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==How to build it== [[File:IMG259.jpg|200px|thumb|right|Step 1: probe parts]] [[File:NIRprobe2.jpg|200px|thumb|Step 3: assembling]] [[File:NIRprobe3.jpg|200px|thumb|right|Step 4: waterproofing]] [[File:NIRprobe4.jpg|200px|thumb|right|Step 5: spacing]] '''Step 1: Cutting acrylic''' Start by cutting the 3/4" acrylic tube into 2 x 1" / 25mm pieces (A1 and A2) and 1 x 3/4" / 20mm piece (A3). Make a slit in A3 approx. 1/3" / 8mm wide by making two cuts that run the entire length of the tube. Acrylic is very easy to cut with a small rotor tool (e.g. a Dreml), but you can also use a small hacksaw. Files or fine sandpaper are good for smoothing rough edges and planing not-quite-perpendicular cuts. '''Step 2: Soldering wires''' Cut the leads on both the LED and the phototransistor about 30% shorter. Solder wires onto the leads, and make sure to note down what colour wire you use for the different leads! These are polar devices and won't work if you wire them up backwards. We suggest you use red for both power / emitter leads, black for the ground lead on the LED, and white for the collector lead on the phototransistor. '''Step 3: Assembling''' Drill a 3mm (or as close as you can get with Imperial units) hole in the center of each acrylic disc. Take two of the discs, carefully lay down a narrow line of acrylic cement around the holes, then insert the LED and phototransistor in the holes, and set them aside to cure. When the acrylic discs with the LED and phototransistor have properly cured, thread A1 and A2 on one set of wires each, lay a fat line of acrylic cement along the edge of both discs and press A1 and A2 firmly into place, creating a lidded, cylindrical chamber for the leads. Leave to cure. Reinforce the seal by laying down another line of acrylic cement along the joint between the tubes and the discs. '''Step 4: Waterproofing''' Waterproof each chamber completely by filling it out with aquarium / hot glue, then immediately string the last two acrylic discs onto the wires and glue them to the tubes with acrylic cement. Leave to cure, then reinforce the seal from the outside with another line of acrylic cement. '''Step 5: Spacing''' Once the aquarium / hot glue has cured properly, take A3, place it between the two acrylic discs holding the LED and phototransistor, and glue the three parts together with acrylic cement, then leave to dry. '''Step 6: Blocking sunlight with PVC (optional)''' Since sunlight includes lots of infrared radiation, it may be necessary to shield your probe. A very simple way of doing this is to take length of PVC tube with a tight-fitting cap, make a hole in the cap for wires, and simply pull the tube + cap over your NIR probe like a lampshade. Making sure the phototransistor is pointing downwards also helps. '''Bonus level: Building the BubbleShakerTM (optional)''' Some biological processes form gas (usually C02), which may cause bubbles to form on the surface of the LED and phototransistor and throw off your measurements. To solve this problem, we've created the amazing BubbleShakerTM - a small off-set motor ('buzzer') extracted from an old cell phone, encased in a short piece of acrylic tube plugged with aquarium / hot glue on both ends. To build one yourself, the first thing you'll have to do is find an old cell phone somewhere, crack it open, and extract the motor - get as much of the wires as you can, it'll make life easier for you when you have to solder extensions on. Solder about 2 ft / 60cm of additional wire (remember the colours) to both leads. Then take a piece of acrylic tube big enough to fit the motor and about 1/2" / 13mm wire and plug one end with glue. Insert the motor in the tube - make sure fits tightly by wrapping the fixed part in electrical / gaffer tape, taking care not to block the rotor. Plug the other end of the tube carefully with glue, covering the soldered joint for extra strength. Attach the BubbleShakerTM to your NIR probe by grinding one side of the tube flat, doing the same to the LED end of the probe, and glueing the two together with acrylic cement. There's a picture of a fully assembled unit [[Media:NIRprobe7.jpg|here]].
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