Obtaining constant voltage from a spike?

hi friends,
Is there anyway to obtain a constant vtg from a spike...
ie, using an ir sensor we are detecting the movement in a room. the ir is allowed to fall on base of a photo transistr.. whenever movement is there the ciruit breaks...now the application is - when there is no one in the room, the circuit works and via a 555, wid a delay we can obtain the spike...but if this spike can be converted to a constant vtg, it can fed to light circuit connected via a relay which can turn off...