Phototransistor bandwidth issue

Are you using the 1K resistor in series with the IR LED? Because based on the datasheet from that page that won't be enough current to really drive it. That will give you about 3.5mA and you need more like 100 mA. So try using a 100 Ohm resistor there instead.

Another cool trick is if you have a camera phone, old webcam or digital camera a lot of times if you point them at your IR LED you'll be able to see it turn on, on screen. Try it yourself by pointing your remote control at your phone and hitting some buttons. Also that might be a good way to see if your detector is outputting anything too. Point it at your IR detector and hold down the volume button and see if you get anything on the Audrino (or even better a scope if you have one).

The wavelength stays more or less the same, but more current = more light or in your case any light at all.

Red might work but I suspect it might be that your LED isn't on.

Good luck!

First, you do not identify what phototransistor you're using, so there is no way anyone can answer your question about collector identification. "flat side" almost certainly identifies the photosensistive face, not collector or emitter.

It doesn't help that you haven't provided a number for the ouput voltage, either. "Low" could mean 1 volt, or .5 volt, or .1 volt.

That said, you need to do the following: put your circuit (without the LED) in a completely light-tight enclosure and see what the output is. Most likely you're picking up stray IR.

If you're still having problems, go back to the data sheet for your phototransistor and look at the dark current spec. If you're using a bc847b, your DC gain could be as high as 450. Assuming "low" means 0.5 volts, that's a collector current of about .5 mA, and a dark current of 1 uA will provide enough base current to do that.