Originally Posted by ahmedragia21

Hi, i wanna know how to calculate the collector resistance of the phototransistor to put to work in active mode for using it in line follower robots ? i got one and here's the datasheet http://www.ram.com.eg/RTE/my_documen...s/PT204_6C.pdf
i tried to put the usual 10k in series but it didnt work ..
thanks

hi,
The PT is a wide spectral range device, that is its sensitive to white light thru near infra red.
Any ambient white light will make it conduct.

If you look at the graph on the datasheet you will see that for a +5V supply you will have a about 2mA flowing thru the PT
and the 10K0, when you have a low to moderate light intensity.

Using Ohms law Vdp across a 10K0 at 2mA is 20Volts!!!, so unless you are using a PT suppy greater than say, 24V,
the Vout from the PT will approx 0V.

If you are using say a +5v supply to the PT and you want the collector/10K0 junction to be around say, +2.5V,
then you require a resistor of about 1K2.

When you do your tests, shield the PT from the ambient light.

Does this help?

Originally Posted by ahmedragia21

yea thanks alot ,
the problem is im using it for a line follower robot on white strip and the background is blue , i tried to use an ultrabright white led , i experimented with a 1k.5 ohm , the swing from white to blue is just few mV , and it wont work when using a compartor .. i wanna increase the difference .

Measure the DC voltage on the collector/resistor junction, when its setup in the line following condition.
[in the actual postion and state on the line, with its covers on,not on the test bench]
you may have to extend the 0V and collector wires to the meter, in order to do this test.

Let me know, the supply voltage and the DC collector voltage?.

Originally Posted by ahmedragia21

thanks for reply:
Red Led
VCC 5V -> Rc 1K -- > Vout 4.50v with no touching , Vout 4.33v when touching ( Blue surace) . the same results with few mV wit 2k and 10k .
with 80K , vout is 0.45 with no touching , with touchin 1.41v

hi,
I see that you are using a RED led light source and the PT is designed for white light.
I know that visible RED light forms part of the visible 'white light' spectrum but the two devices are not well matched, spectrum wise.

As you have a white line on a blue background [with a white light detector] why don't you try a WHITE led light source as a test?

If this gives the signal you want, you could vary the sensitivity of the system by using different values for the WHITE led driver resistor.

Originally Posted by ahmedragia21

the white led worked very good but with only with 80K resistor of the PT , why ?

As I cannot see what you have, I would guess that the reflected light is very small and it gives a weak current in the PT.

So that a high 80K0 resistor is needed to detected the small change from blue to white.

If it works OK are you going to give it full test??