converting calculator to a tally device

Discussion in 'The Projects Forum' started by Colonel Bleep, May 8, 2008.

  1. Colonel Bleep

    Thread Starter New Member

    May 8, 2008
    I'm wiring a switch into a calculator, to make it into a tallying device (i.e.-counting up by one every time the switch is pressed). I'm using a SPDT microswitch connected between the 1 and + keys. There are two terminals for each key. I've joined the two keys at the microswitch common, with the remaining + terminal connected to NC and the remaining 1 terminal connected to NO. The idea is that the microswitch should toggle between + and 1, adding 1 each time it's pressed (of course, the + key also acts as the = key for addition).

    The only problem: the terminals of the + and 1 key do not like being tied together. So then I tried ganging two SPDT microswitches to make one DPDT. The problem with that is, if the switches do not come down at the same time, you might inadvertantly enter 1 on the first press, and another 1 on the second press, thus falsely adding 11.

    There are DPDT microswitches, but they seem to be clunky, expensive and hard to find (most microswitches are SPDT). So what I'd like to do, is have the SPST microswitch operate a low-voltage, low-current circuit that will activate the key terminals (the calculator runs off a 1.5 volt button cell). Maybe something with FETs or a CMOS IC, to keep the current drain down. Any suggestions?
    Last edited: May 8, 2008
  2. Caveman

    Senior Member

    Apr 15, 2008
    For most calculators, you can enter "0" "+" "1" "=" and continue to hit "=" to keep incrementing by one for every hit. Then you only need one button.
  3. Colonel Bleep

    Thread Starter New Member

    May 8, 2008
    Not this one, apparently...but thanks anyway.
    How about a circuit for this?
  4. glene77is

    New Member

    Apr 27, 2008
    Many years ago, I interfaced to a TI-59 by using CD4066 analog switches.

    In your application, you would use a micro-switch
    to trigger a LM555 as a one-shot (for debounce),
    then let that drive a set of CD4066 switches 'on' at the same time.

    Last edited: May 9, 2008
  5. Colonel Bleep

    Thread Starter New Member

    May 8, 2008
    Thanks, glene77is... that sounds like it will do the job.
  6. jpanhalt


    Jan 18, 2008
    Again, not sure of how you have it wired. You might try somehting like this using diode, so the terminal pads won't know they are tied together. John
  7. Colonel Bleep

    Thread Starter New Member

    May 8, 2008
    Thanks for the suggestion, John...actually, I already tried using diodes, but they didn't work with this particular circuit.
  8. Wendy


    Mar 24, 2008
    Just curious, with calculators going for as cheap as a $1, why not find one that only uses one button to increment?
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    The keypad is likely a matrix keypad. You could use transistors controlled by your microswitch to complete the connection.

    I'm not sure how you would determine polarity, save by experimentation.