# Phototransistor common emitter amp

Discussion in 'The Projects Forum' started by tadgoff, Mar 16, 2011.

Sep 27, 2010
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I am working on a heart rate monitor project, and I need some help with the light sensing circuit. Actually I have two questions. Basically I want to use a simple common emitter amplifier like this one. But I'm not sure I am understanding it correctly.

Question 1:
Which direction does current flow? It would make sense to me that the current should flow from Vcc to Gnd via Rc and the phototransistor, but I thought current in a transistor goes in the direction opposite to the arrow in the symbol, right? Are the rules just different for phototransistors, or am I missing something?
If the current does go from collector to emitter (Vcc to Gnd/same direction as arrow) I can see that Ic is the current through the resistor and therefore Vo=Vcc-(Ic*Rc). And Ic is directly proportional to the amount of light, so this is exactly what I want...

Question 2:
What if I want to expand the range of brightnesses this can detect? My professor warned that factors like skin color could change Ic by a factor of ten. He also mentioned a logarithmic version of the first amplifier where the resistor is replaced by a diode. I don't have much experience with diodes, and there's still the issue of current direction from before, so could someone please try to explain it to me? Do I just set Ic=Id and solve the diode equation for Vd? Is this circuit really all I need?