Step 3: Rig the Reciever
Phototransistors work very similarly to microphones in that they vary the current they allow to pass through them when exposed to chaning light levels. So, treat the phototransistor like a mic, and attach it to the MIC port on a laptop, or to the MIC terminals of a preamp, then hook the output of the preamp to the input of an amplifier.
If you are using a preamp, you should be able to screw the leads of the phototransistor right onto the back of the device, but if you are using a laptop, you will need to tape or solder togeather the pins of the phototransistor to the wires of a stripped headphones chord. Remember the ground (-) lead of the phototransistor is the one that has a little flat spot on the plasitc. Look to the photos for help.