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:
- BNC connector
- High impedance
- 0.059 V / pH unit
- 12 VDC power supply
- RJ12 connector
The block diagram for the circuit looks like this:
Most of the circuit is originally described at 66pacific.com.
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.
The small signal from the pH probe is amplified so that it matches a commonly available A/D converter.
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.
Converts the amplified analog voltage to a digital value and makes it available on the 1-Wire bus.
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
This block improves the quality of the supply voltage, which makes it possible to use many different generic mains adapters.
A generic low voltage coaxial power connector commonly used on mains adapters.
The next step is to draw the schematic with all the circuit components. Comments and suggestions are welcome in the comments section below.