some help needed please-comparator

Discussion in 'The Projects Forum' started by tibbles, Jul 15, 2014.

  1. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    hi
    im putting together a comparator output stage, which is working as it should,but the changeover point on the divider variable resistor is very tricky to set, is there any way of padding the variable resistor 22k linear. or changing it(The input is rectified ac),to extend the changeover point ?
    thank you
    doug.
     
  2. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    ps the changeover point is 11k.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Your description is not clear. I presume you mean you are adjusting a threshold voltage with a potentiometer and the adjustment is too sensitive for your liking.

    At this stage a circuit diagram would be helpful if that is not too much trouble.
     
  4. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
  5. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    oops..................
     
  6. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    sorry, the input goes to pin 5
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Ok. There are a number of solutions you can choose.

    1) For ten-times better control, you can replace the pot with a 10-turn pot. If the adjustment is a small screw-driver trimpot, there are 10-turn trimpots available.

    2) If you can live with a reduced control range, you can insert two fixed resistors in the outside arms of the potentiometer. For example, if you want to retain the same 22kΩ pot, by placing fixed 22kΩ resistors on both arms of the pot you will limit the range of the adjustment by one-third the original range.

    You can also change the pot to say a 5kΩ pot and use two 10kΩ resistor on the outside arms.

    3) A third solution would be to have a course control and a fine control. This means have two pots for course and fine adjustments.
     
  8. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    thank you mrchips for your prompt and helpfull reply, i did get a couple of 10 turns in, i'll give one a try first,

    to digress slightly im sure others will have thought of this, if im doing a fairly simple circuit like this. i use a piece of .1" perforated board as my drill guide, i just mark the perfboard with a felt tip where the holes need to be, and just make sure the perfboard dosent move while drilling.no drill press needed.and then just join up the dots with the resist pen, i never could get on with a breadboard.ive done quite complex circuits this way.
    at risk of boring people i havent got a vice suitable for use while cutting pcb board but ive found i can cut even small pieces by putting a couple of thin blobs of bluetack under the board,(and a large hacksaw.)
    thanks again

    doug
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    You are joining up the dots with the resist pen?
    Are you planning on etching your own PCB?

    For one of a kind projects or prototyping I use Veroboard.

    [​IMG]
     
  10. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    yes, its veroboard i use as a drill guide,(somitimes called perf board ) the big advantage of drawing the resist freehand,is you can make the traces and pads a lot bigger on prototypes so you can then add, subtract or change components later.

    i still use transfer or photo- sensitive for the more complex or compact circuits ,incidently ive found the best by far hot iron-on transfer medium ,
    is the dedicated pcb printing film.this transfers all of the print, it dosent like too much heat, so use a piece of card under the iron,

    all this probably old hat to the seniors who ive the utmost respect for, however some of the newbies might find it useful,

    regards doug
     
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