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How to automatically compensate (LDR?) for ambient light so as to eliminate the need for adjusting manually with potentiometer.

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If you don't want to see ambient light, what do you want to see? – stevenvh Aug 17 '11 at 13:16

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You don't explain what you want to detect with your phototransistor, but since it's not ambient light I'll presume that you want to detect some kind of data signal.

For remote control signal reception IR is used so you can use a photodetector which is less sensitive to visual light.

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IR receiver modules also have a bandpass filter around a modulation frequency, often 36 or 38kHz,

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so that the modulator again reduces sensitivity for ambient light, which is often constant, in case of daylight, or quasi constant for incandescent bulbs.
Fluorescent lamps can remain a problem, especially HF (high frequency) lamps which are switched at about 40kHz, right in the receiver's pass band. Relatively speaking they don't emit much IR, but they do emit a lot of energy, so that the IR content may be high after all.

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A third measure is a built-in AGC (Automatic Gain Control), which adjusts the receiver's sensitivity based upon the received signal strength. This can often be seen on an oscilloscope image: while there's no signal received the module sees all kind of noise, but as soon as the right signal is detected this noise is suppressed, and only the actual signal remains.

More:
Vishay's offering of IR receiver modules.

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@stevenh what is an HF lamp? Is that mean high frequency? – Frank Aug 18 '11 at 3:20
@Frank - Yes, high frequency. Something like 40kHz instead of 50/60Hz. More efficient I presume. – stevenvh Aug 18 '11 at 5:26
@stevenh is there an analog method to filter out the sun other than daylight filter? – Frank Aug 18 '11 at 8:15
@Frank - Next to impossible. Sunlight, like diffuse daylight is constant, but so much stronger that it drowns the other daylight level. You'll have to make sure that the sensor doesn't see direct sunlight. Apart from the intensity sunlight also has a high level of IR, which ambient light doesn't have. That can be blocked by (surprise!) an IR filter, but it doesn't change your visual light detection. – stevenvh Aug 19 '11 at 12:55
up vote 3 down vote

If you have some control over the transmitter, a very useful technique can be to modulate or switch it, and then filter the output to reject anything not at the modulation frequency or not synchronized with the switching of the source.

This can take various forms, from passive or op-amp type solutions to complicated DSP ones, to simple synchronous ideas such as "subtract the received level when the source is off from the received level when it is on"

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how do you deal with dynamic range even in modulation? If the signal is weak you are dealing with nAmps signal against uAamp Sun's bias. First phase of your receiver could easily saturate. – Frank Aug 22 '11 at 0:17
@Frank - obviously, you can only deal with it in the range where it doesn't saturate. As for how to deal with dynamic range and modulation... that's pretty much the core challenge in designing a radio - good filters can pull a tiny narrowband or correlated signal out of overwhelming average noise power. – Chris Stratton Aug 22 '11 at 0:32

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