I'm going to assume that you're using a relay similar to this one I found on Digikey.
It lists the control current at 5V as 3mA, which I'll use as a good ballpark number. If we want to know if the relay will turn on, then we want to be sure that the control voltage at the relay will be greater than the minimum control voltage of 3V.
The first check is whether or not we can source the current. In this case, 3mA is well below the typical maximum current source/sink of a microcontroller (20mA), so we're fine. If your relay uses more current than that, you may need to look at using a transistor to drive the relay.
The second check is to see what the voltage at the relay will be after the resistive losses of the wire. If we assume the worst-case resistance of the wire, then that is 0.188 Ω/m, like Ildefonso stated. Note that this is the loop resistance, not the resistance for a single wire.
At 11.5 meters length, you will have a total wire resistance of 2.2Ω. (11.5m * 0.118Ω/m). At 3mA current, this creates a voltage drop of 6.5mV (2.2Ω * 3mA). If you drive the relay using a 5V source, the voltage that the relay would see is 4.993V (5V - 6.5mV), which shows that at DC those wire lengths are negligible.
You would need to be drawing 900mA, or using a wire length of 5km to start seeing a voltage close to the minimum threshold (3V) of the relay.
For powering a sensor, the same calculations will hold true. If you know the DC current your sensor requires, then you will probably find that CAT5 will power your sensor fine. If the sensor has an analog output, there shouldn't be a big issue, and with a digital signal you probably will need to go slower than if the sensor was on the Arduino board.
To answer your other sub-questions,
- You always loose power in wiring, but at low current DC, it usually isn't much. What you're probably meaning is will I lose too much voltage.
- The plenum/shielding would not impact your ohmic/resistive losses.
- If the wire resistance was too high, you could use twice as many wires to cut the resistance by half. The wire in CAT5 is fairly high gauge, so using a low gauge wire would help in a similar way.
- If you couldn't afford to use more wire, but wanted to send more power down the line, you could increase the voltage sent over the wire, and use a DC-DC switching supply to reduce the voltage. This is similar to what goes on in Power over Ethernet, and AC Power Grid Transmission.
To be clear, what you posted is a wiring diagram, not a schematic. A schematic of your circuit would look something like this:
Would a 2N3904 NPN transistor and a 1kΩ resistor work together in this
scenario
1kΩ is probably fine. We don't know the peak and holding current for your magnetic strike (they rarely publish such information from my experience). The 2N3904 is rated for 200mA, likely not enough for your strike. I'd guess that the strike will take around 1A at 12V, so you may want to use a relay to handle the current. For that, you can use your 2N3904, just use it to drive the relay coil (5V, powered from your Arduino, or 12V, powered from your external supply), and use the relay to control the strike. This discussion may be helpful.
What would happen on the Arduino if you had more than two wires that
needed to be grounded
It's not quite clear what you're asking. The Arduino doesn't care how many ground wires are attached (they'll all be at the same electrical potential, or 0V). What the Arduino may care about is how much current needs to flow into its ground pins. As long as you're not passing the current from the strike through the arduino board, you should be fine.
Is the diagram that I have suitable for the strike, or am I missing
anything.
Note the flyback diode in the schematic. Even if you go with the approach of using a transistor to drive a relay and using the relay contacts to power the strike, you'll want a flyback diode across both the relay coil and across the strike itself.
The strike is basically a big inductor - once current starts flowing, it wants to keep flowing. Most strikes have a MOV snubber integral with them to suppress spikes, but a reverse-biased diode will do a better job of eliminating transients. A lot of strikes are made to work on either AC or DC; a MOV works for both, but the reverse-biased diode can only be used when applying DC.
The power supply I linked to.
Should be fine, based on the limited information we have. If the strike needs 2A or less, it should work.
Can you tell if it has two wires or one?
Assuming that you're referring to the strike, I'd guess that it will have two wires, but there's only a couple ways to tell for sure: Ask the reseller or order one.
why do strikes such as this one have 4 wires, instead of 2
So they can be used in either 12V or 24V applications. For 12V, connect the two coils in parallel; for 24V they're connected in series. There should be a (sometimes very small) datasheet that comes with the strike that has connection instructions.
I'd recommend something like the following:
Best Answer
There are a numbers of satellite transceivers, as PCB modules (size of a few coins) or boxed-product, that are low power, easy setting up and come with driver library example code.
Some low-speed equipment is not especially expensive, however, there are data volume charge which may be acceptable if you only send small data once every day.
Two types, those use LEO (Iridium and similar) does not demand very open sky view as sat is high elevation angles. Those use geo stationary sat (Inmarsat and similar) need open sky toward 'generally south', depending on Latitude.
These sat have high, medium and low speed units. Low speed, non-voice units are what you need in terms of cost, easy setting up, low power etc.
SBD Short Burst Data Satellite module
Another similar unit from another brand and service provider
For the ham path, you can use PSK31 HF modem which allows low speed (31 bauds) transmission with small antenna and easy setup than traditional HF modems of higher speed (few kilo BPS). Wiki on PSK31
Ham band need
a) knowledge to choose different freq depending on day, night, seasons, latitudes and solar cycle
b) depends on countries, may not be full legal for machine-to-machine communication not carrying 'ham traffic' (some country defined as self-training activities)
OR, can get a commercial license and use HF band (not sure how hard it is) and same ham equipment
OR use the HF network (fee based or not, 100% sure) for file and email for small yacht at sea. Check SailMail or similar