I have a simple 8 digit calculator and want to use it in a way that instead of pushing the keys on keypad, connecting the keypad wire to an Arduino to control the calculator.
In general I know how the Arduino can read the matrix keypad, but it seems this calculator keypad is not using the scan technique that makes a column wire to 1 and then reading the row wire to detect the digit pressed.
There are 11 wires from MCU to the keypad and there are 24 keys on the calculator keypad.
I checked all 11 wires with an oscilloscope: 5 of them have 1.58 volts and 6 of them are 0 volts. There is no pulse on any of them.
The calculator works with a 1.5 volt coin battery. In my test, I connected an AAA battery and at that time battery voltage was 1.62 volts.
I found that the calculator MCU will convert 1.5 volts to 3 volts for internal use.
My question is what technique is used in this calculator to read keypad?