> 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?

Hi Matt,

Photodiodes are typically used for this application, since they are
available cheaply with good matching. Allied Electronics sells them in
small quantities.

The sensitivity peaks at 700-800 nm and although there are photodetectors
that are linearized, I would only choose them if you needed flat response
since they do it at the expense of sensitivity(and that means less S/N
ratio). If you don't expressly need 645 nm you can get longer wavelength
laser diodes and get a bit more response(just a few percent though).

Typical amps will use the photodiode from ground to the inverting input of
an opamp with gain set with a feedback resistor(output to inverting
input) and noninverting input grounded. Use a rail-to-rail opamp(LMC6484
is a good quad for this). This method of amplification is good in the
kilohertz range but not in the megahertz range, not an issue in your
application.

If you've never built a laser driver before, I'd suggest buying a premade
one and test it thoroughly for output versus temperature(some are pitiful
even though they use the monitor diode for feedback). Also remember the
case of the laser diode usually has voltage on it and also must be
heatsunk. They are also the most static sensitive devices you are likely
to encounter so be good.

Cheerful regards,

Bob

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads