This is a mini strobe light. NE555 is used as a self oscillator. The width of output pulse is changed by the variable resister. 25msec means 40Hz. 6m sec means 166Hz. A LED is connected on the output of this oscillator. Therefore LED twinkles from 40Hz to 166Hz. I lighted a speaker by this twinkle LED. The cone of a speaker is vibrated by 50Hz with 3V-AC power transformer. I adjusted the VR to oscillate 51Hz. Then, I can watch the corn of the speaker vibrates slowly with 1Hz. A strobe scope is used in order to research the vibration.
This is very cheap one. If you are the teacher, you can teach the children that the sound is a vibration of the air. Also you can teach that the concept of phase and frequency with this equipment with this machine. If you are the young man, you can have a time with your girl friend in the dark room.( Last month I looked this light with 5 gentlemen. It was not enjoyable time HIHI) Description add on 10 Dec 2000: I forget to say about most important thing. Such a instrument can be uaed to evaluate the mechanical stability of the parts on the print boad.