555 timer question; up/down freq control;

Discussion in 'The Projects Forum' started by Len Horowitz, Aug 13, 2012.

  1. Len Horowitz

    Thread Starter New Member

    Aug 13, 2012
    I am using a 555 timer to control the speed of a film recorder. The timer operates from 140 to 600 Hz. When enabled, (the + 5V command stays "high"), the timer needs to ramp up to its "set" frequency. When the start command is removed , I need the reverse condition-- The "set" frequency is ramped to "off". Otherwise it will tear the film. I can't find any suggestions for this. the original MAGNATECH control equipment is too elaborate; they count up and count down for synch, rows and rows of ICs- this is not required for my present application . I wanted something simple.
    Maybe it's not possible with a handful of components.

    Thanks for reading.. I have faith in you guys!
  2. ramancini8


    Jul 18, 2012
    Pin 5 is the control pin on a 555. Vary the pin 5 voltage to change the frequency. You can vary this voltage with a triangle wave or two ramps to get the effect you want, but control is limited and not very repeatable from IC to IC. I suspect that the Magnatech hardware is complicated because that is what it takes to get the design to work correctly in high volume applications.
    Len Horowitz likes this.
  3. Len Horowitz

    Thread Starter New Member

    Aug 13, 2012
    Yes, it is not the best way to control the frequency of the 555 in a predictable way..
    The machine already has this 555 circuit so I'll have to try something. Is it safe to ground pin 5?
  4. WBahn


    Mar 31, 2012
    If you can't get adequate performance by playing with the control pin on a 555, the magic words that you want to investigate are "voltage-controlled oscillator".

    When you say that the 555 operates between 140Hz to 600Hz, do you mean that this is the range of the 'set' frequency. In other words, at any particularly time, the timer is configured to run at, say, 500Hz and what you need is to smoothly ramp it from 0Hz to 500Hz upon a start command and then run at 500Hz until the start command is removed at which point it needs to deramp from 500Hz to 0Hz?

    This is the kind of thing that a simple 8-pin microcontroller would be very good at, but if you have never worked with microcontrollers there is some learning curve you have to go through and a little bit of up-front cost (but both can be kept pretty tame pretty easily).

    What kind of time frame are you looking at for the ramp (the time from stop to full speed and vice-versa)?

    If you were to go with a bunch of discrete steps, what do you think the biggest step you could tolerate? For instance, say you are in the middle of your ramp and are putting out 100Hz on your way to 500Hz and you are now going to change the frequency from 100Hz to something higher in one step, could it handle going to 150Hz or would it have to be no more than 105Hz or what?