The Complete Idiot's Guide to constructing a pH meter
Who can construct a pH meter?
    Anyone can construct a pH meter. All it takes is time and determination.
What is a pH meter?
    A pH meter is an instrument that measures the acidity or basicity of a solution.
Why do we use a pH meter?
     We use a pH meter because using strips of indicator paper to measure pH is really inconvenient.
How do we use a pH meter?
     Read your lab manual. It contains detailed information on how to calibrate your pH meter.

Potentiometer Circuit Diagram

This is the basic circuit diagram for the simplified potentiometer (a.k.a. a really simple pH meter).

Parts Needed:
A. a cell that produces an electromotive force (emf)
B. a cell that produces an electromotive force (emf)
C. a battery
D. a variable resistor
E. a double throw switch
F. a movable contact
G. a galvanometer
H. a key
IJ. a resistance wire

How does it work?

Circuit 1contains the battery C, the adjustable resistance D, and the entire length of the resistance wire IJ

Circuit 2 contains the galvanometer G, the variable length of resistance wire IF, the double throw switch E,
 the key H, and either the cell A (the standard cell) or the cell B (the cell of unknown potential) Which cell is
determined by the position of the double throw switch E.
 
    At this point I haven't explained how the cells A or B produce a voltage, but for now just assume that A (the reference) has an emf of 1 volt and B (the unknown) has an unknown emf.
The basic idea is as follows:
    The Galvanometer acts as a null detector for voltage. This basically means that the galvanometer can detect whether 2 voltages are equal. In the current setup (no pun intended), the galvanometer is used to detect a potential difference between Circuit 1 and Circuit 2.  If there is no difference, the galvanometer will not be deflected. If there is a difference, the galvonometer will be deflected.

Why? A galvanometer detects current. If the two circuits are of the same potential, there will be no current (remember: current occurs when there is a flow of charge from a region of high potential to a region of low potential.