Noobie question about calculator board

Discussion in 'General Electronics Chat' started by seal308, Apr 17, 2015.

  1. seal308

    Thread Starter New Member

    Jun 30, 2012

    I know nothing about circuits right now, will be taking my first course next term.
    Recently my Sharp Scientific calculator started malfunctioning.
    Of the number keys only the numbers 1, 2, and 3 worked.

    This frustrated me because I had a exam the next day so I decided to smash it with a iron bar thing that my mom uses in the kitchen.

    I loosed some screws and started prying with a metal ruler and eventually was able to split it into 3 major components. 1. The plastic shell and keys, 2. The physical circuit board where the numbers were, were grey colored circles, and 3. The rubber that was the interface between the plastic keys and the circuit board.

    So I tried pressing the grey circles on the board with my finger and that didn't do anything.
    Then I noticed the rubber interface had small rivets where the plastic buttons were and if you touched that small indentation to the grey circle 1, 2 or 3 would appear.

    So I thought maybe those small groves had small magnets. So I took a refrigerator magnet and tried using that to press on the board, but nothing happened.

    This made me think that they were not magnets. My guess is that they were just pieces of conductive metal then when touched to the grey circles of the board formed some sort of closed connection to allow electricity to pass.

    I then took a multimeter, which I have no idea how to really use and started making it touch the grey circles.
    Just touching only the black tip or the red tip didn't do anything. I noticed I had to use them both together.

    I put the multimeter to measure resistance between one of the working buttons and the rest of the grey squares. The power was on and for some I would read a resistance and for others the resistance would not appear.
    So I'm thinking this means the ones that do have a resistance are connected in some way.

    Also strangly while on measuring resistance, if I kept pressing one of the tips to a grey circle eventually sometimes one of the working numbers would appear.

    I tested this out with the other things on my multimeter and found that if I see the multimeter to read micro amps and did the touching thing it was way more responsive and was easier to display numbers than on resistance mode.

    So my questions are:

    1. Am I correct in thinking that before pressing a button the circuit is open and then when you press a button it makes two pieces of metal touch causing a closed connection to allow electricity to flow.

    2. Why when measuring resistances the resistance between some were 0 and others had a number. Is it correct in thinking that they are connected somehow. Just weird because they seem like random keys to be linked to.

    3. Why was it easier to produce numbers when the multimeter was set to microamperes instead of resistance? I could still produce a number with it on resistance it would just take longer and more times to produce a number.

    I would really like to know number 3, I don't get that one at all.

  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    the buttons are made of conducting rubber,they short together two tracks to make that digit work, you cant measure resistance with power applied.
  3. seal308

    Thread Starter New Member

    Jun 30, 2012
    Thanks for that, learned I shouldn't measure resistance with the power on.
  4. seal308

    Thread Starter New Member

    Jun 30, 2012
    Are there any uses for the conductive rubber.
    I was looking at instructables it said you can use conductive rubber on a phone so that a regular pencil can be used as a stylus.
    But my conductive rubber does not work with the screen.
    Why is that?
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012

    The buttons on a calculator normally stop working when the copper on the circuit board starts to oxidize a little. It will be less bright. The rubber is rarely the problem.

    To re-polish the copper on the printed circuit board with household materials, use a good quality eraser for pencils. Rub the eraser over the copper pads until they are bright. Then remove the rubber dust, close it up and it should work.
  6. ian field

    AAC Fanatic!

    Oct 27, 2012
    If there seems to be some sort of pattern to which buttons work and which ones don't, it could be a crack in the PCB - all too common when people sit down with the calculator in a back pocket. The buttons are arranged in rows and columns, so a crack through just one print trace can cut off a row or a column of buttons.

    When I was at college, the price of even the most basic calculator was a very big deal - these days, even supermarkets have some pretty flash calulators at pretty much pocket money prices. If you really are that hard up - try posting wanted on a few of the free recycling groups - I got a calculator with probably more functions than I'll ever use on Freecycle.

    Calculators can be tricky to repair reliably - and you don't want it to conk out in the middle of an exam.
    absf likes this.