Thread: Adjusting Sensitivity of Photo Transistor?

I have been using photo transistors lately as part of a laser beam detector device. The problem is that my transistor is not sensitive enough. The laser has to be directly into the transistor for it to turn on. In the datasheet, it says that I can use the base leg (that is currently unused) to adjust the sensitivity of it. I don't understand how I would do this. In the past I would always just connect a resistor to either P or N and the other end of the resistor to the base of the transistor to turn it on or off. I never did any half-way stuff.

Thanks!

Actually I am already amplifying the output with a transistor. Perhaps 2? Or perhaps I just have an unsteady hand. I need to do a test with everything set up, not me trying to keep my hand steady.

And I forgot to post this earlier, but here is the datasheet of my photo transistor.

http://www.jameco.com/wcsstore/Jamec...dDS/120221.PDF

Thanks!

Thanks for the help, I didn't really understand that stuff until now. I think my biggest problem is the angle part. I am too shaky to keep it in the right spot. After I get everything installed, it should be alright.

Thanks

Originally Posted by audioguru

The base of a phototransistor is used to decrease its sensitivity.

That doesnt sound right to me..
because ,say that you set the bias current to some fixed level below the actual turn on point ..
then adding light will push it into the active region .
i dont see how you could possibly bias it to make it less sensetive.. :lol:

Infrared peaks at 850nm which is where adamthole's detector is ,
adamthole did not say what wavelength he was using..

This is how it works ..
the collector current increases as the light intensity increases , as it should ..
and THE BASE CURRENT is proportional to the the light intensity

Hi Willi,
A phototransistor has a "dark current" which is its leakage when dark. So it is already conducting and doesn't have a threshold. Adding some base current with a biasing resistor to the positive supply will only increase its idle current without increasing its sensitivity to light.
Using a resistor from its base to emitter will decrease its sensitivity because more light is required to create a voltage drop across the resistor, causing a threshold before the phototransistor begins to conduct.

Originally Posted by chemelec

I would suggest you get a Better Photo-Transistor for the Visable Red Laser, Such as a 2N5777.

Gary

Where would I get one of those, I checked my 3 main suppliers (Mouser, Digi-Key, and Jameco) and none of them had it, or atleast I didn't find it. When I did a search on google for it I just got all of those sites that list as many part numbers as possible, and are basically useless. Thanks for the info though. When I bought the phototransistor I am using from jameco I thought it was best for visable light. I'll have to memorize the color wavelengths.

Thanks!

You could use an LDR, they tend to be easier to use with a laser pointer.