So first off I am totally out of my comfort zone as I have never attempted anything like this, but I have a job I have to keep. Let me say again, I literally have zero electrical experience.
Background: There are 4 different types of trailer plugs that will be put in. One for example is a 7 pin plug with one of the pins being a ground. What I am attempting is to make a device that will connect to a trailer plug and test whether it is fully functional. Preferably each pin on the plugs will use a digital I/O to simply turn the LED On/Off. For example, when the left turn signal is used, a specific LED will turn on signaling that it is functioning properly. I am using an Arduino Mega as I have four different trailer plug types I need to use.
I have a few issues though.
1.) First I am trying to figure out how to reduce the 12-14V trailer plug output to 5V's so the board doesn't get fried. What should I use to do that? Something like this? Voltage Regulator - 5V - COM-00107 - SparkFun Electronics. Will I need to put it on every input wire? Do I even need to do to this? And if so is there a more simple solution?
2.) How do I ground each plug to the board? Or rather what is the best solution to this?
3.) I'll have a simple on/off LED output for each pin of the plug except the ground. However, one or two of the pins on each trailer plug will have to be able to dim a LED as it is testing the brakes to make sure different levels of breaking pressure will work. I was told they would have to be plugged into the Analog pins in order to dim/brighten. Is this true?
Sorry if I'm not using the correct terminology, but I am a beginner who is learning this on the job. I attached what are the four different trailer plugs and what pins go to what.
Why do you need an Arduino for this? A 12 V light bulb, or a resistor (try 2.2 Kohms) in series with an LED will tell you if there is a signal at a plug pin.
Not sure, that's what the boss wanted and I was just following orders. I'll ask him about your suggestion cause it makes a hell of a lot more sense in my mind. If I do it the way you suggested is there a way to make a LED/Marker light up to tell the ground is working?
Is this something you'll plug into the trailer to test whether the trailer circuit is working correctly, or something you will plug in to the vehicle socket to test the vehicle circuit?
I will plug it into the vehicle socket.
Is it basically a miniature light bar, then? I mean, something that mimics what your trailer lights would do if you had plugged them in?
If it is just to test ground and trailer light connections, you can buy these things already made up. Google "trailer plug tester" or similar. Here is one example for US$ 3.99 http://www.harborfreight.com/four-way-trailer-light-tester-66526.html
If this is a test that will have to be done on many occasions I think I would just make up a board with a standard set of tail lights. This has the advantage that the connector is tested at full working voltage and current.
...R
slazer12:
Not sure, that's what the boss wanted and I was just following orders.
Makes us think "the boss" doesn't actually know what an Arduino is, but has heard it performs magic. 
slazer12:
If I do it the way you suggested is there a way to make a LED/Marker light up to tell the ground is working?
Oh, that's easy!
If you simultaneously apply the brake, tail lights and hazard flashers, if they all light up at the same time, the ground is working.
A ground fault causes strange things to happen, notably the stop lights flash instead when you use both brake and turn.
