Hello,

Ok, so when no current is flowing through the phototransistor, we can assume an output voltage of 0.2 Volts.
In the datasheet is given that the output voltage is typicaly max 3.8 Volts at 5 Volts powersupply.
So your output range is 0.2 to 3.8 Volts.
The range is 3.6 Volts, when we divide that by the gain of 440 we will have about 8 mV accross the potentiometer.
Depending on the position of the potentiometer, you can determine the current created by the phototransistor.

Bertus

 

And i changed nothing and it has now started to work as your expecting it to. Must have been a loose connection on the breadboard maybe...

Now, its still moving around a little, is this also expected or would you expect the output to be constant with no environmental changes? Its moving from 0.13 to 0.6v sometimes... Its mostly sitting at 0.13-0.16 but it spikes to 0.6 every 5-10 seconds. Would you expect the AC measurement to be 0 on the output, if i got a AC reading, would that mean it is oscillating?

 

The conductor strips on a breadboard, plus any straggly connecting wires, are pretty good at picking up interference. Perhaps that is what is causing your erratic output.

 

If i'm honest, if it stayed at this output fluctuations and didn't get worse it will be perfectly fine in my finished project, i will have a MCU activate a speaker if it reads a voltage higher than lets say 2v or even 3v, depending on how much smoke is needed to trigger this. So, as long as it doesn't fluctuate to more than my set point it will be perfectly fine. but i'm just trying to educate myself for my own knowledge needs to be honest.

 

Hello,

If you want more stability than on a breadboard, transfer the circuit to a stripboard.
On there you can solder the components.
Here is an example of a stripboard:
http://www.ebay.com/itm/copper-prot...-circuit-board-strip-vero-board-/170860932701

When using such board, I would use a good quality socket for the amplifier chip.

You can also use "the manhattan style PCB construction"
http://www.piclist.com/techref/pcb/manhattan.htm

I also attached a PDF from an other site.

Manhattan construction methode.pdf

Bertus

 

I'm well aware of those stripboards, these have less interference than breadboards then do they? I never really considered this.

Thank you for giving me some information regarding the manhattan style PCB construction! Another morning read over my oats