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Flashing LEDs proportion to Servo position

Discussion in 'Electronic Basics' started by [email protected], Jul 22, 2005.

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  1. Guest

    I have a LED array already made, complete with resistors and 9.2V
    battery pack. I would like to be able to flash the LED's, with the
    speed of the flashing based upon the position of a servo. At 0 degrees,
    it would be on all the time without flashing, all the way to 90 degrees
    with it flashing 15 or 20 times a second.

    To make things harder, this is using the throttle servo from my RC car,
    which also controls the brakes. At 0 degrees full brakes are applied,
    and at around 15 degrees I think, brakes are not applied and the engine
    just sits there idleing. If it's just idleing at 15 degrees it would be
    awesome if i could get it to flash for about 2/3rd's of a second
    (2/3rd's on, 2/3rd's off) and then when accelorating from there go up
    to the 15 or 20 flashes a second. But when applying any amount of
    brakes (from zero to 15 degrees) I would like it to be on the entine
    time. Does this sound to complicated for an amature like me to do? If
    so then a linear change in blinking speed would be OK.

    I googled around, and found out that the signal wire for servos sends
    the angle information via pulses. 1.25ms for 0 degrees, up to 1.75ms
    for 180 degrees, according to:

    However, since my servo is set up with some custom trim levels, is
    there any way to find out what the pulse lengths are at various stages
    (full brakes, idleing, full throttle) so I don't have to just guess and
    check over and over?

    I'm fairly new to electronics, and have no idea where to even start to
    make this fancy "switch" for my LEDs. I was looking at my old 75-in-One
    Electronic Project Lab that I got for my 12th birthday, and the closest
    I found was a variable strobe light, which used a transistor,
    transformer, some resistors, and a pot, to blink a LED at different
    intervals depending on the rotation of the pot.

    But that did it based upon the resistance, not pulses, so I am back to
    the beginning :(

    I would like to learn as much as possible as I make this variable
    switch, so if it is possible, please dumb things down, so I can at
    least partially comprehand your idea/point, and google for more info as

    --Farrell F.

    P.S. In case this information helps:

    I'm using 32 LED's, wired in a series-parallel configuration. 2 LEDs +
    resistor in series, with 16 strings wired in parallel. The LEDs are
    spec'd for FV 3.0-3.4V, so aiming for 3.2V, I went with 160 ohm
    resistors since my power source is 9.6V. Each LED draws ~20mA, so all
    32 LEDs draw ~640mA. Perhaps this information effects the components I
    should select to make this fancy switch?
  2. Andrew Holme

    Andrew Holme Guest

    Two options spring to mind:
    1. Convert the pulse width to a voltage using an RC filter. Then,
    generate the variable flash rate using a voltage controlled oscillator.
    2. Use a PIC Micro-controller. This requires fewer components because
    everything is done in software. The PIC measures the pulse width and
    generates the output pulses.

    You can measure the pulse widths using an oscilloscope. However you
    decide to tackle it, you'll need a 'scope for a project like this.
  3. John Fields

    John Fields Guest

    I've posted a schematic for you, at:

    based on the data at the link you posted. The circuit should do
    everything you want, including the steady "brakes on" function. Below
    15° the LED array should be ON all the time, at idle (15°) it should
    flash at about 1Hz and, at 90°, at around 16Hz.

    The two one-shots (U1A and B) are adjustable so that you can set the
    response of the LEDs at idle and also to avoid overrun at the high
    end. If you need a circuit description, ask and I'll post it.
  4. Guest

    i am unable to follow that URL, any chance you can e-mail me the

    upgrdman at mindspring dot com

    --farrell f.
  5. Guest

    Thanks you so very much, John Fields. If it's not too much trouble, I
    would appriciate it if you would post the circuit description as well.

    On a side note, I'm thinking of buying a new Electronic Project Lab.
    The 75-in-1 I have is limited, and I'm thinking about perhaps gettings
    the 500-in-1 model from Elenco. has some info on it,
    although their price is higher than most places.

    Anyone have any advice? I can barely understand anything above a basic
    schematic, and want to learn as much as possible, in a way that is
    somewhat entertaining. The 75-in-1 model I have is alright, but it's
    doesn't explain the "why" very well in most cases, and it doesn't get
    into the advanced stuff like IC programming and radio. This lab seems
    like a good thing to me, but I am interested if anyone has anything to
    say one way or another.

