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PWM with a 555 Timer...

Discussion in 'General Electronics' started by Len Lekx, Mar 1, 2005.

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  1. Len Lekx

    Len Lekx Guest

    Got a quick question.

    I want to use a 555 timer chip as a pulse-width modulator, in
    order to drive an R/C servomotor. The pulse width will need to be
    varied between 1ms and 2ms for this application.

    What I don't know how to calculate, is how the varying voltage at
    Pin 5 (Control V) of the chip affects the timing. The external RC
    circuit changes the timing by t=1.1RC - that much I know... but how
    much does differences in ControlV alter this?

    Any advice would be helpful. :)
     
  2. Rob

    Rob Guest

    Take a look at http://notrocketscience.mysite.wanadoo-members.co.uk/ibmport.htm
    I never built it, it was only simulated but should work. From memory,
    it takes a variation of around 1/3 of the supply, relative to the
    positive rail to get the variation required. Only vary the voltage
    between 1/3 and the full positive supply.
     
  3. Len Lekx

    Len Lekx Guest

    Thanks. I'll keep that in mind, and look at the website later.
    :)

    Oh - and just FYI... the project I have in mind *is* rocket
    science. I'm planning to use the servos to drive the fins on a
    vertical-stabilization system for my camera rockets.
     
  4. WDino

    WDino Guest

    Have you considered using a Picaxe-08M?
    It is ideal for PWM motor control and as can be software or hardware controlled.
     
  5. Len Lekx

    Len Lekx Guest

    Never heard of that particular device. I *have* been considering
    using a Basic-Stamp... my only problem would be how to write a
    non-multitasking program to vary one of the PWM outputs after a
    trigger event.
     
  6. WDino

    WDino Guest

    Have a look at http://www.rev-ed.co.uk/picaxe/

    You would not consider using a Basic Stamp after trying one of these.
    Extremely easy to use and program.
     
  7. Len Lekx

    Len Lekx Guest

    Looks to me like it's a slightly beefed-up Basic Stamp. Many of
    the same commands and everything. :)

    No Canadian or US distributors yet, either... according to the
    website.
     
  8. Len Lekx

    Len Lekx Guest

    I've been looking at the online manuals... it's got potential. The
    'servo' command in particular would be useful - but I can't find
    anything that will tell me if two servo commands for different pins
    would be mutually exclusive. If I can put (say...) a 1.5ms pulse on
    one pin, and a 1.95ms pulse on another at the same time. If it came
    right down to it, no doubt I could use two chips - each running a
    different servo channel.
     
  9. Andy P

    Andy P Guest


    I use picaxes all the time, andthey're so relatively dirt cheap that
    using two would be ideal, as long as you can tolerate the weight.

    Iv'e got a question for you though...would not making a stabalisation
    capacity with movable fins turn your rocket into a guided projectile,
    which falls under all kinds of new rules that are quite different from
    jsut a rocket that you fire and hope goes where you want it to. I have
    a feeling that the FAA would not appreciate it.
     
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