Picking the right photodiode

[Xardas]

I need to transfer data from my Windows Phone 7 to mC. The problem is that WP7 doesn't have any type of usb client control. So I was thinking, I could connect photodiode to mC, change in voltage will be used to represent data. I have already coded 32 characters using two voltage states, that should be enough. Light will be emited from WP7 screen, just flashing black/white when in sending mode. I would isolate external by building a stethoscope-like fraim for diode. My question is what model of photodiode should I use? Changes in screen light will be every 20 ms. Do I need some kind of amp for voltage from diode ?

 

[Harald Kapp]

With 20 ms resolution almost any photodiode will do the job. Apart from the photodiode you will need an amplifier. Here are some basics on how to amplify photodiode signals:
http://www.ti.com/lit/an/sboa061/sboa061.pdf
http://www.avagotech.com/docs/AV01-0745EN

Harald

 

[Xardas]

Thank you for replying. Will some low part count lm386 amplifier circuit do the job, or I need to build something from the links that you have posted?

 

[Harald Kapp]

The LM386 is not well suited for this task. It is an audio amplifier with comparatively low gain.
You don't have to use any of the Amplifiers in the examples. However, you should use a true operational amplifier preferably with low bias current. Maxim, Linear Technology, Texas Instruments, National Semiconductor (and others): they all offer a wide range of operational amplifiers.

Harald

 

[Xardas]

I will try to find such opamp. I'm from Serbia so I will need to look for equipment in local el. store. Thank you for your help.

 

[Xardas]

Will something like this be useful: 741 DIP OPAMP, 3-18V, 1MHz, GENERAL PURPOSE, DIP8 ?

 

[Harald Kapp]

The 741 is not the best choice, but a good starting point. It is easy to use and relatively benign (meaning it doesnt require fancy circuitry around it).
Go ahead and start with this one. Since your timing requirements are very slow, the 741 may well work in your application.
One drawback of the 741 (and all non rail-to rail-output OpAmps) is that if you operate it from 5 V (as I assume from the description of your application) it will not generate the required logic level at the output. You will need to add a comparator.

However, with 500 nA (max) input current it is not a low current OpAmp. Maybe you can find one of these: http://focus.ti.com/paramsearch/doc...og&familyId=1478&uiTemplateId=NODE_STRY_PGE_T ?
For example the LMC6081 has an input current of only 4 pA, that's more 10^5 times less than the 741.

If you want to search further, here are a few parameters you should take into consideration:
- Single supply voltage (should include 5 V)
- low input current (good: < 100 nA, better: < 1 nA)
- rail-to rail output (if the inputs are rail-to-rail, it is even better
- stable without external compensation

Harald

 

[Xardas]

I will try that, hope it will work...