    Thanks again,
    --Farrell F.
  6. John Fields

    John Fields Guest


    From the link you gave, it turns out that your servo, with an input
    pulse width of 1250µs corresponding to a rotation of 0° and 1500µs
    corresponding to 90°, will need a pulse about 1300µs wide to rotate to
    15°, your approximate idle/no brakes position. Looking at the timing
    graphically, we have something like this:

    _________________________ ______
    T1___| |________...___________|

    __________________________ ______
    T2___| |_______...___________|

    ______________________________ ______
    T3___| |___...___________|


    Where T1 is the input pulse with maximum braking applied, T2 is the
    input pulse with the engine idling and the brakes off, and T3 is the
    input pulse with the servo at full throttle. the 20ms time is how
    often the input pulse repeats, and stays constant at 20 milliseconds.

    Since we have two states we want to differentiate between, (the first
    being when the brakes are on and the second being when they aren't)
    what we need is a circuit which can tell the difference between when
    the input pulse is less than 1300µs wide and when it's greater than
    1300µs wide. U1A, (a monostable multivibrator with a period of
    1300µs) U2A, (a "D" type flip-flop) U3A, (a NAND gate) and U4A (an OR
    gate)do that, like this:

    __________________________ ______
    U1-6___| |_______...___________|
    ___ ________...___________
    U3-3 |_________________________| |______
    _________________________ ______
    U1-4___| |________...___________|

    Assuming that U2-5 starts off low, if the pulse from the servo, U1-4,
    goes low before U1A times out, its inverted version at U3-3 will clock
    the high on U2-2 through to U2-5, and U2-5 will stay high for at least
    one 20ms cycle of the servo. If the next pulse from the servo is
    shorter than 1300ms, a high will again be clocked though U2A, with the
    result that U2-5 will remain high as long as the pulse from the servo
    is less than the period of U1A, which is 1300ms, which corresponds to
    15°, the idle/no brakes boundary. Since U4 is an OR, if any of its
    inputs are high its output will also be high. So, as long as U4-1
    remains high, U4-3 will remain high, (regardless of what's happening
    on U4-2) the gate of Q1 (a logic-level MOSFET) will be driven high,
    and the LED array will be continuously illuminated. That, then,
    fulfills the first requirement, which is that the LEDs stay ON
    continuously any time the brakes are applied, which will be any time
    the servo's input pulse is less than 1300µs wide.

    Now, assume that the servo's input pulse width increases to greater
    than 1300µs. The timing will look like this:

    __________________________ ______
    U1-6___| |_______...___________|
    ___ _____...___________
    U3-3 |____________________________| |______
    ____________________________ ______
    U1-4___| |_____...___________|
    U2-5 |_____...__________________
    U4-3 |_____...__________________

    Notice that now, since the servo's input pulse went low _after_ U1A
    timed out, U1-6 will be low when U3-3 goes high, so U4-1 will go low,
    allowing whatever is on U4-2 to control what happens on U4-3.

    Since we have a pulse width of 1300µs with the engine at idle and
    1500µs with the engine at full throttle, we have a difference of up to
    200µs between when U1A times out and the pulse input to the servo
    times out, and we'd like to be able to use that difference to make the
    LED array flash slowly when the engine is idling and flash quickly
    when it's at full throttle.

    In order to do that we can use an up counter to accumulate
    high-frequency clocks during the time between when U1-6 goes low and
    the input pulse to the servo goes low, and then load that count into a
    down counter which counts continuously and loads itself with the count
    on the output of the up counter every time it gets to zero. If we use
    4 bit counters we can have up to 15 different flash rates, with U6
    being the up counter and U7 being the down counter.

    Assuming that we want a 200µs difference between the two pulses to
    result in a maximum count of 15 means that if we enable the counter
    for 200µs we'll need to accumulate high-frequency clocks at the rate
    of 15 clocks per 200µs, which is a frequency of:

    1 1
    f = --- = ----------- = 75kHz
    T (200µs/15)

    Since we'll want to accumulate clocks using the high frequency 75kHz
    clock, but display the speed-variable flash at a much lower frequency
    we can use something like a 4060, which has an integral oscillator and
    a fairly long ripple chain to good advantage here, and U5 is that
    counter. Since we need 75kHz and Q3 is the highest frequency output
    available from the ripple chain, we'll need to set the oscillator to 8
    times 75kHz, or 600Khz, with C3, R3, and R4, according to:

