alternative for a LDR

What is an alternative for a LDR?
I need a light sensor with less respond time, not too big.
Or are there many different LDRs? And what is the respond time of them?

A phototransistor will respond much more quickly than a cadmium sulfide photocell (Light Dependent Resistor).

LDR's can be replaced with phototransistors. The principle is different but the results are similar (plus, LDR's contain cadmium which is now a banned substance under RoHS directives so they are becoming increasingly difficult to find). You can see a sample phototransistor interface circuit on our Gadget Shield schematic on page 3 at the top-center. The more light that shines on phototransistor T17, the lower the voltage at the analog input A1.

The QuadRAM shield: add 512 kilobytes of external RAM to your Arduino Mega/Mega2560

IIRC LED’s can be used to ‘see’ light too, unfortunately no schematic or link :frowning:

I think i will buy a phototransistors, but there were many types of them.
He must at least have an angle of 120 degrees, and recognize light from a distance of 200 meter.

I did a survey... not very well informed, but if you are blind, I am a "one eyed king" who may be able to help...

If you are in the States, or willing to order from there, I think it is Digikey which has a site that lets you filter searches, to get the phototransistor YOU need....

Alternatively, Sparkfun does a good job of finding specific components which are "about right" for "most (hobbyist) people's needs".

thanks for you replay.
Is this one a good when I need a vieuwing angle of 120 degrees, a detect distance of 200m and a minumum respons time?

a detect distance of 200m

It's depends more on power of source , than on phototransistor. Actually on both, transmitter and receiver, but as power of signal decreasing proportionally square! of distance most likely your transmitter would be impossible to detect with any sensor. What is it?

200 meters is too far for a sensor.

One of the key data points you need to make decisions with is: optical wavelength. If you know the frequency of light you are wanting to "see", then you find a photo-transistor with sensitivity to that wavelength. Peak ranges normally go from 820 to 940 nm, so you want a light source to be close to the PEAK value for the phototransistor for best results.

The only way a sensor can achieve a range of over about a foot is to have an amplifier in it. The problem with that is that a large gain results in the output saturating under normal light level. Therefore the light is normally modulated (turned on an off rapidly). This allows the amplifier to be tuned to only amplify signals of that modulated frequency and ignore most of the ambient light. This is how TV remote controls work. However the sort of range you get with this technique is only about 5 meters.

If the light is a point source, like a laser then you can get a considerable increase in the range. However, you pay for this in the narrowness of angle of the beam. Indeed using a laser modulated light source and tuned amplifier you can get a range of 200 meters, but it has to be aligned perfectly. You can relax the alignment requirement a bit by having a sensor array like a video camera.

The remote controll thing sounds good, but i may be a littte difficult I guess.
Is the ligth transister i sent good for an angle of 120 degrees, and can it detect a light frequentie?

200 meter was a little exaggerated.

It says
Wavelength Typ: 935nm
Viewing Angle: 150°
So it is just over 120°
If you look here you will see it is in the IR portion of the band so it will not peak on visible light

The remote controll thing sounds good, but i may be a littte difficult I guess.

No you can get one in a single package, that is sensor and amplifier, again Infer Red