Simple Solution

Hydroponics is all about growing without soil. In many ways this simplifies the lot of the gardener, but it gives them added responsibility for providing plants with the right level of nutrients. As water with nutrients tastes, feels and looks much the same as plain water, a testing instrument called an “EC meter” or “CF meter” is used.

HOW TO MAKE YOUR OWN HYDROPONICS NUTRIENT METER.

Hydroponics is all about growing without soil. In many ways this simplifies the lot of the gardener, but it gives them added responsibility for providing plants with the right level of nutrients. As water with nutrients tastes, feels and looks much the same as plain water, a testing instrument called an “EC meter” or “CF meter” is used. The hand-held ones tend to look a lot like a truncheon with party lights down the side and indicate the nutrient level in EC or CF units (.1 EC unit = 10 CF units). The way they work is a bit complex, but we’ve figured out how to replace a lot of the esoteric circuitry with an ordinary Arduino. Not only can you build a CF meter cheaply, but you can also easily modify it for a garden automation project. All the meter does is measure the resistance of the water. Lots of nutrients means a lower resistance. If you just use an ordinary resistance meter, the current passed through the liquid changes the nutrient’s chemistry and your reading goes inaccurate very quickly. So CF meters measure the conductivity in short pulses, swapping direction each time they make a measurement to reverse the chemistry.

The process

Make a rust-proof probe by carefully cutting the carbon rod from the centre of an AA zinc/carbon battery. Clean it, clean everything else you got the mess on, then boil the rod in water for a few minutes to get the last of the battery compounds out of it. Break it into two pieces, notch the ends with a file, then wrap and tightly twist a bare copper wire into each notch. Thoroughly embed the whole thing in your favourite waterproof plastic (eg, Sugru, polymorph or silicone) leaving just a couple of millimetres of each rod sticking out about 10 mm apart. No nutrient solution must get in or it will corrode the copper. That’s your sensor and you can hook it up to the A1 and A2 Arduino pins. A slightly better idea is to wire things up to a socket that can be detached easily from the Arduino—this project isn’t complicated enough to require a circuit board. Instead of the impressive collection of op-amps and comparator chips in a commercial meter, we’re going to use a couple of resistors and an Arduino to do the measuring. Arduino has pins to measure voltage and these can also be used to output a voltage. This circuit simply alternates two pins between measuring and volting. Add a couple of resistors as per the diagram so we can compare the voltage dropped across the probe with the voltage dropped across a known resistance. The device built here uses two 2.2K Ohms resistors (red, red, red bands) but you use a value of roughly one tenth of the resistance you measure when you put your probe in: a very weak—0.1CF—test solution. So if your probe measures the solution at 35K Ohms, use a 3.3K Ohm resistor and so forth. Put one across A0 and A1 and the other across A2 and A3. You might feel happier building it on a breadboard first. 

Meter code

At this point you can use the Arduino development kit (“IDE”) from http:// arduino.cc to load in the CF meter’s code. Instructions to do this for your Arduino and computer types can be found there. Under [Tools] is the “Serial Monitor” and this will show you the readings that the meter is reporting. The first number is the raw reading from the sensor, the second number (if present) is the CF value. Calibration is a bit tricky and, unless you have EC buffer solutions handy, you’ll want to borrow a real EC meter. We use CF units rather than EC units because they’re largish integers to which both the Arduino and humans find easier to relate. Dunk the sensor in the very weak 0.1CF solution to get the “baseValue”—make sure you completely cover the exposed probe tips and edit it in to the line saying: const int baseValue=340; // Value of weak 0.1CF solution The “cfTable[]” code below that line consists of a series of [sensor reading, CF reading] pairs. The first pair should be your baseValue and zero. To get a few other pairs, ideally you would mix up slightly salty water, measure its CF value with a borrowed EC meter and then test it with your probe. If you expand or reduce the table do remember to alter the TABLE_ENTRIES count. If you can’t borrow a real meter, 1 gram of table salt thoroughly dissolved in 500 ml of deionised or distilled water will have a CF of about 40. Take half of that, dilute it back to 500 ml again and it’ll have a CF of 20. Repeat for CFs of 10 and 5. Finally, dilute 5:1 to get a CF of 1 and use these solutions to calibrate the probe. Once you have entered the values into the code, save it and upload it to the Arduino again. Now when you dunk in the probe, you should see the CF reported in the Serial Monitor. If you’re using this to make an automation system and know what you’re doing, you can probably use the project pretty much as-is. The important thing is to remember to call the neutral () routine as soon as the measurements are done to turn off the sensor and prevent chemistry happening. It’s all open-source software under the GPL so feel free to share and enjoy.

