Hi all, long time no see
I'm wondering if I can please get some help: I'm trying to make "accurate" (as in highly granular / large range of values) light readings from a BPW34 photodiode, through a TL074, in to an Arduino Mega. Refer attached image for circuit.
The approach I'm going for is using the op amp as a transimpedance opamp: the BPW34 apparently has a very linear current response to light, so I'm wanting to convert that current signal in to a voltage of 0v to 5v (which the Arduino analogue input would interpret as values from 0 to 1024)
As it is, the arduino reports about 920 in ambient light, and 850 with a bright lamp shining right at the diode and nothing i do to alter the gain seems to make any difference. The fact it's inverted doesn't bother me so much.
Note the 5K resistor in my circuit diagram: it's my understanding that is meant to adjust my gain: looking at the BPW34 datasheet, and using the Wiki page on transimpedance amps, I see the maximum current is about 1000uA, so using the formula:
I get a value of 5k for my gain resistor. I've tried a range of resistors though: 500R, 2k, 10k, 50k, and there doesn't seem to be much difference: the Arduino still reads 920 to 850. I tried switching the direction of the diode (just in case) and it just sits at 820 or so when reversed
What I'm expecting to see is a range of values from, say, 100 to 900? 100 to 500 even would be better.. I'm expecting to see a BIG difference from halving or doubling my gain resistor though: that way I can "tune" it to the range of light I will be dealing with.
The fact that it is inverted even though I'm using the non-inverting input doesn't really bother me though I don't exactly understand...
I'm wondering if my problem is related to the unused pins on the TL074. I will eventually use three of the op amps on the chip but for the mean time, I have three unused op amps on the chip: the negative input is connected to the output, and the positive input is connected to ground. (shown in circuit diagram too)
I can see there are alternate ways to wire my op-amp, where I connect the positive leads of diode to Vcc for reverse bias but I believe that decreases the range of sensitivity doesn't it? In other news: the voltage created by the diode goes between 200mv to 400mv.
Thanks for your help!
I'm wondering if I can please get some help: I'm trying to make "accurate" (as in highly granular / large range of values) light readings from a BPW34 photodiode, through a TL074, in to an Arduino Mega. Refer attached image for circuit.
The approach I'm going for is using the op amp as a transimpedance opamp: the BPW34 apparently has a very linear current response to light, so I'm wanting to convert that current signal in to a voltage of 0v to 5v (which the Arduino analogue input would interpret as values from 0 to 1024)
As it is, the arduino reports about 920 in ambient light, and 850 with a bright lamp shining right at the diode and nothing i do to alter the gain seems to make any difference. The fact it's inverted doesn't bother me so much.
Note the 5K resistor in my circuit diagram: it's my understanding that is meant to adjust my gain: looking at the BPW34 datasheet, and using the Wiki page on transimpedance amps, I see the maximum current is about 1000uA, so using the formula:
I get a value of 5k for my gain resistor. I've tried a range of resistors though: 500R, 2k, 10k, 50k, and there doesn't seem to be much difference: the Arduino still reads 920 to 850. I tried switching the direction of the diode (just in case) and it just sits at 820 or so when reversed
What I'm expecting to see is a range of values from, say, 100 to 900? 100 to 500 even would be better.. I'm expecting to see a BIG difference from halving or doubling my gain resistor though: that way I can "tune" it to the range of light I will be dealing with.
The fact that it is inverted even though I'm using the non-inverting input doesn't really bother me though I don't exactly understand...
I'm wondering if my problem is related to the unused pins on the TL074. I will eventually use three of the op amps on the chip but for the mean time, I have three unused op amps on the chip: the negative input is connected to the output, and the positive input is connected to ground. (shown in circuit diagram too)
I can see there are alternate ways to wire my op-amp, where I connect the positive leads of diode to Vcc for reverse bias but I believe that decreases the range of sensitivity doesn't it? In other news: the voltage created by the diode goes between 200mv to 400mv.
Thanks for your help!