, it's prolly limited to 10 milliamps if that.

    So i want something simple, feed it with the 2.1 volt signal from the alarm,
    and output 3.6 volts, at a higher current,

    i'm thinking along the lines of something that will switch a seperate power
    input when it recieves the pulse for the flashing led, and have that output
    12 volts, then i can drop the voltage to 3.6, and the current to what ever i
    need to run how ever many led's i want,

    i know a solid state relay would prolly be ideal, but that's pricey just to
    flash a few more led's, is there a cheaper solution, were only talking say
    75 milliamps here if i want to run 3 led's, but hte input must be 2.1 volts
    at around 10 milliamps,

    i could use a relay, but as the alarm will be armed when were sleeping in
    the motorhome, something ticking all night long won't be popular.
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Please view in a fixed-width font such as Courier.


    12V
    ---
    |
    +--------+
    | |
    / |
    10K |
    / |
    \ e
    from >-----+ | |/
    alarm | +------| 2N4403
    | | |\
    --- / c------+---------+--------+
    (existing\ / ~~ 1.5K | | |
    LED) --- / / / /
    | \ 330 330 330
    | | / / /
    | | \ \ \
    150 | | + | | |
    +-/\/\-+ | --- --- ---
    | |2N4401 | 3.6V \ / ~~ \ / ~~ \ / ~~
    | ----- | --- --- ---
    | / \ | - | | |
    | e c | | | |
    +---+ | | +---------+--------+
    | +------+ |
    --- ---
    /// ///
     
  3. Fred Bloggs

    Fred Bloggs Guest

    This will be less stressful- all resistors 1/4W except the 51 ohm which
    is a 1W metal oxide film placed near the PNP:

    Please view in a fixed-width font such as Courier.


    12V
    ---
    |
    +--------+
    | |
    / |
    10K |
    / |
    \ e
    from >-----+ | |/ 2N4403
    alarm | +------|
    | | |\ 51
    --- / c--/\/\---+---------+--------+
    (existing\ / ~~ 1.5K 1W | | |
    LED) --- / / / /
    | \ 180 180 180
    | | / / /
    | | \ \ \
    150 | | + | | |
    +-/\/\-+ | --- --- ---
    | |2N4401 | 3.6V \ / ~~ \ / ~~ \ / ~~
    | ----- | --- --- ---
    | / \ | - | | |
    | e c | | | |
    +---+ | | +---------+--------+
    | +------+ |
    --- ---
    /// ///
     

  4. Maybe. I think I'd want to know how the LED is connected. Wouldn't be
    surprised to see it between an internal Vdd/Vcc and an open
    collector/drain or micro port pin.

    Maybe the easiest way might be to replace the existing LED with a
    photodarlington output optoisolator.


    Best regards,
    Spehro Pefhany
     
  5. Fred Bloggs

    Fred Bloggs Guest

    We'll "prolly" never know. I'm not going to second guess him.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    If it is an open drain , then connecting the '4401 emitter/resistor
    junction to the drain will work the same- disconnect the GND there of
    course. The '4401 CB junctions standoff the 12V. If the current limiting
    resistor of the open drain is on the low side-it will still work. So
    make that:

    Please view in a fixed-width font such as Courier.


    12V
    ---
    |
    +--------+
    | |
    / |
    10K |
    / |
    \ e
    (+)>---------+ | |/ 2N4403
    | +------|
    | | |\ 51
    --- / c--/\/\---+---------+--------+
    (existing\ / ~~ 1.5K 1W | | |
    LED) --- / / / /
    | \ 180 180 180
    | | / / /
    | | \ \ \
    150 | | + | | |
    +-/\/\-+ | --- --- ---
    | |2N4401 | 3.6V \ / ~~ \ / ~~ \ / ~~
    | ----- | --- --- ---
    | / \ | - | | |
    | e c | | | |
    | | | | +---------+--------+
    (-)>--+---+ +------+ |
     
  7. CampinGazz

    CampinGazz Guest

    Thanks for that guys, i'll get the trannies tommorow and have a bash,
    one thing tho', i was thinking of removing the origional led, the circuits
    here still show it, what components would i need to change to take just the
    2.1 volt feed that would have pulsed the led to trigger the transistors, and
    then run all the led's from the output's??

