I designed this as a substitute for a non-contact laser tachometer.
It uses an IR emitter/detector pair mounted in a hand held wand.
The resulting signal is sent to the computer's microphone input and the waveform displayed with a sound card oscilloscope.
Step 1: The Circuit
The circuit is a simple voltage source for the emitter and detector.
Input voltage can be anywhere between 7 and 35vdc. (See the 78L05 datasheet.)
R1 limits the current to the emitter.
R2 & R3 are pull-up resistors.
T2 is added as an output driver.
R4 is used to limit the current and voltage to the sound card. Anything over one or two volts can damage the card. A 2.2 Megohm resistor gives an attenuation of around 25:1.
Schematic created using Eagle 6.4.0