Overview & Theory of Operation

A pH electrode works like a small battery.   When placed in the aquarium, the electrode puts out 60 millivolts (mV) for each pH unit that the aquarium water is above or below neutral (pH 7).  For example, the pH electrode puts out 0 volts when immersed in water that is pH 7, 60 mV at pH 6, 120 mV at pH 5, and so on.  The polarity reverses if the water's is above pH 7, so the electrode puts out  -60 mV at pH 8,  -120 mV at pH 9, and so on.   (For a more detailed discussion of pH and its measurement please see Omega Engineering's pH Primer and pH Electrode Basics.)  Fundamentally, all that's required to measure pH is to measure this electrode voltage.  The problem is that the pH electrode has a "source impedance" of about a billion ohms — it's as if you were trying to measure the voltage from a tiny 60 mV battery that had a billion-ohm resistor in series with it.  Because of this high source impedance, simply measuring the pH electrode voltage with a common voltmeter is out of the question.  Fortunately, this high source impedance signal isn't a problem for the inexpensive operational amplifier (op amp) integrated circuits (ICs) that are now cheaply and readily available, like the Texas Instruments TL082 which is the heart of this circuit.  When the pH electrode's output is connected to the TL082 op amp, it is simultaneously converted to a low source impedance signal and amplified about 17 times.  After passing through this stage a 60 mV signal equals one volt (17 * .06 = 1), so a change of one pH unit produces a change of one volt at the circuit output.

Because this circuit uses a cheap, easy-to-find DC power supply that doesn't provide plus and minus voltages plus ground, an artificial "reference ground" must be developed.  (This is covered in more detail below.)  Because this reference ground is set to be exactly 7 volts above the power supply negative voltage, the circuit output is 7.00 volts with a pH 7 input.  The only other difficulty is that the voltage from the pH probe gets more negative (that is, lower) as the pH gets higher.  By inverting the signal in the next stage, it's turned right-way-around so the voltages get smaller as pH gets lower, and a pH 6 solution gives an output of 6 volts, not 8.  This output can then be read with either a digital voltmeter or a digital panel display, the voltage being identical to the aquarium pH.

For the controller circuit, the output voltage is compared with a user-set reference voltage, the "turn on the CO2" pH-level.  A comparator circuit accomplishes this, and turns on both a light and a valve-actuating transistor at the appropriate, user-defined pH.

One limitation of this circuit is that it will only read pHs in the range of about 3.5 to 10.5.  This is a far broader range than any aquarist will ever encounter in the aquarium, and it still allows the use of the standard pH 4 or pH 10 electrode calibration solutions.

Fundamentals

This is not a complicated or subtle circuit.  It should be easy to build.  Those who don't have a lot of experience with electronic circuit assembly would do well to build the circuit in the sequence described below.  Build the regulator first.  Look at its output with a voltmeter and adjust it to the appropriate level.  If there's a problem at this point, it will be  easily recognized and fixed.  Build the reference ground circuitry and set it to 7.00 volts.  Proceed to the amplifier, and so on in the sequence given.  By proceeding methodically and sequentially through the circuit, much confusion and difficulty can be avoided.

There is really only one critical area in the whole circuit, and that is the connection of the electrode to the amplifier.  Keep the leads in this area as short and direct as possible to avoid problems.  (This is discussed in detail below.)

The rest of the circuit is non-critical.  Do try to keep wiring runs as short and direct as possible, especially around the op amp inputs.  Careful layout makes assembly, testing, and troubleshooting much easier.

An old trick to avoid problems with voltage drops and noise on the supply lines is to use a "star ground," with the separate ground wires all linking straight back to the supply, not daisy-chained across the circuit board.  This "star" approach can be used on all three of the supply lines here:  Vreg (the + supply), the - supply, and the reference ground.  Separate, direct wiring runs from the supply to each IC, voltage divider, and so on may be overkill, but practically guarantees problem-free operation.  If you're not confident about the subtleties of wiring, please consider using the star technique.  (A look at Morrison's classic book, Grounding and Shielding Techniques, may also be illuminating.)