BPW 34 , opinions on simple circuit requested

Hey, I’m making a simple little project to trigger my camera by laser trip/lightning detector
I’m using a BPW34 pin photodiode and came up with a very simple circuit to detect an adjustable level of light, however after doing some research after the fact i haven’t seen this exact circuit done before and I’m wondering if I am overlooking something that makes my circuit a bad idea in general. So basically asking your opinion on this circuit and any insight on things i may not have thought of

If you look at the attached photo of a circuit diagram, I am directly connecting a digital Input to the #2 terminal of the 1M Potentiometer/cathode of bpw34, I monitor the pin and have an output to another pin to indicate the status of my input then just turn the potentiometer to where the input isn’t triggered, any additional light (aka lightning or my laser ) then triggers the input,

some things that i am aware of-

  1. logic is inverted, signal is low when it sees light, high in absence of light

  2. I understand this isn’t a digital output from my simple setup and its just a matter of my analog signal being just above or below the threshold to see it as high or low, not an issue for me as I’m adjusting the potentiometer every time to compensate for ambient light

  3. at higher ohms the sensitivity is higher but the rise time is slower, but for both of my uses that doesn’t matter, lighting is slow enough i think it will still trigger, and the laser line being broken is what i’m looking for, which it still has a much faster fall time at higher ohms

  4. at very high ohm pot settings and very bright light (direct 5mw green laser) the signal goes ever so slightly below 0 (about -0.4v when my project is being actively powered, and -1.3 max when entire project is unpowered), i know the digital inputs have some over/reverse voltage protection diodes but the amount of current from the bpw34 is exrtremely small, i believe less than 100 microamps, so even when i have 3x photodiodes in parallel (for larger hit area) i don’t think it will bring much risk of damage and havent given me an issue yet



connecting a digital Input to the #2 terminal of the 1M Potentiometer/cathode of bpw34

Did you mean #3 terminal? If not, is anything connected to #3?

Nothing is connected to #3, I’m using the potentiometer as a variable resistor, sort of like a variable pull-up for the signal, the bpw34 Pulls it low with light

i haven't seen this exact circuit done before

This is a standard reverse bias configuration, but it is unusual to take the signal across the photodiode.

You can read all about various photodiode circuits, their problems and partial solutions here.

I guess you're making some sort of beambreak trigger.
Wouldn't you just use a 3-pin IR remote control receiver and modulated (38khz) an IR LED for that.
Then you don't have issues with ambient light.
Such a setup could have a reaction time of 1/2000 sec, and a beam diameter of ~5mm.
Lasers look cool in movies, but could be impractical for beambreak.

I meant this circuit alone, without any kind of amplifier or buffer directly to the high impedance input, everything else I’ve seen is a bit more complicated, if just using the core of it alone

And yup main use is a beambreak, I went with this route to keep the code as simple and fast as possible, (will be a relatively quick moving object breaking the beam) once it’s in “waiting” mode it does nothing except wait for an interupt on that pin, then directly sets another pin high to release my camera shutter, largest delay so far is the reaction time of the camera of around 60ms,
Laser is also going to be included in the long time exposure on purpose for cool factor lol

edit additional info on timing
With this current setup, I’ve timed the delay between laser being broken to output triggered with a logic analyzer and I get a period of about 30-70us, which is great for triggering a flash at nearly the exact moment the laser is broken, just wish the camera was faster to respond, at the speed of what I’m photographing (~30-40 mph/ ) I need to be framed a few feet down from where my laser is broken

everything else I've seen is a bit more complicated

That is because of the several problems posed by the circuit you posted, discussed in the photodiode amplifiers link above. But have fun.

But that’s why I’m curious if there’s anything wrong with doing it this way in this particular situation connecting to an arduino,
As far as I can tell I haven’t come across any of those problems but it seems too simple and good I be true lol, I feel like I’m missing something but it keeps working fine

I’ve attached 2 photos of my o-scope of the signal with 3x bpw34 in parallel, pot to highest setting of 1M, the crisp one is simply a 6in piece of wire next to my breadboard, the fuzzy one is my circuit, both are 20mv/div, which from my experience is pretty neglible considering my very noisy city setting, not sure what the fuzz is it’s only clear at my 20mhz o-scopes fastest setting

edit won let me attach photos from my iphone

The problem with a photodiode is ambient light.
That could be daylight and/or mains powered light with 50/60hz AC ripple on it.

A lot of light on the photodiode (a laser), and a low pull up resistor value to just keep the pin LOW could help.
1Megohm seems waaay to high.

Problem with a laser is the setup. Not easy to keep the spot on the photodiode, depending on distance.
A common 3-pin IR sensor and an IR LED have a wide opening angle (easy to setup), while still having a 5mm ⌀ beam.

Try with your IR TV remote, about 5meters away from the TV.
You can angle the remote quite a bit, or be in an angle from the TV.
But a finger between remote and receiver will block reception.

What are you detecting.

First and main use I'm designing it for it to trigger my camera for a long exposure of my bike as I pass by, will be manually or wirelessly triggered to start and the laser will stop it,
Aligning nthe laser is certainly the hardest part during my test setup, but did a real world application and over the course of the 30 or so feet the beam travels it actually gets to be about 5 inches in diameter, current method of eliminating erroneous light is that pot 1M is the highest it can be, depending on the value of that it takes a certain amount of light to bring the signal low enough to trigger the digital input change
I've tried a few other methods with an opamp bu so far this seems to work best, just so long as my source is brighter than any other ambient sources

One drawback I've seen so far, I assume is just how a photodiode works, is that the frequency response goes way down as sensitivity increases, testing with another led as a source of pulsing light, at 1M I can only get to about 5khz until the square wave is basically a triangle, when the pot is way down probably around 1k I can get over 200khz but I need to make the source led many times brighter
Is there a work-around to keep the sensitivity and the frequency response at the same time?

the frequency response goes way down as sensitivity increases

That is explained in the document linked in reply #3.

One of the many problems with your circuit, which are solved by more advanced designs.

Can you give me an example? I’ve read that document a few times and tried a few of the circuits, but that fundementality doesn’t seem to change, both reverse biased and in photoconductive mode, also I found the inverting amp configuration lowers sensitivity and is better non inverting, however these are all just methods of amplifing the signal, which helps a bit but with a low value resistor (<10k) reverse biasing the photodiode it still suffers from reduced sensitivity to the point where it won’t noticeably change on a 5mv scale so no matter how much amplification it wont pick up weak source of light

Another note as I try and increase the frequency response, with a reduced sensitivity for faster response, the actual signal I'm looking for is much smaller, which would be solved with one of those amplifier circuits but with such a small margin of signal to noise ratio to work with, The amplifier has a much harder time amplifying anything useful

Also realizing this was probably better off in the general electronics section now, the part that pertains to my project works fine I just didn't know if I could damage anything this way

40mph is a distance of < 2cm in 1/1000sec (the timing you can achieve with common 38khz modulated IR).
Not sure why you want to take the much harder path of a laser with a photodiode + amplifier.
Good luck.

This is also something I didn't realize at first, I initially did the math in my head and got a much larger distance traveled per ms, I thought I was dealing with several feet at that speed lol