Blank cd

Posted: June 17, 2009 in Log!c Puzzles
Tags: , ,

There is a blank cd. You are given two colours of paint, say black and white . A sensor can recognize the colour painted on the disc and produce an output. Paint the disc in a way such that you can find the direction of rotation by looking at the output.

Please try to solve this problem.If not I’ll mail u guyz later.

  1. Aadya says:

    could u plz , explain this puzzle a lil’ more, only if its permissible!!!

    • Sure… Imagine that u hav a cd with u.Ther is a motor and ur cd is fixed in the rotor (the rotating part of the motor). Befor that u hav to colour it with black and white paints.
      Now ur cd is rotating. And ther is a sensor which senses the colour in the cd and prints the pattern in which colouring is done in ur cd (eg., BWBWBW if u colour ur cd with alternate colours)
      Now the sad part is u ll get the same result irrespective of whether cd rotates in clockwise or anti-clockwise direction. So the qn is 2 find a colour pattern in which u r supposed 2 colour ur cd such that by the o/p we can find the direction in which it is rotating.
      Hope that nw u hav understud the qn.. Comment me for more clarifications…

  2. Srinivas Reddy says:

    I will paint in 33% white, 33% blank and leave the rest clockwise.
    If the output has blank after Black, it is rotating clockwise, else if the output has blank after White, it is rotating anti-clockwise

  3. Benjamin says:

    does the sensor know the ‘size’ of each color? Meaning you could have something such as WWWBWWWBBBB…. ?

    • Ben,

      Sensor need not know the size of colour sectors, assuming, sensor is not triggered in time basis. It is always on, and it logs if it encounters a color change.
      So number of sectors does nt really matters.

  4. Benjamin says:

    damn!!! πŸ™‚

  5. Benjamin says:

    And could the time to go from one colour to another be calculated?

  6. Shilpa says:

    I will paint in 50% white, 50% black in clockwise direction so that first half is white and the second half is black.
    Now If the output has white and then Black, it is rotating clockwise, else if the output has black and then White, it is rotating anti-clockwise.

    • Shilpa,

      Good try but you missed a point that, there is no exact start point or end point in the disc. And the sensor doesn’t read from the starting of the disc rotation. That would make it simple :). Imagine the disc is spinning and from one instant sensor starts reading. With the pattern of the output you must sort it out.

      With your solution BWBW and WBWB both can be possible results :). You are close to the solution. Give some more try. good luck


  7. shivi says:

    divide cd into 8 sectors and paint 4 sectors with white then 1 sector with black then adjacent sector with white and again remaining 2 sector with black…so cd now looks like W W W W B W B B….now if in output after 4 whites one black come and then one white come cd is moving in anti clockwise direction …..else if after 4 white 2 black comes cd is moving in clockwise direction

  8. Frisky says:

    Hey Murali,
    You mentioned that sensor is not triggered in time basis. It is always on, and it logs if it encounters a color change.So making 8 sectors with color (WWWWBWBB) will make a display of WBWB only .How can you figure out direction from that.

    • Deepak,

      we need to observe the complete pattern of the reading, not just a chunk of it. If we divide the disk into n sectors, we must observe consecutive n readings.
      So with a result of decreasing “W” and increasing “B” would mean one way and increasing “W” and decreasing “B” would mean the other direction.
      Hope you understood. Do respond me for further queries.
      Thanks for your time.


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