phototransistor sensibility


I'm trying to use a phototransistor to "read" the light transmitted by some leds. It looks that the phototransistor is not sensible enough to read the light I'm emitting.

I built this circuit:

+5V----Resistor 18kohm----+------PhotoTransistor------GND | | Arduino

Question (i'm not an expert as you can see): how can I increase the sensibility? Increase the resistor? How much?


What phototransistor are you using?

Post a link


I tried with two. One is this
With this one I extracted the phototransistor (it is actually combined with an IR emitter) and I used a 10Kohm resistor.

Then I tried with a different one (for which I don’t have a technical reference, but it looks like a LED), with a couple of different resistor (10 and 18 Kohm), but I always got the same results…

If you have a particular mode to suggest…Thanks,

Decrease the resistance. Based on the datasheet, something a little bigger than 100ohm would be fine.

I would use a pot and vary it until I found the perfect resistance for the application.

As ritalinkid says decreasing the resistor in the LED line will give you more light to play with. However the resistor in the collector of the transistor should be increased to increase the sensitivity. You see the smaller the amount of current flowing through the transistor the less light it takes to switch it. Try using 100K or bigger. The down side is that the smaller the current the slower is the response but for human movement speeds that should not be a problem.


I actually tried a 120Kohm resistor. The situation has been improved, but it looks it is not enough. Should I increase the resistor again or should I buy more powerful leds?

Isn't this in the wrong part of the forum?

Well perhaps your phototransistor don't work in the same wavelength as your LED. You said you have taken a phototransistor from an IR receiver. This phototransistor probably work in the IR part of the spectrum if your LED is one working in visible part of the spectrum perhaps they don't match.

Are you using the phototransistor you first mentioned, or the photodiode you secondly mentioned? Their uses would be different. In your ascii-diagram, it looks like a standard hook-up for a photodiode.

This document may help:


Ah, light sensors…
I did a project for uni last year with this idea, we had to track infared targets and fire at them.

A couple of things.
firstly, how far away are these sensors? im guessing some distance.
in which case, that first sensor you were talking about:

These sensors have an effective range of approximately 6mm

made more for encoders and the like, not transmitting data.

Then, the other thing being an LED like device is what we used for our project, but note they are fairly directional and being even 10 degrees off directly looking at the transmitter, will significantly impact the response.
An oscilloscope was ESSENTIAL for us testing this stuff out.

Finally (its been said, but its important), you’re using a photo transistor to receive, but whats doing the sending? See, that receiver looks to be infared to me, but if youre just using some stock visible leds or something, you’re unlikely to get much love…
Youll need some IR leds to transmit too, which you can check are working by pointing your mobile phone camera at them and they should look white.

Oh, another bad thing about upping the sensitivity too much, sunlight and floruescent lights will effect the sensor and give you readings, so if you make it too sensitive, the signal will be half garbage anyway. (once again, scope was really useful…)


First of all thanks for all the replies.

What I'm trying to build is

It actually uses some LEDs a an IR phototransistor. May be I'm reading it in the wrong way....



No need to shout.

Sensitivity of LEDs to what?

Nothing is perfect.

Photo transistors have perfect sensitivity, one absorbed photon creates one electron hole pair.

Problem is you needs lots of electrons to generate a current that gets noticed above the thermal noise. The other thing is that the junction doing the absorbing of the photons is small. So to get more electrons then increase the junction size, but that is down to the device manufacturers.