# Analog Electronic Circuit to Drive a Bright LED from a Photodiode

Thread Starter

#### gebseng

Joined Nov 24, 2017
27
Hi everyone on the forum,

I am currently building a electro-mechanical TV and need help in designing an electronic circuit for that purpose. Here is a description of what I am trying to do:

I need to design an analog electronic circuit that can drive a bright white LED from a photo diode. By that I mean that the LED should be able to change its brightness according to the brightness induced to the photo diode. This should work with a maximum frequency of 2 MHz. The circuit should also include a potentiometer to gamma correct the signal.

I need this circuit for a mechanical television system I am building, as described here: http://gebseng.com/11_big_paul/

Since this system (the spinning Nipkow Disk) blocks out 99,9886 percent (sic!) of the available light, the photo diode needs to be very light sensitive, and the LED needs to be very bright.

The photo diode should be used in "reverse bias" mode to be as sensitive as possible. Probably the photo diode could be an "Avalanche" type to be more sensitive.

Here is an example of a photo diode we could use:
https://www.digikey.com/products/en?keywords=SD100-12-22-021-ND

Here is an example of a bright 28W LED we could use:
https://www.digikey.com/product-detail/en/opulent-americas/XHP70A-00-0000-0D0BN240E-SB01/1672-1050-ND/6012737

If you have a design for a circuuit that could be re-purposed for my task, it would be great to hear from you!

thanks,

geb

#### danadak

Joined Mar 10, 2018
3,918
Two aspects of the design -

1) Photodiode sense and conversion to digital value.

a_A TIA would handle the photo diode converting its current to a voltage.
b_Follow that with an A/D to generate a digital value.

2) Drive LED with PWM to control its average current/brightness.

a_Digital value from A/D is written to PWM.

Challenges is meeting 2 Mhz with photo diode amp over wide dynamic range
and A/D speed all in one part.

The PWM portion easy in generic UP. The TIA separate external part. The
ADC can be done onchip by either interleaving two SARs or overclocking
one SAR but result lower resolution. This would be PSOC 5LP for both
PWM and 2 SARs.

Here is an example - https://community.cypress.com/thread/13920

Here is wideband TIA - http://www.ti.com/lit/ds/symlink/opa857.pdf

So what is the resolution you want in the photo measurement ?

Regards, Dana.

Last edited:
Thread Starter

#### gebseng

Joined Nov 24, 2017
27
Hi Dana,

Thank you so much for your extensive and thoughful reply!
The resolution I want in photo measurement is around 256 greyscales.

Thread Starter

#### gebseng

Joined Nov 24, 2017
27
Hi again, I am making slwo progress here. I found out that most bright LEDs used for lighting purposes are of the "luminophore" type and have significant afterglow, which does not work here.

I found an LED without luminophore:
https://www.cree.com/led-components/media/documents/ds-CLX6F-FKC-1352.pdf

But it is very, very dark, only outputs 6 lumen (as opposed to the 4000 lumen I would need).

Do you know if any non luminophore LEDs are out there? An array would work for me, but not 1000, maybe 10 or 100.

thanks,

geb

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