I take it you will be measuring backscatter from the light source.
When trying to detect very small amounts of light it is often
advantageous to "bias" the sensor by applying an amount of light
that just brings the sensor to the point where it begins to
produce a useable output. An LED can be used as the bias
illumination. You can also increase sensitivity by using a convex
lens to gather light and focus it on the detector.

Most often a silicon detector is used. You can make a current-
to-voltage converter using a single op-amp. Ground the non-inverting
input. Connect the silicon detector between ground and the
inverting input. Connect a 10 Megohm resistor between the output
of the op-amp and the inverting input. This will create a circuit
that will give you 1 volt out for every tenth of a microamp
generated by the silicon detector. If you want more gain, then
connect a standard non-inverting amplifier circuit to the
output of the current-to-voltage converter.

Because the detector is forced to operate as a pure current
generator (voltage across detector is forced to be zero volts),
the detector will have the fastest possible response time,
and the most linear output that the detector is capable of.

It is imperative that the op-amp used is something like the
LF356, which has an extremely low input current. A 741 or an
LM324 will not work well in this application.

Fr. Tom McGahee


----- Original Message -----
From: Matt Brauer <brauer@GENOME.STANFORD.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, January 12, 2001 3:41 PM
Subject: [EE]: Need advice re. laser photodetector


> Hello,
>
> I am trying to develop a device to measure multi-angle light scattering
> through a microbial culture to determine the biomass of the culture. I
> have not worked with lasers or photodetectors, and I'm wondering if anyone
> can point me to some good resources for possible circuit designs. (I hope
> to use a 645nm laser diode, and sequentially monitor the output of 8
> photodetectors through a multichannel ADC such as an ADS8344.)
>
> Has anyone done anything like this? What are the considerations for the
> photodetectors to use?
>
> Many thanks,
> Matt Brauer
>
> --
> Matthew J. Brauer
> Department of Genetics
> Stanford University School of Medicine
>
> --
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>

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