Before jumping into the electronics part you need to get familiar with the circuit so it holds no surprises.
This isn't too hard. Grab the schematics off github or the files section and take a look.
I'll wait while you download and view the diagram....
Ok, got it? Now I'll talk you through it from left to right.
On the left are four squarish boxes that represent the MAP sensors. I'll assume you are following my exact build. Anyone doing their own mods should know what they are doing, right? The diagram is set up so that the pins in the diagram correspond to their real-life locations. This makes it easy to wire it up and check your work as you go.
Beginners please note that wires drawn crossing each other in the diagram are only connected if they have a dot on the junction. Rectangles are resistors (its a Euro-style diagram), US style uses squiggly lines but they mean the same.
The sensors are really simple (electrically speaking). They each need a wire to +5V and Ground to do their thing. Then they will constantly output a voltage (the third wire) that is proportional to the pressure sensed. Less voltage is lower pressure down to 0V at 150Mbar above absolute vacuum, and up to almost 5V at atmospheric (sea level, nice weather, no hurricane touring the neighborhood).
The signal wires from the sensors are connected directly to the Arduino analog inputs A0 through A3 where the software can measure the voltage they give off. Keep these short in your project and don't go through a connector but solder directly to the board or to a pin header. This reduces interference.
Now comes the Arduino. It too just needs V+ and Ground to work. Actually, you can feed it anything up to 18V (It will make its own 5V if it has to) so you could directly connect it to the vehicle power. If you want to go this route, feed the higher voltage into the Vin pin. Use the +5V for the sensors. You'd probably fry them with anything even slightly above 5V. The schematic uses a model of the arduino with all the pins in the proper places. This is an Arduino nano v3 type.
The next thing you may notice is the four switches. These are just plain and simple push buttons that make contact when you press them. One side is connected to an Arduino digital pin and another is wired to ground. This keeps things nice and simple to hook up.
However, if you like to play around with stuff then be very careful you NEVER set the Arduino pins connected to these switches to output HIGH (5V) or the switch will short the pin to ground when pressed! If that happens you may be lucky and find another pin that still works, but don't bet on it.
The lone resistor
R4 is a resistor that is in there as protection for the Arduino. The power used to drive the backlight LED of the display is limited by this transistor so the Arduino doesn't fry. Some LCD modules have this resistor, some don't. I figure a slightly dimmer backlight is better than a Kentucky Fried Arduino. In fact my second prototype screen came with a resistor already. I noticed it was rather dim after I fired it up. I just cut out the resistor and it now draws 28 mA, which is perfectly acceptable to me and my Arduino.
The dynamic Duo: R3 and C1
These connect to the contrast setting on the LCD display. You often see some sort of adjustable resistor or (POTentiometer) used for this but I thought I'd rather do this in software.
R3 is there to limit the current flowing into C1. C1 is used to smooth the signal coming from the Arduino. This is a pulsating PWM signal that can make these types of LCD flicker horribly and even become unresponsive. Don't ask me how I know. These two together do a great job at the frequency the Arduino software does PWM. This is known as an R/C network and you can forget I said that.
Last but not least is the display. Its a HD44780 compatible 20 characters by 4 lines display. These are easy for the Arduino to drive and can be tricked into displaying nice bar graphs with up to 100 segments. They can also be read in the dark or even in bright sunlight (if you turn off the integrated backlight!) In fact, there are displays without a backlight altogether, you may prefer using one and can then skip the backlight wiring.
This display has a 4-bit parallel mode of working which we use that saves 4 pins. The other lines we need to connect are signalling lines that tell it t o turn on and accept commands offered up on the four data lines we are using. The other four data lines are happily hooked up to the +0V.
I've got the Power!
Finally there's the issue of power. As I explained above you could just plug vehicle power into the Vin pin of the Aruino (that's +12-14.8v) which should work just fine. In theory its scary as hell, but it works for a lot of folks. The MAP sensors we have are automotive and can handle some abuse. I have run Arduino's on worse (Joule thief anyone?).
What I recommend for beginners is just hook up a 9V alkaline block, which will happily power your circuit for up to 7 hours, more if you are diligent with the on-off switch!
It goes in the V+ line between the battery and Vin on the arduino. Its a simple single pole single throw rated for peanuts and low voltage.