user37840: My project is to use an Arduino to control the lighting of a small apartment. My preferred option is to replace each wall light switch with a 4x4 membrane keypad (https://www.amazon.co.uk/Mirocle-Matrix-Membrane-Keyboard-Arduino/dp/B01LMU6OPG/ref=sr_1_19?s=electronics&ie=UTF8&qid=1504376458&sr=1-19&keywords=membrane+keypad). The UX would allow any keypad to control any light, eg. pressing 1-4 will set light 1 to brightness 4.
The problem I have is this will result require 4 x 8 = 32 pins, so I'm looking for alternatives.
So my questions:-
1/ Is there a simple pll to serial converter that I should be using to interface the keypads?
2/ Can I wire multiple keypads onto the same set of 8 pins? Only one keypad will ever be touched at one time.
3/ Are there simple serial versions of the classic membrane keypad.

user28709: As far as I know, you can connect multiple keypads of this type to a single set of 8 pins.  However, it seems like a lot of wire needs to be strung to get this to work, for each switch, 8 wires for the keypad, and two, or more, for power control.
Were I to do this project.  I would likely handle each switch independently, embedding a Pro Mini in each switch box along with an HC-12, or some such.  This means no wires are needed, any pad could control any switch, and expansion is much easier. 
Of course, this is only what I would do, and you understand your own requirements far better than I.

user359: Or the 8 pins 4 will be connected to output pins to be enabled one at a time and 4 will be connected to input pins that get shorted to the input pins when the button is pressed.
The way to get a keypress from this will be to pull a single output pin high and then check all of the inputs, if any of them are high then the corresponding button is pressed.
If you want to treat all keypads the same then you can simply put them in parallel. If you want to treat each of them differently then you can parallel either the inputs or the outputs. But the other set needs to have a S2P or P2S shift register to get the proper data or make the correct pin high. 
Though it will probably be simpler if each got their own microcontroller to poll the keypad and use a different longer range protocol to communicate to the main controller.

37840: <p>My project is to use an Arduino to control the lighting of a small apartment. My preferred option is to replace each wall light switch with a 4x4 membrane keypad (<a href="https://www.amazon.co.uk/Mirocle-Matrix-Membrane-Keyboard-Arduino/dp/B01LMU6OPG/ref=sr_1_19?s=electronics&amp;ie=UTF8&amp;qid=1504376458&amp;sr=1-19&amp;keywords=membrane+keypad" rel="nofollow noreferrer">https://www.amazon.co.uk/Mirocle-Matrix-Membrane-Keyboard-Arduino/dp/B01LMU6OPG/ref=sr_1_19?s=electronics&amp;ie=UTF8&amp;qid=1504376458&amp;sr=1-19&amp;keywords=membrane+keypad</a>). The UX would allow any keypad to control any light, eg. pressing 1-4 will set light 1 to brightness 4.</p>

<p>The problem I have is this will result require 4 x 8 = 32 pins, so I'm looking for alternatives.</p>

<p>So my questions:-</p>

<p>1/ Is there a simple pll to serial converter that I should be using to interface the keypads?</p>

<p>2/ Can I wire multiple keypads onto the same set of 8 pins? Only one keypad will ever be touched at one time.</p>

<p>3/ Are there simple serial versions of the classic membrane keypad.</p>
 28709: <p>As far as I know, you can connect multiple keypads of this type to a single set of 8 pins.  However, it seems like a lot of wire needs to be strung to get this to work, for each switch, 8 wires for the keypad, and two, or more, for power control.</p>

<p>Were I to do this project.  I would likely handle each switch independently, embedding a Pro Mini in each switch box along with an HC-12, or some such.  This means no wires are needed, any pad could control any switch, and expansion is much easier. </p>

<p>Of course, this is only what I would do, and you understand your own requirements far better than I.</p>
 359: <p>Or the 8 pins 4 will be connected to output pins to be enabled one at a time and 4 will be connected to input pins that get shorted to the input pins when the button is pressed.</p>

<p>The way to get a keypress from this will be to pull a single output pin high and then check all of the inputs, if any of them are high then the corresponding button is pressed.</p>

<p>If you want to treat all keypads the same then you can simply put them in parallel. If you want to treat each of them differently then you can parallel either the inputs or the outputs. But the other set needs to have a S2P or P2S shift register to get the proper data or make the correct pin high. </p>

<p>Though it will probably be simpler if each got their own microcontroller to poll the keypad and use a different longer range protocol to communicate to the main controller.</p>