    Is the output dependant on running 3 led's?? can i run more or less? thinkng
    at first to just run one led, then later add 2 in the awning light over the
    living area entry door, and at a later date again i want to add an led on
    things like the under floor storage locker doors,
    but at the moment i'm busy building the actual motorhome, so just want the
    singel led just to let people know it is alarmed, and add more led's later
    on.
     
  8. The output of Fred's last circuit does depend on the number of LEDs, since
    all the current goes through that 51 ohm resistor.

    You can use a slightly different circuit (mostly the same, only the output
    stage is different):

    (view in courier, of course)

    +----------+---------+---------+------------+ 12V
    | | | | |
    |< e | | | |
    +--b-| | | | |
    | |\ c | | | |
    | | | | | |
    | | |< e |< e |< e |< e
    +------+------b---------b---------b------------b-|
    | |\ c |\ c |\ c |\ c
    | | | | |
    .-. | | | |
    470 | | | | | |
    | | | | | ... |
    '-' | | | |
    | | | | |
    | | | | |
    |/ | | | |
    Input --+----| 2N4401 | | | |
    | |> V V V V
    .-. | - LED - - -
    | | | | | | |
    150| | | | | | |
    '-' | | | | |
    | | | | | |
    +------+----------+---------+---------+------------+ GND


    All PNP transistors 2N4403
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Its a current mirror, using a PNP transistor in place of the resistors. That
    way, you can add lots more LEDs simply by adding another transistor/LED
    pair.

    Regards,
    Bob Monsen
     
  9. Sure. But note that if you'd used an 88-cent (one-off/Digikey) Clare
    LDA110 instead of the 2N4401 & 150 ohm resistor it would work either
    way ((*and* with either polarity of connections to the previous LED).
    All the rest could stay the same.

    Best regards,
    Spehro Pefhany
     
  10. <snip>

    How close do you estimate the current matching will be?

    Best regards,
    Spehro Pefhany
     
  11. CampinGazz

    CampinGazz Guest

    Trying something different now, sorry to be a pain, but i've been thinking
    of using a cheap ssr, and found opto isolators...

    would a MOC3020 be any good to do what i want?

    it has an input of 1.2 volts, so i'd have to use a resistor to drop the
    input from the alarm's led output down from 2.1 volts,

    But then i'd feed 12 volts through the output terminals, the switched output
    via resistors to the led's, which complete the circuit to chassis earth,

    This way i could run upto 4 led's if i wanted on one opto isolator (100Ma
    max current)
    this particular isolator is a triac type, would it work on 12 volts DC going
    through the switched terminals?

    or am i grasping the wrong end of the stick again,
     
  12. No, it will never turn off, since it has an opto-triac output. Triacs
    do not turn off until the current drops almost to zero, which won't
    happen with a DC supply (at least until the power is interrupted).
    They are used on AC where there are typically two zero crossings every
    1/60 or 1/50 of a second.
    Very unlikely you'd need to do anything other than just wire it across
    the existing LED (which would then stay off).
    Only the turning on part.
    Here is a more expensive (about $6.50) part that will do the job with
    a minimum of components. You need a resistor in series with each LED
    (or series string of LEDs) on the output,of course.

    http://www.irf.com/product-info/datasheets/data/pvd13n.pdf


    existing +12
    LED+ o o
    | |
    | |
    .--|---------|--.
    | | | |
    | | +--+ |
    | | | | |
    | V ||-+ | |
    | - ||<- - |
    | | ||-+ ^ |
    | | | - |
    | | +--+ |
    | | | | +---+
    '--|---------|--' | |
    | | ___ | V <-- eg. 3 series LEDs
    | <------+-|___|---+ -
    | More .-. Rb |
    o LEDs | | |
    existing | |Ra V Ra = (12V-Vf)/I
    LED- '-' -
    | | Rb = (12V- (Vf1+Vf2+Vf3))/I
    V V
    - -
    | |
    === ===
    GND GND


    Best regards,
    Spehro Pefhany
     
  13. Fred Bloggs

    Fred Bloggs Guest

    I wonder if he can use the MOC's as a weak optocoupler- holding current
    spec'd at 100ua typ and trigger current at 30ma for the '3020- don't
    have any to experiment with at the moment:

    Please view in a fixed-width font such as Courier.



