How to bias a photo-transistor (the resistance in the collector) given that:

Vcc = 5v
Vce(sat) = 0.4v
   min: 0.2 - 1.6
   max: 0.6 - 2.4

I'm using LTR-301 photo-transistor. I want to use it in the Active mode, to get the heart pulses and what about Vce, if it's low, (it's supposed to saturate the photo-transistor, I don't want that to happen) I need the range of values of Vce to calculate the thresholed in the Comparator (to convert the signal into a pulse train)

Do I need amplification if Vce is big enough?

The Project Schematic


If you know the collector current you want the calculation for Rc [assuming Vce(sat)] is:

Rc = (5v - 0.4v) / Ic

However with a part like this with a wide operating range you may be more interested in the switching speed. A lower value Rc will give faster switching. On the spec sheet there is a graph with example Rc values from 1k to 10k showing the typical switching speeds. So if you need fast switching stay closer to the 1k range, if you need low power operation and can tolerate a slower switching speed then stay more toward the 10k range.

The data sheet also gives some spec's per Bin number. This is a common way of sorting optical parts for matching purposes (if needed). If you have the actual component the Bin number (letter) might be printed on the part or the package. This could give you a closer idea of the actual tested specifications.

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  • \$\begingroup\$ Thank you, I hope that you'll help me, with the modification that I made \$\endgroup\$ – Ismail Mar 17 '15 at 19:11

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