How can i increase the range of values that a light sensor gives?

I have constructed 2 ceptometers that measure light intensity. Each ceptometer consists of 50 photodiodes that are connected to 2 copper wires (+ and - of the photodiodes are on the same orientation). Those wires are soldered with cable (+ to the copper wire where the positive side of the photodiodes is and the ground wire soldered to the copper wire where the negative side of the photodiodes is). Then, pins were soldered to the cables so as to can be connected to the breadboard.

The paper where the protocol for the construction is present, states that a 1.5 ohms resistor needs to be connected to the copper wires so as to enhance the range of the signal that the ceptometers will give. However, it mentions that this step can be followed later and the resistor can be connected to the wires (that is something that makes sense). I need to mention that one more ceptometer is present that was constructed by another person the previous year and it has the 1.5 ohms resistor soldered on the copper wire. It is something very important to keep in mind for the rest of the text!

I decided to use an arduino uno for my experiment and I connected the 3 ceptometeres (the old from the previous year and the 2 that were constructed by me) to a breadboard. Firstly, I applied light to the ceptometers with a projector and I tried different resistors from 470R to 5K but the values I got were the same. I thought that a kind of signal disturbance or noise may has been caused and I made 3 opAmp circuits in 3 different breadboards. When the breadboards were connected to the Arduino I tried the same resistors again but the values I got were the same. For the old ceptometer that has the 1.5 ohms resistor soldered to the copper wires the change of resistors altered a lot the signal and it gave a wide range of values. Here I have to say that when I applied to the 1.5 ohms resistor to the new captometers I got a pure 0 as a value. And it was the same when I placed them under sunlight.

When I converted the output of the Arduino to voltage signal by multiplying it with 5 and then divide it with 1023 I had the same voltage signal as when I measured it with the multimeter. And it was the same under both artificial and sun light. So, I can imagine that it is something that depends on the resistor that is not applied to the copper wires. I mean that maybe this is the case that I do not get a wide range of values like the old ceptometer but only 20 units

My question is: Do you know any way to enhance the range of the output or do you have any recommendation? Thank you a lot in advance!

With kind regards,

Dimitrios

• Can you amend your question to include a copy of your schematic? A sketch would do if necessary. It is hard to visualise a circuit which is done purely by description in words. – Nick Gammon Nov 5 '20 at 23:40
• A schematic would really help. Have you been able to confirm the voltage output range of your setup outside of the Arduino, by measuring them with a multimeter for example? – StarCat Nov 6 '20 at 8:16
• Hi Nick and StarCat. Yes StarCat I measured the voltage with a multimeter and it was the same that the Arduino gave me after the conversion of the Arduino output to volts. For example, when I got a value of 90 (from the Arduino) and then did the conversion ((90*5)/1023) what I got was 0.44 that was the same that the multimeter show in voltage. But when I did the calibration with a quantum sensor the value of 90 corresponded to 600 to 690 μmols. – Dimitrios_Lioilios Nov 6 '20 at 11:25
• This is something that will affect my experiment since 100 μmols of PAR is an important range but from the arduino it is described as one value. – Dimitrios_Lioilios Nov 6 '20 at 11:25
• I have uploaded the schematic on my question. Thank you a lot both of you! – Dimitrios_Lioilios Nov 6 '20 at 11:27