Stand alone

If you want to make a stand-alone meter out of it, you can add a line of LEDs to the Arduino. The “pinTable[]” array contains a list of the pins you have LEDs on, put in the order in which you want them to light up. You can add spare analogue pins if you wish to make the line of LEDs even longer, just update the LED_TABLE_ ENTRIES number accordingly. The LEDs will strobe until the meter measures something and then light the appropriate LED. A value corresponding to a point part-way between two LEDs causes them both to blink. Because we’re only lighting one LED at a time, we can get away with just one resistor to control the drive current. So all the LED cathode leads (usually lead on the chunkiest half of the metalwork inside an LED) are wired together, then to GND. The anode LED leads go to your chosen Arduino pins. Diamond age ad I powered mine from a USB emergency phone charger, but you can use a 9V battery connected to GND and Vin instead. Some form of switch is probably in order and then your stand-alone meter is complete. If you can get to a 3D printer, a printable case for the LED-bespangled Arduino Nano version is here: http:// www.thingiverse.com/thing:704520 and the software can be downloaded from GitHub here: http://github.com/ VikOlliver/CF_Blinkenlights (look for “Download ZIP”). CF changes by temperature, but by less than 1 unit for 10°C, so for most practical purposes we can ignore the temperature, insert the probe, and measure away— unless it freezes, of course.



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A man of many sheds

Sitting on the edge of a little paddock in the river mouth settlement of Kakanui, ten minutes drive south of Oamaru, is a ramshackle shed where Lindsay Murray creates things of rustic beauty.
The shed has a long history. It was built in 1875 as the house and shop of a nurseryman, George Packwood, who came from Scotland to plant and tend trees around the mill manager’s house in Kakanui.
When Lindsay, an artisan wood worker and sometimes blacksmith took it over in 1993 the back wall had fallen off, half the roof was collapsing and the floors were rotted through in the living quarters. As far as Lindsay was concerned, it was perfect.
It had the remnants of the original dwelling: a kitchen with no running water but with a working coal range, and two other rooms which have served, over the years, as sleeping quarters and a sitting-cum-dining room. This is where Lindsay stays overnight when working on a major project. It also serves as temporary accommodation for visiting artisans and is currently home to Rolands Selis, a highly skilled leather worker.

VERSATILE SHED OF THE YEAR COMPETITION!

Do you have New Zealand’s most interesting shed?
Whether it’s the cleanest, most cluttered, an epic man cave, or simply one-of-a-kind, we want to see it!
We love them all, and now it’s your chance to show off your shed and win!
Think your shed has what it takes?
Enter today and claim the crown!
HOW IT WORKS:
We’ll select the Top 5 sheds. The finalists will be showcased on Facebook, and the public will vote – most likes & comments wins!
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A $500 Prezzy Card. A feature spot on the cover of The Shed Magazine, 2026 Calendar
HOW TO ENTER:
Send a high-res JPG image of your shed
Email your entry to: [email protected]
Entries close: July 31st
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August 1st – 10th. The winner will be notified by email.

Lights galore

If you are interested in adapting and using discarded bits and bobs to make something useful, then building lights, table lamps, and the like, offers an endless source of relatively inexpensive projects to amuse you. What’s more, they make unique gifts and nice conversation starters for visitors. Lamps are also a good way of supporting a decorating theme. Whether you are into an Art Deco, Arts and Crafts, modern or industrial look, it is nice to enhance your chosen theme by constructing a light from an object that is reminiscent of it.
Of course you are dealing with 240-volt electricity, which is potentially lethal, so it is essential that you use common sense and particular care when wiring up lights (see the safe wiring tip box). The advent of 12-volt downlights powered by small transformers has also made the construction of lights much safer. Lights can now be wired in such a way that the 240-volt transformer is remotely located or secured and independently earthed. There is no electrocution risk posed by the 12-volt wiring in the rest of the light.