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PhotoTransistor Amplification

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kad_911

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hello,
I have a small circuit which has a phototransistor which sends a output voltage when detecting InfraRed light. However, the infrared light coming to the phototransistor is very weak and i only get >1V as its output...

is there anyway to amplify that voltage to atleast 3V ?

thank you..
 

do you require analog or digital level?

by analog , mean , any signal is transmitted thro infrared.

srizbf
26thjune2010
 

No data is transferred. Just an IR beam is received by the photo transistor. the IR comes reflected from a white surface. thats what makes it to become weak.
 

First measure what logic you get as an output. HI or low, then you may decide wether to use NPN or PNP general purpose transistors to amplify the output. Any ways, in case of confusion, posting your present schematic will fetch better answers. Cheers
 

This is not the whole schematic.. but this is basically what im taking about..

6_1277566444.jpg


the rays from the IR diode isn't enough to bias the Q1 transistor...
 

many suggestions are possible .
the simplest one is change Q1 to pnp type.
now monitor the output.

srizbf
26thjune2010
 

wat else?

the problem is that im not getting any voltage from the photo transistor..
isnt there a way to increase the output of that?
 

what is the voltage in collector of photo transistor in quiescent state?
(12v? measure it)
so when light falls on , the collector goes to 1v . right.

srizbf
26thjune2010

Added after 14 minutes:

did you make Q1 as pnp ?

and connect a resistor between (phototransistor collector + R1) junction to base of Q1.

and check the voltage .

srizbf

26thjune2010
 

nothing happens to the photo transistor when light falls! because i dont get enough light!!

the phototransistor and IR diode are like this. and they're very close to the ground (which is white)
67_1277620745.jpg
 

please test without any additional transistor Q1.
just have irdiode and phototransistor alone with R1 and supply.

measure the voltage at collector of phototr when light falls and when no irlight falls.

what is the photo transistor type you have chosen?
its type no? is ir sensitive?

what is the value of R1?
do you want 12v only or if it is a 5v supply is it enough?

srizbf
27thjune2010
 

thanx for the replies,

the phototransistor is a L14G2.
and R1 is 100K

the i get a VBE (without VCC) when the light falls directly to the phototransisror as =~2 Volts.

but when i try to reflect it, i dont get anything.
and yes a 5V is enough..
 

Version1:

IRDiode:

+5v--->330 ohm --->irdiode anode---catode---->Gnd.

phototransistor:

+5v---->R1(15k or22k) -->phototr_collector---phototr-emitter --->Gnd.


measure the voltage .


srizbf
27thjune2010

connect and test.
 

    K

    kad_911

    Points: 2
    Helpful Answer Positive Rating
hi changed as you said. but still nothing. that transistor just doesnt detect the reflected IR!!
 

first test the circuit by directly pointing ir LED on phototransistor.

the distance should be close possible.

then test for 'reflected' light .

what is the object meant for reflecting ?

srizbf
27thjune2010
 

when i point the IR LED directly, Vbe becomes 2 ~ 3 V.
nothing when reflecting.

im using a white glossy cardboard floor to reflect.
 

measure Vce (not Vbe).
two readings of ,at collector to ground voltage.
i) when no light falls on
ii) when ir light falls on(no reflected . only direct)

have you connected collector and emitter of phototransistor properly ?
do not interchange the terminals(an usual mistake)

check whether 'white glossy board ' is reflecting or absorbing types for infrareds.
(may be from data sheet)

srizbf
27thjune2010

srizbf
27thjune2010
 

Vce is =~ 5V when IR is off.
it becomes 0 when light falls in.
its working as it should!

and yes the surface reflects.
 

so now you have what you want .

srizbf
27thjune2010
 

no........... it only works when its directly aimed at the photo transistor. Nothing happens when it reflected.
 

may be your angle of led and photo transistor is not in proper value.

point LED and photo tr as shown
to reflect properly , try

LED PHOTOTR
\ /
\ /
\ /
\ /
'theta'

where theta is approximately 30 to 35 degrees.


srizbf
27thjune2010
 

    K

    kad_911

    Points: 2
    Helpful Answer Positive Rating
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