It depends on the colour and the LED technology. The energy of red light is about 1.5eV and blue is 3eV. Add to that internal resistance of the device. An ordinary diode (not a LED) emits infrared around .6eV, which is the voltage drop of a silicon junction.

Martin

On 2013-08-07 20:02, Ed Kelly wrote:

Oh, thanks.

That was dumb I didn't remember that!

Is it really 2 volts drop for an LED? I should know this stuff...
Ed
Ninja Jamm - a revolutionary new music remix app from Ninja Tune and
Seeper, for iPhone and iPad
http://www.ninjajamm.com/


Gemnotes-0.2: Live music notation for Pure Data, now with dynamics!
http://sharktracks.co.uk/

    ------------------------------------------------------------------------
    *From:* Mikael Fernström <mikael.fernstrom@ul.ie>
    *To:* Ed Kelly <morph_2016@yahoo.co.uk>
    *Cc:* Charles Z Henry <czhenry@gmail.com>; Epic Jefferson
    <jeffreyconcepcion@gmail.com>; pd-list <pd-list@iem.at>
    *Sent:* Thursday, 8 August 2013, 0:26
    *Subject:* Re: [PD] electro-mechanical piano (player piano) -

    Arduino, Solenoid Issue

    note that you have to subtract the voltage drop over the LED, hence
    it's R = (Vsupply - Vled)/ Iled, e.g. (5-2)/0.02 = 150 Ohm

    /Mikael


    On 8 Aug 2013, at 00:19, Ed Kelly <morph_2016@yahoo.co.uk

    <mailto:morph_2016@yahoo.co.uk>> wrote:

    Check Ohm's law.

    V=IR, so the resistor you choose is the voltage you provide to the
    LED divided by the current it draws.

    e.g. if the LED draws 20mA and you power it from 5V, then the
    resistor you need is 5/0.02 = 250 ohms in series with the LED.
    This current is drawn from the positive voltage supply through the
    resistor and then the LED, and then the transistor.

    This is a fairly good tutorial:
    http://www.ehobbycorner.com/pages/tut_transistors.html
    Ninja Jamm - a revolutionary new music remix app from Ninja Tune
    and Seeper, for iPhone and iPad
    http://www.ninjajamm.com/


    Gemnotes-0.2: Live music notation for Pure Data, now with dynamics!
    http://sharktracks.co.uk/

        ------------------------------------------------------------------------
        *From:* Charles Z Henry <czhenry@gmail.com
        <mailto:czhenry@gmail.com>>
        *To:* Epic Jefferson <jeffreyconcepcion@gmail.com
        <mailto:jeffreyconcepcion@gmail.com>>
        *Cc:* pd-list <pd-list@iem.at <mailto:pd-list@iem.at>>
        *Sent:* Wednesday, 7 August 2013, 20:41
        *Subject:* Re: [PD] electro-mechanical piano (player piano) -

        Arduino, Solenoid Issue

        On Wed, Aug 7, 2013 at 12:05 AM, Epic Jefferson
        <jeffreyconcepcion@gmail.com

        <mailto:jeffreyconcepcion@gmail.com>> wrote:

            Hey Charles,

            it seems like this might work. i got some pnp transistors
            and built the circuit from julianvogels site.
            The only problem is that the LED on the test circuit
            barely lit up. I think it's because the transistors are
            not for 20mA, none were available. i'll check another
            electronics store to see if i find some.


        I think you just need smaller resistors.  Every transistor in
        a 3-pin package I've ever seen could run 20mA or much
        greater.  Swapping the transistors will have no effect on the
        amount of current.

        Chuck


                There are two ways to solve your problem:

                The proper one is to use PNP transistors or P-channel
                mosfets (remember
                I already told you about that ? :))

                See this document, you can find the wiring at the end:
                http://julianvogels.de/wp-content/uploads/2013/06/stromkreis_transistorschaltung_final-1024x627.png

                http://julianvogels.de/extending-pwm-output-pins-with-a-texas-instruments-tlc5940-led-driver/


                The good enough one is to put a pull-up resistor (10k
                works) on every
                NPN transistor base, and use the TLC as a pull down.
                In this case, the
                on-time on the TLC corresponds to the off-time on the
                solenoid. Also
                when the arduino reboots and every time the BLANK is
                issued, every
                solenoid will act for a veeeeery short time. This can
                be a big problem
                in your project. I did this for a 96 channels
                motor+led strip system,
                and I regret not using PNPs instead.


                Enjoy,

                --
                Charles



                Epic Jefferson wrote:
                > Hey guys,
                >
                > updating on this project. I got the pwm shields and
                i've hit a wall. The
                > driver circuit I'm using to control the solenoids
                via arduino is this one
                > from instructables
                >
                (link<http://www.instructables.com/id/Controlling-solenoids-with-arduino/)>)
                > and
                > it uses a single pin to control the pwm signal.
                >
                > The pwm shield
                (link<http://www.practicalmaker.com/products/arduino-shields/pwm-shield-assembled>)
                > is
                > based on the tlc5940 which requires each pin to have
                it's own ground
                > instead of a common ground across all drivers. This
                is a problem because
                > all of the information i've found suggest that the
                signal from the pin
                > controls the gate (transistor - TIP102). But i
                think, in the case of the
                > tlc5940, the 5v supply is constant and the ground is
                being controlled,
                > that's why it works perfectly for LED's but seems to
                be ill suited for this
                > circuit.
                >
                > Any suggestions on how to modify the instructables
                circuit for use with the
                > shields? or would the circuit have to completely change?

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            www.avmachinists.org <http://www.avmachinists.org/> Puerto

            Rico based Art Collective/ Non-Profit Org

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