using photodiode in binary mode - Page 1
Forum section: electronics / beginners

author: Raydius
author_rank: Newbie
timestamp: June 13, 2023, 08:52:11 am
content: Hi everybody,I use this schematic to do digital light detection. (see attached picture)I want to tune detection level.My issue :I never used photodiode. I wasn't able to find any theoric document to help me at to calibrate this diagram.I think understood how the component works physically. But, I don't have any formula to calculate how many current the photodiode to allow to run following the luminosity.Please, anybody help me to find document or explain me how I have to do.

author: Faranight
author_rank: Unknown
timestamp: June 13, 2023, 09:04:05 am
content: What do you mean by binary mode? Do you mean to output a binary (on/off) signal with some hysteresis depending on the light level?Personally, I would just connect the photodiode to atransimpedance operational amplifierand feed the output signal into another opamp configured as aSchmitt trigger(positive feedback).You can then use two trimpots in place of the feedback resistors - one on the first amplifier to select the desired brightness, and the other on the second opamp to adjust the hysteresis range.I've added some schematics of a light sensor that I used in one of my projects.The J5 connector has a photodiode connected to it off-board. there's no schmitt trigger though, as I used the signal to feed an ADC on a microcontroller.EDIT: Added linksEDIT2: Schematics

author: Raydius
author_rank: Newbie
timestamp: June 13, 2023, 09:36:50 am
content: Thank you @Faranight,By binary mode, I mean output on/off. I don't need hysteresis. The captor monitor a flash detection.For my application, I need cheap and smallest solution. These are raison I choose this diagram.The diagram works but is too sensitive. I need to tune the level detection.I would like to begin my tune by theory to understand.

author: bidrohini1
author_rank: Contributor
timestamp: June 13, 2023, 10:59:07 am
content: You're trying to switch the transistor with the photodiode, right? Here is a circuit that does the similar thing. It is explained in detail. You can see this for a better understanding:https://www.build-electronic-circuits.com/photodiode/

author: tggzzz
author_rank: Super Contributor
timestamp: June 13, 2023, 11:16:57 am
content: The only time photodiodes are used in "binary mode" is as avalanche photodiodes, to detect individual photons. I doubt you are doing that! (There are also SiPM and PMTs, also irrelevant to your needs).You will probably want to considerambient light power relative to the light power in the signal you wish to detectdark current - similar in many ways to very slowly changing ambient lightsignal level - relative to dark/ambient current and noisesignal bandwidth - affects the type of receivers that can be usedthe "link budget" - optical power emitted then lost in transit, then converted to current/voltage, relative to noiseFirst piece of information: a typical starting point is a Si PIN photodiode operating in a reverse-biassed mode, where the photodiode current Ipd= 0.5 * Po+Idark, where Pois the optical power falling on the photodiode and Idarkis the dark current. The dark current depends on the temperature but not the reverse bias. The optical power depends on the diode's area and the source's brightness.Solid general information is available in The Art of Electronics, and more speciallsed/advanced information in Phil Hobb's website and bookhttps://electrooptical.net/