Arduino Forum

Topics => Device Hacking => Topic started by: ElectronicNoob on Jul 10, 2018, 06:36 pm

Title: Reversing calculator keypad
Post by: ElectronicNoob on Jul 10, 2018, 06:36 pm
Hi, I just dissasembled a calculator and wanted to connect the keypad to my arduino. Honestly I have no clue how to do that. I think I know the idea behind matrix keypads, but the keypad works bit different.

The only things I know:
-The keypad is piece of paper
-There is specially crafted rubber pad, that helps to make connection between paper and finger
-The keypad is connected via 11 pin FFC/FPC
-There are 25 keys on the keypad
-I have no success with checking conductivity between each pin pair with pressed button

Does anybody have any tips that would help connect the keypad with mine microcontroller?

Images:
Front:
(https://i.imgur.com/slq0bTa.jpg)

Back:
(https://i.imgur.com/gE4CI8Jg.jpg)
Title: Re: Reversing calculator keypad
Post by: Paul_KD7HB on Jul 10, 2018, 07:36 pm
Hi, I just dissasembled a calculator and wanted to connect the keypad to my arduino. Honestly I have no clue how to do that. I think I know the idea behind matrix keypads, but the keypad works bit different.

The only things I know:
-The keypad is piece of paper
-There is specially crafted rubber pad, that helps to make connection between paper and finger
-The keypad is connected via 11 pin FFC/FPC
-There are 25 keys on the keypad
-I have no success with checking conductivity between each pin pair with pressed button

Does anybody have any tips that would help connect the keypad with mine microcontroller?

Images:
Front:
(https://i.imgur.com/slq0bTa.jpg)

Back:
(https://i.imgur.com/gE4CI8Jg.jpg)
Explain your method of testing the conductivity. The rubber contacts are carbon filled, so are resistors. The printed contact pads have infinite resistance until a rubber pad is pressed onto it. Then the resistance will no longer be infinite. Over time these things stop working for various reasons.

Is such a keyboard usable with an Arduino? Depends on the resistance you read with your ohmmeter when the pad is pressed down. Pressure also has a bearing on the resistance.

Paul
Title: Re: Reversing calculator keypad
Post by: ElectronicNoob on Jul 10, 2018, 09:07 pm
Firstly, thanks for the answer.

The first method I was trying was to remove the secure thing from the FPC tape, then put ohmmeter probes between pin 1 and 2, then move the second probe from pin 2 to pin 3, 4, 5 and so on, before every probe move i was trying to press every button and check for connection, just like in casual matrix keypad but as I said it gave no results.

Now i attached the probes like on the image:
(https://i.imgur.com/aLrSyVU.png)

Normally the ohmmeter shows about 20M Ohm, when i press some buttons it drops to 5M Ohm, but I'm not sure if it's measurement error because i need to hold the paper upside-down. I probably need an FPC connector to get reliable results.
Am I measuring it in wrong way?
Title: Re: Reversing calculator keypad
Post by: Paul_KD7HB on Jul 10, 2018, 09:14 pm
Your measurements seem pretty normal for that type keyboard.

Paul
Title: Re: Reversing calculator keypad
Post by: ElectronicNoob on Jul 10, 2018, 09:26 pm
Is such a keyboard usable with an Arduino? Depends on the resistance you read with your ohmmeter when the pad is pressed down.
So, if these measurements are correct is the 5M Ohm value good enough? If so how can I recognize which key was pressed? I assume that it works quite different from the casual matrix keypads, because there are actually 11 pins on the keypad, while there are 5 rows and 5 columns
Title: Re: Reversing calculator keypad
Post by: Paul_KD7HB on Jul 10, 2018, 09:55 pm
It is not usable without some type of interface circuitry to convert the change from 20meg to 5 meg into some value, either 0 volts or 5 volts. I am not engineer enough to conceive of such a circuit.

Paul