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Adjust the Photo transistor sensitivity

Thread starter #1
I am using IR photo transistor as sensor and IR LED for light source. IR LED with wavelength of 940nm and used 20 ohms resistor to control the intensity of light .
Photo transistor of 940 nm wave length of peak sensitivity and here 22K ohms resistor is used. any object in front of Photo transistor should detect ....this is the basic functionality.
in the normal case it is working fine, but if i masked the IR LED by masking tape, it is stopping detection of object...
i thought may be light intensity is not reaching to Pt's.
i changed the IR LED with more intensity, but then also Pt's are not working.
if more intensity also it is not working and if it is less intensity also it is not working.... how to handle this???
is there sensitivity factor is affecting on it???
how to handle this ???? please help me to resolve the problem....


Well-Known Member
Most Helpful Member
Show us a schematic of what you have.

Why are covering the LED with masking tape and expecting it to still work?


Active Member
Trying to use a simple light level detect for something like that often gives problems.

The better method is to use modulated light, eg. switch the LED at a moderately high frequency such a 1KHz or 10Khz, then capacitively couple the signal from the detector via simple high-pass filter from the detector & amplify that.

It eliminates ambient light levels from the system and gives better operation over a wider signal range.

Also look at active photodetectors for the input stage rather than just a simple bias resistor, eg.
Thread starter #7
thanks for reply... by using IR pass filter on LED side is it possible to make LED light to fall on Phototransistor?? if LED is covered by masking tape??
or am I need to look into sensitivity of phototransistor???


Well-Known Member
Most Helpful Member
Is your IR light source on all the time or pulsing. If pulsing, what frequency?

What voltage do you have available at your phototransistor? What do you want the output to look like? 5v when IR is sensed?, 0v, 10v?


Well-Known Member
Is the "masking tape test" an Infrared industry accepted equivalent for a dusty IR receiver?

I'm asking because I don't know. But my gut feeling is masking tape could well have a significantly higher attenuation factor to IR than a much thicker layer of dust.


Well-Known Member
Most Helpful Member
A quick test to see if your IR LED is lit up is to look at it through your phone camera. Phone cameras see IR as bright light. It will also tell you how much your masking tape attenuates the signal. I suspect too much.

Thread starter #11
What are the parameters I need to consider for selection of phototransistor which should give better result even though the intensity is reduced from transmitter side...

please help me to choose best phototransistor , it should work in dust environment also.....

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