Hi, I have the following IR emitters / detectors:
Through experimentation I've determined that at near-zero range a 10K pull-up resistor does what I require (analogRead is < 50 for low, > 1000 for high). Is there any way I could have calculated that from the data sheets? Even just roughly as this took a reasonable amount of time to find out - however I only have to do it once per sensor.
Thanks in advance,
There are few application notes out there that describe how to use a photo transistor. This is one:
http://www.fairchildsemi.com/an/AN/AN-3005.pdf
Is there any way I could have calculated that from the data sheets?
Yes but you would have to have known other stuff as well.
Basically you need to look at the on state collector current at the light intensity you are receiving (I bet you don't know this). This is given in the electrical optical characteristics table for one value and as a graph in Figure 4. Then use this current to see what voltage you would drop across your pull up resistor.
Basically the larger this resistor is the greater the voltage drop for any given current down it, so making it bigger gives you more signal. Making it too big brings problems with noise, coupling into the A/D and speed of response. However 10K is about right. 