1) why do I need a resistor there?

You probably don't need the resistor but it probably won't hurt either.  Most of these solid state relays, such as the one in reply #8, accept a wide range of input voltages.  They are not rated for a specific current, so a current limiting resistor is not needed to protect the relay.  It is not likely that the relay will draw enough current to damage your Arduino.

2) I supose the CN3 and the arduino goes to the ground from those not connected cables... but where do I connect the ground?

The 'ground' symbol just signifies a common connection point.  Connect all of the wires that go to a ground symbol together.  You do not have to connect the ground to anything else.

Don

you'll have to condition the signals to talk to the Arduino

That's the purpose of RS-232 Driver and Receiver chips.  Do a search for 'MAX232'.  The original MAX232 chip required external capacitors but there are other variants that do not.

Don

From the ancient technology department - wouldn't an  MC1489 Line Receiver do the trick?  I might even be able to find one around here someplace.

Don

Are we supposed to figure out how your modified sketches might have affected the LCD memory without the benefit of knowing their contents?

Don

... making me wonder if the seller posted some wrong images or there's a simple way to invert images.

No, the seller probably did not post some wrong images.  The displays are available in both configurations.  
and
No, there's no simple (or hard) way to invert images.

Don

[digital]

It escapes me why uP companies don't put a decent Analog output on their chips.

Perhaps it is because uPs are digital devices.   I haven't seen too many analog ICs that have decent digital outputs for a similar reason.

Don

The KS0066 is compatible and you can use it with the 'Hello World' examples.  

Don

[must]

I guess I misinterpreted your original question.

1.  Is it possible to use the LiquidCrystal library with a different hardware interface?

That depends on how different the hardware and/or the interface is.  If you must use PB6 and PB7 then I think you are out of luck.  You don't need to implement R/W since it is not used by any of the current LiquidCrystal routines.  This leaves six lines which can be accommodated by PB0-PB5.

2.  If so, what is the best way to implement ...

You would have to have some way to program your chip with a bootloader.  You would also have to use a version that runs at your 8MHz clock frequency (I believe that there is one available) and hope that your clock frequency is accurate enough.

... and where?

???

It would be far easier for you to write your own LCD routines for your current hardware/software setup.  You can get most of what you need by examining the LiquidCrystal.cpp file.

Don

You specify which Arduino pins correspond to which LCD pins when you invoke the library with one of the four possible commands.  

From the reference material:

LiquidCrystal()

Syntax:
LiquidCrystal(rs, enable, d4, d5, d6, d7)
LiquidCrystal(rs, rw, enable, d4, d5, d6, d7)
LiquidCrystal(rs, enable, d0, d1, d2, d3, d4, d5, d6, d7)
LiquidCrystal(rs, rw, enable, d0, d1, d2, d3, d4, d5, d6, d7)

Since you are using seven interface pins (four data lines, RS, R/W, and E) you would use the second example.  Just substitute your seven desired Arduino pin numbers in the sequence specified in the syntax example.

Don

But I have one question before I start:
In the above example there is a display used with 1 line and 24 columns. In this turorial (http://www.ladyada.net/learn/lcd/charlcd.html) it is said: The good news is that all of these displays are 'swappable' - if you build your project with one you can unplug it and use another size. Your code may have to adjust to the larger size but at least the wiring is the same!
Does that mean, that I could simply by another LCD Display for example with 2 lines 16 columns instead. Nothing of the wireing would be different. And the program I have to write will roughly be the same, exept for the number of lines and columns used.
Right?

It looks like no one has answered this question for you.

... if you build your project with one you can unplug it and use another size.

That is correct.  All you have to change is the argument (the stuff in parentheses) of the lcd.begin() statement.  You may also have to change some of the cursor positioning.

Don

Normally I would follow the instructions on this site http://www.ladyada.net/learn/lcd/charlcd.html  for controlling an character LCD.

The problem is, since the Arduino is already in use there are only few pins left:
- Pin 5 / T1
- GND
- +5V
- PIN 19 ADC 5
- PIN 7
- PIN 6
- TX
- RX

What is keeping you from doing just that?

Make these connections:
LCD <--> Arduino
RS          5
RW          Gnd
E           19
DB4         7
DB5         6
DB6         1 (tx)
DB7         0 (rx)

And implement it with this:
LiquidCrystal lcd(5, 19, 7, 6, 1, 0);

You may have to disconnect your USB cable (after downloading your sketch) and power the Arduino via PWRIN.

Don

Can you post a sample of a polling technique that would work for this application?

I could probably come up with one.  I can make time to do that if you come here and mow the grass, paint the bedroom, clean out the garage....

Don

[interrupt]

I was under the impressions I would need an external interrupt to be able to poll the PPM signal

An interrupt is used to deal with asynchronous events, events whose timing is not under your direct control.  If you want to initiate something periodically, you generally use a polling technique.  It looks like you want to use the latter.

Don

What is the nature of the signals that are generating your interrupts.  The two 'external interrupts' on the 328 can be configured to select the desired signal level and edge.  The rest of the I/O pins can also be configured to generate an interrupt whenever the logic level changes.  They aren't nearly as flexible as the 'external interrupts' but perhaps detecting a logic level change is all you need.

Don

Robert M:

Go back and carefully read reply #6.  There's no way you are going to build your own reliable and safe triac controller for the price of a solid state relay.

See if you can find a 'hamfest' in your local area.  These are basically electronics flea markets.  You should be able to find reasonably priced soild state relays by the dozens.  Don't be afraid to purchase 'takeouts' -  devices removed from equipment.  I bought some 240v/20A ones for my home heating system about 30 years ago and they are still working perfectly.

Don