• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Choosing PhotoTransistor

Thread starter #1
Hi everyone
in my last project i need to detect 670 nm laser (P~mW) with phototransistor for laserstop system
what kind of phototransistor recommended for my work??
 

gophert

Well-Known Member
Most Helpful Member
#2
Hi everyone
in my last project i need to detect 670 nm laser (P~mW) with phototransistor for laserstop system
what kind of phototransistor recommended for my work??
Is it constant wave or are you pulsing the laser?
If pulsing, what frequency?

Is the sensor open to interference to room light? Outdoor light?

What are you "stopping" - is it just mechanical movement or are you protecting a human (I.e. is it a safety-related device?).
 
Thread starter #3
Is it constant wave or are you pulsing the laser?
If pulsing, what frequency?

Is the sensor open to interference to room light? Outdoor light?

What are you "stopping" - is it just mechanical movement or are you protecting a human (I.e. is it a safety-related device?).
I try to use continuous laser (670 nm and power about mW)
this laser stop will work in Textile Industry and should stop if a very thin yarn pass front of laser
 

gophert

Well-Known Member
Most Helpful Member
#5
I try to use continuous laser (670 nm and power about mW)
this laser stop will work in Textile Industry and should stop if a very thin yarn pass front of laser
Most natural color (unpigmented) polymer fibers do not absorb far red or even near-IR wavelengths. Even if it does absorb the laser frequencies, the thin fiber will not cause much of a shadow to the detector unless your detector has a very good way focus the beam. However, too much focus and a 50um diameter fiber could be damaged by the laser. I'm hoping your yarn is dozens or more 50um fibers.

Most photo-Transistors are black dyed plastic to cut off visible wavelengths (below 750nm) so make sure you get one that is clear with no cut-off filter dyed into the plastic.

Alternatively, instead of a phototransistor, a simple red LED (preferably clear lens) can be used to detect light as well. Depending on your laser power, a 10k to 10M ohm (connected to ground) and LED in series to 5V supply will allow you to detect the laser power (or lack of when the fiber is masking the laser beam).
 
Thread starter #9
i want to detect a row of yarn by laser pass, that by pass every yarn in front of laser, phototransistor send data and cause to turn off system, my problem is to find proper phtotransistor, Absorption spectra show that my yarn has maximum absorption in 670-700 nm
 

dknguyen

Well-Known Member
Most Helpful Member
#10
i want to detect a row of yarn by laser pass, that by pass every yarn in front of laser, phototransistor send data and cause to turn off system, my problem is to find proper phtotransistor, Absorption spectra show that my yarn has maximum absorption in 670-700 nm
What's the problem? 670-700 nm is easily covered. Do you actually need something more selective? You could use a pair to reject ambient lighting/common mode light.
 
Last edited:
Thread starter #12
Red? so use a red LED for the light source.
I really don't think the color is going to make much difference. Your yarn is not clear at 650nm.

I think "very thin" is more of a problem than color.
What is very thin?
the diameter of yarn is about 0.1 mm and 0.01 mm range
and about wavelength of laser
I know that the maximum absorption is not absolutely 650 nm. but the best wavelength is about 650~700 nm
 
Thread starter #13
What's the problem? 670-700 nm is easily covered. Do you actually need something more selective? You could use a pair to reject ambient lighting/common mode light.
actually i have to use laser because of long pass of light (about 2 meter) the phototransistor constantly send a voltage to system, if a yarn pass from laser, the intensity of laser decrease and i can detect it
 

dknguyen

Well-Known Member
Most Helpful Member
#14
actually i have to use laser because of long pass of light (about 2 meter) the phototransistor constantly send a voltage to system, if a yarn pass from laser, the intensity of laser decrease and i can detect it
You might want to use a photodidoe with an transimpedance amplifier and comparator in that case so then you can adjust the switching threshold instead of a phototransistor where you have to find one with the right threshold.
 
Thread starter #15
You might want to use a photodidoe with an transimpedance amplifier and comparator in that case so then you can adjust the switching threshold instead of a phototransistor where you have to find one with the right threshold.
i check it, thank's
but in the patent of one company that produce this system they use phototransistor and because of this i insist of phototransistor, but if can work with photodiode and other thing more easier i do it
 

dknguyen

Well-Known Member
Most Helpful Member
#16
i check it, thank's
but in the patent of one company that produce this system they use phototransistor and because of this i insist of phototransistor, but if can work with photodiode and other thing more easier i do it
Well a photo-transistor is easier to use than a photodiode but that doesn't matter much if you can't find a phototransistor with the exact threshold you need. You have a lot more control and flexibility with a photodiode. A photodidoe is also much faster, but that probably doesn't matter for your application.

http://www.ti.com/lit/pdf/tidu535
 

Latest threads

EE World Online Articles

Loading

 
Top