    T = 2.5 R3 C3

    Where T is the period of 600kHz:

    1 1
    T = --- = --------- = 1.67E-6s = 1.67µs
    f 6.0E5Hz

    Arbitrarily choosing C3 as 100pF and rearranging

    T = 2.5 R3 C3

    to solve for R3 we have:

    T 1.67E-6s
    R3 = -------- = ---------------- = 6680 ohms
    2.5 C3 2.5 * 1.0E-10F

    6650 is a standard 1% value and is shown on the schematic with a 5%
    cap. R4 needs to be approximately equal to twice R3, so 13.7k will
    work there. However, depending on your application and the pulse
    width of your servo control signal, the frequency may need to be
    adjusted in order for the thing to work properly.

    Now, in order to gate the 75kHz counter (U6) properly, we need to
    first clear it, and then enable it to count by supplying it with a
    low-going pulse with a width which will allow it to count 75kHz
    clocks, with the number of clocks counted being proportional to the
    difference in timeout times between U1A and the servo's input pulse.

    The CLEAR pulse is generated by U2B and U3B, and is a single pulse
    with a maximum width of about 13µs generated every time the servo's
    input pulse goes high, while the enable pulse is generated by -ORing
    the output of the 1300µs one-shot, the inverted servo input pulse, and
    the complementary output of U1B, a one-shot set to time out about
    1500µs after the start of the servo's input pulse. The enable pulse
    timing with the servo pulse timing out close to 1300µs is:

    --1300µs-->| |
    ______| |
    U1-6 |___________________________
    SERVO |__________________________
    _______ ______
    U6-4 |____________________|


    and with the servo pulse timing out far from the 1300µs one-shot:

    --1300µs-->| |
    ______| |
    U1-6 |___________________________
    SERVO |________
    __________________________ ______
    U6-4 |_|

    -->| |<--13.3µs

    Which gives us exactly what we want.

    Consider: with the engine idling, the servo pulse will terminate close
    to 15°, (1300µs) which will allow U6 to count for a long time and fill
    up. Consequently, when free-running down-counter U7 counts to zero,
    its carry output will go low and load the outputs of U6 (hex "F") into
    U7, while when the servo pulse terminates close to 90°, (1500µs), U6
    will only be able to count up to 1, which will be loaded into U7 when
    U7 counts to zero. Since it takes longer to count from F down to
    zero than it does to count from from 1 down to zero, when the engine
    is idling it will take U7 16 times longer to count down to zero than
    when the engine is at full throttle. Now, to flash the LED array, the
    carry output from U7 is used to clock U8B, which is a "D" type
    flip-flop wired as a divide-by-two. Assume that U7-13 went low
    earlier, loaded the new count value into U7, and has just gone high.
    If U8-5 was low before U7-13 went high, it will now go high and the
    LED array will light. While the array remains lit U7 will be counting
    down, and when it gets to zero, U7-13 will go low and load the new
    value into U7. it will also toggle U8B, causing U8-5 to go low,
    turning off the LED array. Since U7 is free-running, the
    load-toggle-count cycle will continue indefinitely with the flash rate
    depending on the width of the servo input pulse.

    U8A is a "D" type flip-flop wired to extend the count range of U5 in
    order to get a low-frequency clock to drive the LED flash counter, U7.
    U8-9 toggles at about 36Hz, which means that with U7 loaded with hex
    "F" the LED array will flash at about 1Hz, with the array on for about
    1/2 a second and off for half a second. With the array loaded with
    '1', it will flash at about 18Hz with the array on half the time and
    off half the time.

    In order to adjust U1A and B what you'll need to do is to connect the
    servo control pulse to the circuit at U3-1 while the car is idling and
    adjust R1 until you get about 1 flash per second out of the array,
    then rev the car up and adjust R2 until you're happy with the flash
    rate. If you try to make it flash too fast the counter will roll over
    and it'll start flashing slowly again, so what you'll need to do is
    back off on R2 until it starts flashing quickly again. Also, if your
    servo's characteristics are different from those in the link you
    supplied, it may be necessary to change C3, R3, and R4 in order to get
    the thing to work at all.

    Finally, there were a few errors on the schematic so I fixed them and
    I'll repost the revised one to alt.binaries.schematics.electronic in a
    few minutes.
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