Hi! I'm a complete beginner, so please bear with me.
I'm trying to replicate what this person did to make an EMF detector:
Only I'm trying to connect a 0.75 mm2 solid copper wire directly into an analog input with a dupont connector pin.
I figured not having a resistor there would make this more sensitive, but should still give predictable readings.
I am concluding that the pin connector is doing it's job properly, because when I entwine a jumper connected to GND to the copper on the bottom, I get a reading of 0.
I don't want any LED indicators or LCD screens showing anything, I just want to read the values and feed it into an application I will be making.
The problem is I don't understand the values that are coming through. Holding it in the middle of the room and not near anything special is giving me readings fluctuating between 300 - 600. Putting the antenna near the spotlight in the roof is giving me readings fluctuating between 0 and 1023. I'm thinking maybe the field near the spotlight should be more stable? Maybe I'm wrong.
I don't see why there should be readings as high as 600 in the middle of the room either?
Any tips on something obviously wrong here is highly appreciated 
My code is as simple as this:
int inPin = A0;
int val = 0;
void setup()
{
Serial.begin(9600);
}
void loop()
{
// put your main code here, to run repeatedly:
val = analogRead(inPin);
Serial.println(val);
delay(500);
}
Your EMF sensor is a voltage sensor and there are varying levels of EMF everywhere you look, as you discovered. Many posts on the forum warn of using an AD pin with no ground reference as an ANTENNA. That is what you have with an EMF detector and it is doing what you asked it to do.
It is working exactly as expected.
Some people use the ADC readings as part of a random number generator, but they are actually not very good for that. Otherwise, have fun!
The linked article says,
"The numbers are unitless, thus are are only relative to themselves."
Yeah. I was basing everything on the assumption that the field from eg. the spotligh is constant while turned on, and hence that the value reading is in a narrow range while antenna is in same spot.
Also realize that this is not a high performance receiver. If you were to look at the actual incoming signal, for example with an oscilloscope, you would see that EMI is actually a complex waveform. You are only seeing a mashed up, truncated version of that with this circuit and processor.
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Great to hear that the connection is at least good 
The person in the linked article attached his copper wire to a resistor leg inserted into the analog input. The other resistor leg was inserted into GND. I was assuming this would just "consume" some of the signal from the wire, but I'm probably wrong?
He seemed to be getting a pretty "stable" reading from the socket and lamp though, which gave me the impression that those fields where pretty constant and readings would be also in a narrow range?
What are you hoping to do with this device?
No, you are correct and now you see why that is important!
This is probably going to sound crazy.
I want to have about 30 wires hooked up and spread out, one for each letter in the alphabet ++, to make a sort of keyboard that can register keystrokes by EMF fields emerging and subsisting.
In case non-physical entities can generate emf fields, they would be able to input text directly into my application.
How many AD pins does your Arduino have?
It has 6, but I am planning to use 2 x 16:1 multiplexers on the breadboard - and the wires plugged in there - to read the 30 inputs.
I recommend you start with just the 6. Use arrays in your code.
Keep in mind that random variations of EMF in the environment will also lead to "text" being directly entered into your application.
I have been on/off the forum since 1300 UTC and this is the first laugh of the day! 
Bunches of "ghost detectors" on the Internet.
You may wish to modify your "antenna" to provide a reduction in sensitivity using a 100K linear potentiometer:
Yeah, I was kind of assuming that there would be little natural emf picked up, and that emf from technology would be pretty constant in power and reach, so I could have a somewhat stable baseline reading to compare against for each wire end.
It can be. That is how humbucking guitar pickups work, they have two coils that cancel the magnetic fields around the pickup by electrically subtracting the signal from the two. You need to make something similar, but operating on an electrostatic principle instead.
There is more than "a little" EMI around and it's mostly not "natural", it's mainly from the technology around us.
Do ghosts not know Morse code? Then you only need one wire.
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I know it's crazy
I just think running around with one detector can never prove anything. If anything intelligent comes through on a system like this, it can't be anything other than an intelligence behind it.
Thanks for your suggestions, I will look into it!
Strictly speaking, this is not true. Suppose you have a random letter generator that will generate 8 letters. Every so often, it will produce some English words. What if you have a real human being using a very noisy channel and sending letters? Also you will get an occasional full English word and mostly gibberish.
So unless you actually know the source, how would you tell the difference? Maybe you have heard of the monkeys writing Shakespeare example?
It is definitely possible (by definition, actually) that a random generator could say, "I am Frank's uncle" or something like that. Then what are you going to decide?
Okay maybe it could be hacked by a human. I guess the more controlled the environment the better.
I still think anything lengthy and grammatically correct, especially if it's relevant to any questions etc, would be extremely unlikely to be random.
At least it's a step forward 