    FROM

    (-) ALARM (+)
    | |
    | |
    | |
    | MOC3020 |
    | +-----+ |
    | 2| |1 |
    +------|<|------+
    | |
    1M | |>| |
    GND>-------+----/\/\----+---------| +------+------<12V
    | | 4| |<| |6 |
    +-----||-----+ +-----+ |
    | 0.001U | |
    | | 0.1U |
    +------------|---||-------+ |
    | | | |
    | +--+--+ | |
    | | | | |
    | +--TR----THR-----+ | |
    | | | | |
    | | | | |
    | | TLC555 RST--+-----+
    +-----GND | | |
    | | VCC--+ |
    | | | |
    | +-------OUT------+ |
    | | |
    | | 1.5K 10K |
    | +--/\/\-+-/\/\---+
    | | |
    | | e
    | | |/
    | +------|
    | 2N4403|\
    | c
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    | LED 1/2 WATT |
    | |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    | LED 1/2 WATT |
    | |
    | |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    LED 1/2 WATT
     
  14. Clever. ;-)

    They used to occasionally use thyristors like that. What were they
    called.. remote base transistors?

    You could avoid the TLC555 etc. and just drop a power MOSFET in there.

    Best regards,
    Spehro Pefhany
     
  15. Fred Bloggs

    Fred Bloggs Guest

    Okay- the OP should include a current limiting resistor for the TRIAC in
    the event it does trigger fully on:
    Please view in a fixed-width font such as Courier.



    FROM

    (-) ALARM (+)
    | |
    | /
    | 100
    | MOC3020 /
    | +-----+ \
    | 2| |1 |
    +------|<|------+
    | |
    1M | |>| |
    GND>-------+----/\/\----+---------| +------+
    | | 4| |<| |6 |
    +-----||-----+ +-----+ |
    | 0.001U | |
    | | 0.1U |
    +------------|---||-------+ /
    | | | 1K
    | +--+--+ | /
    | | | | \
    | +--TR----THR-----+ | |
    | | | | |
    | | | | |
    | | TLC555 RST--+-----+------<12V
    +-----GND | | |
    | | VCC--+ |
    | | | |
    | +-------OUT------+ |
    | | |
    | | 1.5K 10K |
    | +--/\/\-+-/\/\---+
    | | |
    | | e
    | | |/
    | +------|
    | 2N4403|\
    | c
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    | LED 1/2 WATT |
    | |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    | LED 1/2 WATT |
    | |
    | |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    LED 1/2 WATT
     
  16. Fred Bloggs

    Fred Bloggs Guest

    Please view in a fixed-width font such as Courier.

    FROM

    (-) ALARM (+)
    | |
    | /
    | 100
    | MOC3020 /
    | +-----+ \
    | 2| |1 |
    +------|<|------+
    | |
    | |>| |
    +-----||-----+---------| +------+
    | 0.001U | 4| |<| |6 |
    | / +-----+ |
    | 470K |
    | / |
    | \ /
    | 680K | 1K
    +----/\/\----+ /
    | | \
    | --+- |
    | ---- |
    | |^ | |
    GND>---+----------++ +-----------+ +---<12V
    IRF510 | |
    MOSFET | |
    | |
    | |
    | |
    | |
    WIRE PAIR-------> | |
    TO LEDS | |
    | |
    | |
    +-------------------------+ |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    | LED 1/2 WATT |
    | |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    | LED 1/2 WATT |
    | |
    | |
    | |
    | |
    | - 3.6V + |
    | 330 OHM |
    +----|<|----------/\/\----------+
    LED 1/2 WATT
     
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