Editing BioBoard/Documentation/Optical loss
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*how much can you afford to invest? | *how much can you afford to invest? | ||
This design uses an 850nm plastic LED (Everlight HIR204 - $0.43) as the photoemitter, and a matching phototransistor (Optek OP506B - $0.80) as the photosensor | This design uses an 850nm plastic LED (Everlight HIR204 - $0.43) as the photoemitter, and a matching phototransistor (Optek OP506B - $0.80) as the photosensor; the required circuitry is limited to a couple of resistors. Here's a quick [[Media:NIR_probe_schematic.jpg|hand-drawn sketch]] of our NIR probe to give you an overview of the construction described below. | ||
==Things to keep in mind== | ==Things to keep in mind== | ||
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Optional: cell-phone motor (BubbleShaker Technology) | Optional: cell-phone motor (BubbleShaker Technology) | ||
==How to build it== | ==How to build it== | ||
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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. | 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: | 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:NIRprobe5.jpg|here]]. | ||
=Interfacing and measuring= | =Interfacing and measuring= | ||
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=Calibrating= | =Calibrating= | ||
Calibrating measuring equipment is an important part of any scientific pursuit, because the accuracy of your calibration determines the reliability of your data. However | Calibrating measuring equipment is an important part of any scientific pursuit, because the accuracy of your calibration determines the reliability of your data. However, determining absolute accuracy may be somewhat difficult, so you might want consider whether you actually need absolute values; in a lot of cases, you are likely to be less interested in the absolute biomass than in the relative change in biomass over time. | ||
To get an absolute measure of biomass in a live microbial culture, you can use several different techniques: | To get an absolute measure of biomass in a live microbial culture, you can use several different techniques: | ||
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* desiccating samples to measure total dry organic matter by weight | * desiccating samples to measure total dry organic matter by weight | ||
All of these techniques will require special equipment, though, and multiple measurements over time to create enough data to draw reliable calibration curves and compare your NIR probe values to those curves. We're not going to go into detail with any of | All of these techniques will require special equipment, though, and multiple measurements over time to create enough data to draw reliable calibration curves and compare your NIR probe values to those curves. We're not going to go into detail with any of those here, but merely describe the ways in which you can adjust your probe if you need to. | ||
The easiest way to calibrate this NIR probe is by tweaking the Arduino sketch. Use a voltmeter to measure the voltage coming off the leads from the phototransistor directly when the probe is just sitting in air (should be close to 5V), and simply enter that value instead of the default IMAX value in the calibration settings: | The easiest way to calibrate this NIR probe is by tweaking the Arduino sketch. Use a voltmeter to measure the voltage coming off the leads from the phototransistor directly when the probe is just sitting in air (should be close to 5V), and simply enter that value instead of the default IMAX value in the calibration settings: | ||
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If you have problems covering the whole spectrum, the other two things you can adjust relatively easily are the distance between the LED and the phototransistor, and the resistance on the circuit. A low resistance potentiometer instead of the 100Ω resistor on the emitter lead would allow you to adjust the sensitivity of the phototransistor directly, and provide a way of tuning it without having to disassemble and rebuild it. You can also decrease the light intensity of the LED by increasing the value of the resistor; 470Ω seems to be a good mid-level. | If you have problems covering the whole spectrum, the other two things you can adjust relatively easily are the distance between the LED and the phototransistor, and the resistance on the circuit. A low resistance potentiometer instead of the 100Ω resistor on the emitter lead would allow you to adjust the sensitivity of the phototransistor directly, and provide a way of tuning it without having to disassemble and rebuild it. You can also decrease the light intensity of the LED by increasing the value of the resistor; 470Ω seems to be a good mid-level. | ||
= | =Making it cooler= | ||
Tuning to different substances | |||
Multi-channel measurements | |||
=Geeking out= | |||
Reduction of light passing through a mass | |||
Absorbance vs. scattering | |||
=Links= | =Links= | ||
*Optek | |||
* | *Wikipedia | ||
*TruCell .pdf | |||
* | |||
* |