Arduino Mimicking Voltage Dividing Keypad / Matrix Still very new to Arduino, so bare with me. I've been all over Google looking for ways to hack my alarm keypad. This is my first time using the Arduino to hack something  :smiley-confuse: 

The keypad has a 13.7v input. Every other contact I can touch shows 5v or less. I realized quickly that the key buttons were producing various voltages. A quick search revealed that it was a voltage divider. I can see the resistors on the ladder. I though about mimicking the ladder via the Arduino and using a common ground. 

I can see two shift resistors that say HEF4094BT. My guess is that these power the led's on the board.
I can also see a chip that reads KSL1 r1342 that I can't seem to find a datasheet on.

I have a few questions, but I guess I need to tell you my end goal. I want to make my keypad an IoT device. I want to be able to read the status of the alarm, and possibly mimick the keypresses to disable the alarm or activate it.

1st. I was thinking I could share a common ground, and read a led state for high/low. Like the "Armed" Led. Thought I'd ask the community before I go and short something  :( 

2nd. How can I mimick the keypad's output voltages? Maybe a DAC? If so, what do you recommend? 

Other Information: It's a DSC PC1550 - 6 zone alarm. (...and I'm also guessing the Rx/Tx are some DSC proprietary serial connection for security :/ )

Cheers,
Robert.
Reading the leds would work. Depending on the voltage going out of the shift register, you might need to add a buffer chip, to lower the voltage.

For the keypad I'd probably go with a variable resistor, better known as a digital potentiometer.

You could probably add a way to also read the keypad, from the arduino. Which might be useful, maybe.