Im currently not is a position to be able to post photos. But i think its less my wiring and more my understanding of resistor values i need and the gain i need.
Im aware that the photodiode draws somewhere in the range of a few micro amps.
A PIN photodiode does not -draw- current, it sources current.
What do you mean, you don't know what is happening?
You can take some DC voltage measurements.
well firstly i have some code which im pretty sure works fine
void setup(void) {
Serial.println();
}
void loop(){
int lightsensor = analogRead(A0);
float voltage = lightsensor * ((5.0/1023.0));
float vResult = (voltage - 5) *(-1);
Serial.print(vResult);
Serial.println();
delay(1000);
}
now when i use my circuit which is basically the same as the one in the link page 29 left hand side. Although ive used ground at anode because board cant do -5. (i might have to purchase a voltage converter which i know exist)
Something is actually happening. Like i put my finger over the diode and it just goes to its limit which is expected because it goes dark.
But say i shine light on it and it just saturates to to one specfic value and its not even at the end of the range, it just stops at 1.31 (ive set it up so it goes from 0-5. The light isnt even that bright.
Im using 1 mega ohm resistor.
picture is too big to upload sorry.
A great book on Op Amps and comparators:
Thanks for the FREEBEE ! I downloaded it.
@OP,
Any single-supply OP AMP should work.
My personal preference is the LT-1215. (SEE ATTACHED DATASHEET).
What is the part number of the photo diode you are using ? Do you know what the forward voltage is ?
Are you sure it isn't 1.31 V ?
Can you post a link to a datasheet ?
You need to know ID to calculate R1 if you let Vo = some arbitrary value.
Vout = R1 * IDiode => R1=Vo/ID
Let ID= 5 mA
Let Vo = 4 V
R1 = 4/0.005= 800 ohms
LT1215.pdf (339 KB)
Im pretty sure the current im getting from the photodiode is somewhere in the range of a few micro amps. Im thinking perhaps because im only putting 5v in to drive the op amp, i might actually be reaching the rail -> so once i reach it, nothing else registers - but im not sure.
the photodiode im using is this one:
perhaps my understanding of electronics and whats going in here just isnt that good.
Im not actually sure whether the resistor is the problem.
I suspect that my photodiode is actually just saturating out. I might try to put a neutral density filter.
I make use of the LT1215
If youre after an efficient NFB loop non inverting, look into the TL071, very effective audio amplifier unit, (yes i know that was your intended purpose), alround known as a more modern successor to the 741, very capable opamp.
Did you actually do the math for the resistor ? What value are you use for the op amp feedback resistor ?
I just have 1 M ohm. Im pretty sure the value of that resistor changes nothing at all considering ive tried many.
MrDropsy:
I just have 1 M ohm. Im pretty sure the value of that resistor changes nothing at all considering ive tried many.
You are wrong it is vital to setting up the amplifier's gain.
OK then what kind of gain do i want then.
I have changed my circuit to the first shown in:
http://www.ti.com/lit/an/sboa035/sboa035.pdf
Or at least as close as i can get it to that.
quite close to giving up as i cannot get this to function they way i want it to. Possibly just a lack of knowledge
Hello 
for your application you must use a R2RI (Rail to Rail In) and RRO (Rail To Rail Out) operational amplifier to have a full 5v swing output
but you are using it as a I/V converter and you must pay attention to an important parameter: the input bias current
each op is not perfect and a current appears at the V+/V- input (near 100nA for some standard opamp but depend of the temperature and tecnology of the opamp like Jfet bipolar etc)
but in your application with a 1M feedback resistor 100nA give = 1M * 0.0000001 = 0.1V of offset voltage.
1M is a big gain for a photodiode amplifier and even a very low ambiant light can give 1mA of photocurrent and saturates the output.
the light variation is to the square of the distance this is why the best way is to use a logarithm amplifier but it's very complex even if some circuits are ready to do that.
for the opamp check the microchip MPC601/602 it's a good general purpose rail to rail amplifier with 2 pico amp bias current and feedback resistor gain. (the rail to rail in version is MPC6L01)
MCP6L02
for my optical application i'm using a special op with very low bias current but expensive! 8€/unit
Thank you very much what kind of resistance should i be using then? I think i get a few micro amps out of the photodiode.
Ill be sure to check the suggested one out.
for that you must know what is the maximum photodiode current value at the maximum luminous intensity.
after you can use the Vout = R*I photodiode formula.
example if you have a 1mA maximum photodiode current value and your supply rail is 5V
you must choose a 5/0.001 = 5K resistor (nearest value 4.99K 1% E96 serie)
but note that the bias current must be very low compared to the photodiode current. And even if the op amp is a rail to rail out there's a little voltage drop at the saturation voltage (near 50mV) so you never get exactly the supply rail. you must think about this for your ADC full scale value
Ahh makes sense, thank you very much. Im not sure what you can recommend, but i've run some tests and determined that my photodiode is actually part of the problem. It seems to saturate out under a
light source:
Im using a Olympus CLV-S20 which is a 300 Watt light source. There is just too much light for the photodiode to handle.
If i use a Pentax EPK-1000
Which is a 100 watt light source. I am able to get a nice linear result (light intensity vs voltage)
Is there a photodiode you can recommend that can operate very well under high intensity light.
The light sources im using are medical grade.
Im thinking i need a better photodiode for my application or a neutral density filter to reduce the intensity.
Hi
1 - LM358. is cheap and quite linear
2 - For better precision go with a TL072 that is largely used in industry
3 - If you want to amplify a digital signal try a transistor buffer. Cheap and very reliable.
Regards
2 - For better precision go with a TL072 that is largely used in industry
Not a good choice.
If you give it +/- 30V you can only get +/- 24V output.
Also the magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 volts, whichever is less. - The first part is standard the second isn't.
The recommended operating voltage is +/- 15V.
They are not too clever from a single rail 5V supply.
right Grumpy! this is why the choice must be R2R amplifier like I've explained before.
LM358 is ok for mic amplifier with supply/2 bias but not for single supply and rail voltage out application.
MrDropsy, you must use a neutral filter over certain level of light.
use the datasheet of the photodiode with it's sensivity and maximum current value. with these informations you can know how much luminous powet it can handle.
look here it's a good spec
PhotodiodeAmplifers.pdf
and here
You were right about TL072 Grumpy_Mike, sorry about that.
Nice file Genesis92.
I do think that LM358 will do the trick but for a 5V output, input power must be around 7,5V, might be a problem.
If the range could work, lets say, from 0- 2.5V, then it will go perfectly.
Of course RTRIO opamps are much more expensive and usually don't come in DIP packages