Spectrophotometer

[George L.]

Hello,

I am doing microbiology experiments with bacteria in school and I need a quick and easy way of determining roughly, how many bacteria are in a test tube. The easy option is to use a spectrophotometer, basically shine a light through the test tube and a sensor on the other side will get a light level reading and determine the "murkiness" of the liquid and from that you can determine the number of bacteria. I don't have a $1,000 spectrophotometer, but it seems like it shouldn't be too hard to make.

I am thinking an LED or Laser diode from a laser pointer at one side of the test tube and a photoresistor at the other side. The voltage at the photoresistor can determine the number of bacteria. The question is how sensitive can this "makeshift" sensor be. To .01V (multimeter)??? Also this would be powered by a battery which would slowly loose power and change my readings from test to test....Would a wall wart as the supply solve this problem?

Suggestions, ideas, and guidance would be really appreciated.

Thanks,

George L.

 

[Olihou]

I am not sure how it works as you have described --- how the "murkiness" would relate to the number of bacteria ? Will the liquid itself, say the thickness, affects? How about the stray ambient light ?

The electronics part should be easy --- just a stable set of light source and and sensor-amplifier. LED may be a good choice. Laser may not as it covers too small an area .....

 

[evandude]

I would expect that to deal with ambient light and other factors (like the solution itself, and the test tube, etc) you would take a couple of readings, one control sample (with just empty solution in it) and then another of the actual sample, and just look at the difference.

Your worry about the battery voltage is unnecessary if you just use a voltage regulator. Your accuracy can (within reason) be as high as you want it to be, if you only need to look at a small range of voltage changes, then just shift and scale it to a larger range to read it with a multimeter, etc. If you use some opamps and a potentiometer to zero the meter with the control sample, you should be able to do it quite simply with just analog electronics.

 

[Sceadwian]

This is called a 'colorimiter'
The EXACT spectrum of light created and it's brightness as well as the brightness observed in each spectrum is key to it's results. The only fasion I can see this being used for a biological reason is for density measurment in a mono culture. And even then you would require the known spectrual measurments for that partciular bacterium (and only that bacterium) in that medium. if there is more than one kind of bacteria or ANY other form of interference the measurment is useless.