Aquaponics systems need a good water quality, and one of the parameters is pH value, which needs to be within a certain range in order for the fish and plants to thrive. This is of course also true for fish-only aquariums, but since I have an aquaponics system set up already, this is what I’ll focus on. There are a few pictures of the system in this blog post: Debian on NSLU2 With USB Hard Disk and Homeplug Network (I’m using a Debian NSLU2 but it should be possible to use an Unslung NSLU2 too).
The small NSLU2 computer has a USB connector and with a DS9490R 1-Wire to USB adapter it’s easy to collect data from a 1-Wire unit onto a hard disk. Then I just need a pH-to-1-Wire adapter, which is what I want to build and describe in the upcoming blog posts. A 1-Wire bus has a much longer reach than a USB bus and that is very convenient for many purposes.
I’m going to build a circuit with the following specifications that will interface to a common pH probe immersed in the aquaponics water:
Input
- BNC connector
- High impedance
- 0.059 V / pH unit
- 12 VDC power supply
Output
- RJ12 connector
- 1-Wire
The block diagram for the circuit looks like this:
Most of the circuit is originally described at 66pacific.com.
BNC Connector
The pH probe that is supposed to be connected to the voltage amplifier only sends out 0.059 V per pH unit and using a coax cable will keep out noise. Coax cable naturally terminates in a BNC connector.
Voltage Amplifier
The small signal from the pH probe is amplified so that it matches a commonly available A/D converter.
Ground Reference
The voltage signal coming from the pH probe is both positive (pH < 7) and negative (pH > 7). Since the power supply for the circuit is from an ordinary 12 VDC mains adapter and the signal is going to an A/D converter, the ground reference for the voltage amplifier is raised to ensure positive voltages only for the converter, assuming that a normal one is only able to handle positive voltages.
A/D Converter
Converts the amplified analog voltage to a digital value and makes it available on the 1-Wire bus.
RJ12 Connector
A common type of connector for the 1-Wire bus. Only 2 pins are used although 6 are available. The pinout is shown on this page: RJ12 Pinout
Voltage Regulator
This block improves the quality of the supply voltage, which makes it possible to use many different generic mains adapters.
DC Connector
A generic low voltage coaxial power connector commonly used on mains adapters.
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The next step is to draw the schematic with all the circuit components. Comments and suggestions are welcome in the comments section below.
