Timers in GH

Hi,

We're using grasshopper and firefly and we have a surface that starts in a negative (concave) position and every time something passes by the motion sensor the surface becomes more positive. Also we want the surface to begin moving back to negative after a certain period time that nothing passes by the motion sensor.  This way the surface is constantly moving back and forth.

We are trying to figure out how to use the timers to do this?  Any suggestions??

Thanks!