speed of photodiode

Physical shielding has been relied on for many, many years. Without LASERs, even.

Put the light source and the receiver into two black tubes, glued so they are parallel. Point this assembly at a red bike reflector. A feature of those reflectors is that they bounce most of the light right back at the source. A lot of door announcers in stores work on this principle. A focused light bulb in one tube, an LDR or phototransistor, also with a lens, and a reflector on the other side of the doorway.

I've seen similar set up's used multiple times for 'sprint's' (single bike drag racing to get a time)
but don't see how it would work as a lap counter for multiple bikes?
Wouldn't RFID's be a better idea?

Depending on the type of the photodiode, load resistance can affect the
response speed. For maximum bandwidth, we recommend using a 50 ? coaxial
cable with a 50 ? terminating resistor at the opposite end of the
cable. This will minimize ringing by matching the cable with its
characteristic impedance. If bandwidth is not important, you may
increase the amount of voltage for a given light level by increasing RLOAD.
In an unmatched termination, the length of the coaxial cable can have a
profound impact on the response, so it is recommended to keep the cable
as short as possible.
Vout = Iout * Rload
Thanks :slight_smile: