Optical detector help needed
Nige
find a schematic at http://www.portal.freeuk.com/detector.jpg.
I borrowed this circuit from a Sharp Optoelectronics application
note, together with a bit of guesswork and memory from my
electronics course 20 years ago. (Plus guided by whatever parts
I had in my electronics box).
My problem is I want to increase the sensitivity of the
circuit. It fundamentally works ok : in darkness Vout=0.5V,
and shining a Maglite directly into the photodiode gives
Vout=7.5V. But I want to be able to detect lower intensity
light.
Please can you suggest any modifications that will improve
the sensitivity? Alternatively can you point me at any
simple circuit designs that will do the job better?
Thanks.
Jim Thompson
Nige <mal...@freeuk.com>,
In Newsgroup: sci.electronics.design,
Article: <3CFE7A9A...@freeuk.com>,
Entitled: "Optical detector help needed",
Wrote the following:
Just try increasing R1 until you get the sensitivity you want. Keep
in mind that you will sacrifice dynamic range.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
For proper E-mail replies SWAP "-" and "_"
I love to cook with wine. Sometimes I even put it in the food.
Bill Sloman
As Jim Thompson says, increasing R1 is the first thing to try.
There is a classic op amp circuit for the job, and Burr-Brown's
application notes for their low leakage op amps OPA111, OPA121, OPA129
all show it as an example. The application note for the much faster
OPA655 discusses why this circuit is noisy at high frequencies.
The data sheet for the Burr-Brown OPT101, which combines a photodiode
with a single supply op amp, makes even more interesting reading.
-----
Bill Sloman, Nijmegen
Nige
> |the sensitivity? Alternatively can you point me at any
> |simple circuit designs that will do the job better?
> |
> |Thanks.
>
> Just try increasing R1 until you get the sensitivity you want. Keep
> in mind that you will sacrifice dynamic range.
Thanks for the speedy response.
I've now taken R1 up to 4.5M in various steps, but I'm still not
getting sufficient output (I think. Wish I'd borrowed that 'scope!)
What defines the limit?
Nige
>
> Nige wrote:
> > Please can you suggest any modifications that will improve
> > the sensitivity? Alternatively can you point me at any
> > simple circuit designs that will do the job better?
>
> As Jim Thompson says, increasing R1 is the first thing to try.
>
> There is a classic op amp circuit for the job, and Burr-Brown's
> application notes for their low leakage op amps OPA111, OPA121, OPA129
> all show it as an example. The application note for the much faster
> OPA655 discusses why this circuit is noisy at high frequencies.
>
> The data sheet for the Burr-Brown OPT101, which combines a photodiode
> with a single supply op amp, makes even more interesting reading.
That sounds tempting. Dual supplies put me off using an op-amp. (As
an aside, is using a potential divider on the supply viable if you
don't want much current or linearity from an op-amp?)
Cheers,
Nige
Jim Thompson
Entitled: "Re: Optical detector help needed",
Wrote the following:
|> |Please can you suggest any modifications that will improve
Yep, Borrow that 'scope ;-) Then try it dark and with the light level
you want to detect... you should see movement at the output.
Kevin McMurtrie
wrote:
Use an op-amp in a current to voltage converter setup. You'll have a
nearly infinite range of gain. You can also put filters on the feedback
to control the bandwidth. It's a classic demo circuit so it should be
easy to find in photo diode spec sheets and electronics books.