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.

  • Wow! Following one link, to another via the comments led me to [link]github.com/dougkpowers/pc1550-interface which makes me feel even more hopeful! I might be able to skip hacking, and move directly to Interfacing! The article didn't really state whether I should connect the wires directly, or use resistors. I'm assuming that the 13.3v is regulated by the arduino, but I'm not sure about the clock and data pins? - thanks @jwpat7 Commented Mar 8, 2016 at 12:58
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    @RobertCole The Arduino I/O pins can take only up to 20mA of current, and a MAXIMUM of 5.5v. But that is not normal operating condition. That is assuming you are using an Arduino Uno, but otherwise the ratings don't change much. Anyway, a simple woltage divider can solve your problem
    – sassoPera
    Commented Sep 5, 2016 at 22:24

1 Answer 1


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.

  • Glad to hear that my common ground would work for the leds. But I can't see the logic in the buffer chip? Digital potentiometer makes sense [link]sparkfun.com/products/10613 and then pulse various resistances in relation to the resistance of the keypad? Commented Mar 8, 2016 at 12:15
  • The leds might be supplied with 12v. Connecting this to the Arduino will break if. So in that case you need to convert the 12v signals to 5v signals, using a buffer chip, or some discrete circuitry. The the voltage is around 5v, I would at least put a resistor between the led and the Arduino. PS depending on how the led is wired (resistor 'before' or 'after' led) you might have to connect your wire to the associated resistor. The pot. you linked is what I meant. But you have to measure the resistance of the keypad (while powered off) to see if the 10k version is okay.
    – Gerben
    Commented Mar 8, 2016 at 14:21

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