I am trying to prototype with a phototransistor, BPV11(http://www.vishay.com/docs/81504/81504.pdf ). I have it connected as a common emitter setup with 5v to the collector and the emitter to ground through a resistor. I am trying to read a voltage proportional to the received light so the resistor has to be a lower value to set it in active mode. I have used about every resistor value between 500 ohms and 100 ohms.

My problem is that as soon as I apply power to the circuit, the oscope shows the voltage jump to 4+ volts immediately even inside of a dark box. There is no voltage change no matter if light is introduced or if it is in a dark environment.

I attached a jpeg image with a quick shot of the circuit, it is very basic.

 

http://www.fairchildsemi.com/an/AN/AN-3005.pdf

It might help if you read it.

 

yeah I have read that, it is where I got most of my information from.

 

Try looking more closely at the suggested resistor value.

 

Your circuit is common-collector that has no voltage gain so it is not sensitive. (You said it is common-emitter but it is not).
Use common-emitter if you want voltage gain then it will be sensitive.

Since your photo-transistor turns on hard without IR light then either it is connected wrong or it is shorted.

 

well darn is suppose it is common collector haha, i was reading from the sheet wrong...oops

The circuit is simple, I dont think anything is connected wrong unless I have place the BPV11 in backwards but I am sure it is connected right

 

Okay I switched the circuit around so it is in the real common emitter setup.

The voltage output still does not change, although the voltage level is lower at ~820mV

 

I have not found a datasheet that specifies which pins are assigned to 1, 2, and 3. Pin 2 is most probably the base. as to what pin is the collector and emmiter is not known. flipping the transistor around in the common emmiter configuration might help. You can even go as low as 100 ohms on the resistor.

 

the first link is the datasheet for the phototransistor and on page 4 is specifies the emitter as the flat side

 

I tell you what guys, I flipped the transistor around, opposite of what I believe the datasheet was showing and now when I apply light from a IR led the voltage drops from high to low and when I place a piece of paper in between the voltage jumps back up.

I had a similar issue with a datasheet for a laser diode, it seemed like their anode/cathode image was flipped but that is a separate issue lol

 

oh and I upped the resistor to 100k

 

So there's the datasheet. And the center pin is not the base, but rather the collector.
Are you sure you are baising the Base(Pin opposite Flat side) and not the collector(center pin)??

Glad something's happening.

 

oh man i feel like an idiot, i must have dyslexia at times, i swore that the base was the center pin

*hits head

lol