working with a phototransistor

[ahmedragia21]

Hi, i bought this photo transistor and connected it as showen in this schematic and it doesnt really work , i used an ultrabright led to shine light on a surface (black and white ) and measure the analog value , no voltage swings between white and black just few mV between both colors.

I tried 1k , 10k , 100k resistors and doesnt swing and 180K resistor it swings but with +- 2V between white and black ...

i dont know how to choose the value of this resistor ?

https://www.electro-tech-online.com/custompdfs/2007/12/PT204_6C.pdf

here's the datasheet

 

[Pommie]

I think that is a IR (infra red) photo transistor. I can't see the data sheet at the moment.

Mike.

 

[ahmedragia21]

cant work with red light spectrum ?

 

[dknguyen]

Have you tried using a regular light bulb (with more IR than a visible LED) and a black/white material that you re sure is not transparent to IR?

 

[ahmedragia21]

yea, i tried that , but what's a good value of R to really work on that and do most experiments ?

 

[Pommie]

I'd start with 10K and try pointing a TV remote at it.

Mike.

 

[audioguru]

Its response peaks at infrared and is half the sensitivity with violet light at the other end of the visible spectrum. Red is at 80% to 90%.

It is sensitive to direct light. With reflected light it needs a very bright light, a very close and reflective surface and a very high value (1M?) for its collector resistor. An amolifier might be needed.

 

[ahmedragia21]

thanks , but from where did you deduce that a 1M can work ? from which part from the datasheet ?

 

[audioguru]

thanks , but from where did you deduce that a 1M can work ? from which part from the datasheet ?

The datasheet says its collector current is from 0.7mA to 5.07mA when a very bright light shines on it.

You said the transistor did not have an output with a 10k or 100k collector resistor. It is 10 times more sensistive with a 1M collector resistor.
The 1M includes the load you have on it. What resistance is the load?

 

[ahmedragia21]

i connect it to an LM324 compartor to compare its output voltage to a reference (( used in line followe robot to detect the color ( white or green) ).
i will try with 1M and ultra brigh red led .

thanks

 

[crashsite]

You also might try putting some bias on the transistor base to kind of get a little collector current flowing before you apply the light and see if that gives a better resuld.

 

[audioguru]

Google has hundreds of links to line follower robot circuits like yours. They use a 10k collector resistor on the photo-transistor and have enough sensitivity.

Maybe yours doesn't work because something is connected wrong?
Check the LED current and the pins on the photo-transistor.

 

[crys]

try connecting the collector to the black end (negative) of a multitester and the red end (positive) to the emitter.

try covering the phototrans
it will show a very high resistance

try applying light to the phototrans
it will show a very small resistance

 

[ahmedragia21]

crys, and hence what could be the resistance value ?

 

[audioguru]

I notice that the datasheet for the Taiwan photo-transistor does not say how tight is its focus. Maybe it has a very narrow window to accept light.
Then its numbers will look like it is very sensitive when it is just focussed instead.

 

[cdstahl]

possibly ambient light is saturating the device and your added light isn't doing much?