Electronic Circuits and Projects Forum

24th June 2005 03:20 AM

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!

24th June 2005 04:35 AM

The base of a phototransistor is used to decrease its sensitivity.
A laser beam is very directional so of course must be shining directly on the phototransistor.

You could amplify the output of the phototransistor with an ordinary transistor. Then you might have sensitivity to temperature changes or ambient light.

24th June 2005 05:22 AM

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!

24th June 2005 11:46 AM

Hi Adam,
Your phototransistor has a focussing lens in it. Therefore the laser must be directly in front of it for max sensitivity. If it is off to the side only 10 degrees, then the output is half. Maybe you could shine a laser beam on it with it directly in front of you, but far away, for target practice or something. :lol:

The phototransistor is most sensitive to infrared. At the red wavelength of a laser its output is almost only half.

24th June 2005 01:07 PM

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

Gary

24th June 2005 01:10 PM

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

24th June 2005 01:40 PM

I think your Biggest Problem is the Wrong Photo Transistor.
The one your Using is best for UV Light.

Thats like using a Skate Board instead of a car to go get your weekly groceries.

24th June 2005 02:45 PM

Originally Posted by chemelec

I think your Biggest Problem is the Wrong Photo Transistor.
The one your Using is best for UV Light.

Sorry Gary, no. This phototransistor responds best to light with a long wavelength, like infrared. Ultraviolet has a short wavelength. Visible light is in between. :lol:

24th June 2005 03:28 PM

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:

24th June 2005 04:04 PM

audioguru: Sorry Gary, no. This phototransistor responds best to light with a long wavelength, like infrared. Ultraviolet has a short wavelength. Visible light is in between.

I Thought this guy was using a "Visible RED Laser Light"?

If So, it appears Off of the Sensitivity Region. According to that Graph that was also Posted by someone else????.

Correct me if I'm Wrong!

Gary

Electronic Circuits and Projects Forum

Adjusting Sensitivity of Photo Transistor?

Adjusting Sensitivity of Photo Transistor?