The standard Polifemo photocells can be used in a "double" configuration for Track Sprint applications. The objective is to only get a pulse from the torso of the runner, not the hand.

Attach the Polifemo photocells so that they are stacked as described in the picture.

http://www.microgate.it/eng/ti...ell_polifemo_two.asp

You want the beams about 8 inches apart.

Attach a jumper from the black (ground) output of the top cell to the green (signal) output of the bottom cell. Attach the signal line to the timer to the green (signal) output of the top cell. Attach the ground line from the timer to the black (ground) output of the bottom cell.

You now have a set of cells in series.

Both beams have to be broken to send a pulse to the timer.

The Polifemo photocell has optocoupled output so there will be no feedback issues.

Both cells have to remain aligned for this to work.