OK, no problem. I believe I understand what you are saying (hopefully 
).
Here are the important things to know when interfacing an INPUT to any Arduino.
The signal level must be less then the processor operating voltage, for R4 that's 5V
In general, it is not a good idea to have devices that are powered connected to an unpowered Arduino.
Yes, thank you. That was the general gist of what van_der_decken was saying above. I seriously did not consider that concept, and I am glad it has been brought to light, to avoid damage. I think PLCs tend to be more robust, so I presume one must be extremely careful with something like Arduino.
Not really, so I'm not sure you understand yet about inputs.
OK Jim. I will bear that in mind. I will tread carefully, as always.
There are hundreds of posts here that use or recommend a 10k series resistor in series with the input when connected to a low impedance voltage source.
Using the input protection diodes should be avoided if you can.
They are basically only designed for ESD protection, and their max rating is usually not listed in the datasheet.
If needed, then add another pair of Schottky diodes externally.
Leo..
Not realy a fixed resistance, but more like a temp dependent leak current.
Could be more than 1000 Megohm at room temp, and not to ground.
The analogue inputs leak/drift to about ⅓VCC.
Leo..
Just to come back to this, I just got a photoelectric switch, and this literately has a clicky clicky type output. It clearly has a power-relay type effect. So when it activates, it has a kind of power relay interface. This means it only needs INPUT_PULLUP configuration on Arduino.
It was not what I wanted ideally. It works, and I will use it, but I prefer sensors that just give a voltage signal, because they would last longer (due to no actual mechanical switch). This was why I mentioned my industrial engineering background. A lot of the sensors used were simply providing a signal voltage, that was fed directly into the control PLC system.
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That’s quite an engineering description
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Maybe say you were hoping for solid state controlled signal.
It was not what I wanted ideally . It works, and I will use it, but I prefer sensors that just give a voltage signal.
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How many operations are you expecting per hour ?
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Any reasonable relay contact should give years of operation especially if the contact current is small.
Yes, LarryD, you do have a good point. I am maybe being a little too picky. Any time I hear the "clicky clicky" I get a little disconcerted due to mechanical motion within the device. I find it is essential when dealing with AC circuits. If DC is being used I just feel better if it is switching electrically/ electronically. I guess it makes me feel better 
- We had some equipment that had relays (17 relays) switching in a flying capacitor configuration.
The relays were similar to below. - Each relay operated every 10 seconds (closed for 200ms), 24/7.
In 15 years we had zero mechanical relay failure.
Mind you, when you stayed in the same room as the equipment, you went unstable.
Do you mind me asking your background? It probably does not really matter to be honest. Mine is high-speed production, and mainly pneumatic systems operating at high speed. Mostly PLC operated. A lot of it was low level consumer media stuff (e.g. tape... VHS or audio).
I feel that I saw many errors in regards to both power relays, or power transistors being used in ways that should have been reversed! I go all the way back to VHS/ audio tape lines, in the 90s. So I go a way back, quite far.
I could be wrong, because many of the power transistors were, quite possibly, being used as AMPS (possibly to do with supply and take-up motors)! If they were, then I am wrong. The only way to do it would have been power transistors, if it is to do with amplification.
I am pretty certain that many of them could have been reversed though. It is hard to say due to how long ago it was.
- Several jobs, 50 years, all in industrial electronics.
Including 35 years in the oil/gas and electricity power utilities.
PLC and SCADA.
Oh, OK cool. That is a cool resume! I did electronics but mainly in tuning things like 3.5 inch floppy systems, with oscilloscopes, etc. I tuned some circuits associated with motor speeds on VHS loading lines. That is why I believe that the power transistors were possibly more for motor control speed, rather than switching. This is 90s though, so it is hard to remember all details.
- Lots of us old timers here sharing what we have learned and giving back to this community.
Excellent community, might I add! This is one of the best I have ever come across.
In another topic, asking a similar question to yours:
would-i-need-an-current-limiting-resistor-in-series-when-looping-back-the-digital-io-pins
I did a test where I measured the current in to / out of an Arduino digital input.
The current was around 10nA, with spikes of 100nA as the input changed state.
- This thread might be of interest to you:
That depends.
With ATmega INPUT pins, the pin samples such a tiny amount of current per read that being connected to VCC (like through a button or transistor) with no resistor is not a problem. With newer devices, I expect that the samples are even smaller.
What to be concerned about is voltage, not current.