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PH Meter Circuit Aquarium (led display)

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Any aquarium enthusiast knows how important it is to know and control the conditions of the aquarium water for life within it to run smoothly. One of the factors that determine such conditions is the acidity or pH. Unfortunately, pH meters of good quality are hugely expensive. With this in mind I decided to design and build a pH meter, bringing together the precision and reliability with no great cost. The result is the assembly described below, whose main features are:

PH Meter Circuit Features

Accuracy: better than 1%.
ph Measuring range: pH 0 to pH 9.99.
Probe type: standard.
Alarms pH too high and too low adjustable.
Outputs for connecting to a power module that activates the appropriate device if pH is too high or too low.
Equipment needed: making material (photosensitive plate virgin, exposure unit, developing material and attacked, welder, tin, etc..) Printed circuit boards and digital multimeter.
Low voltage power supply through a 220V-AC / AC-9V adapter.
Approximate cost: 7000 pesetas (feeder and tube not included).

PH Meter Circuit Aquarium (led display) ph metre

Since we will use a standard pH probe, we begin by describing its operation at the electrical level. In theory, a pH probe provides at its output a voltage inversely proportional to the pH of the liquid, with the zero neutral pH. That is, for pH 7, the output voltage is 0V; when the pH is greater than 7, the output voltage is negative and when the pH is less than 7 positive changes being approximately 60mV per pH level. For example, to pH 5, the output voltage would be about 120mV and pH 8.5 would be approximately-90mV. I have said that this is the theory, because the reality is that, depending on various factors (quality and aging of the probe, temperature, etc..) May not give exactly 0V to pH 7 or that increases in voltage or not 60mV per degree of pH. Therefore, the circuit must be calibrated regularly so that its measure is reliable.

Source: PH Meter Circuit schematic pcb files alternative link: ph-meter-circuit.rar

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