A smartphone strobe tachometer / timing light.
Suppose you have a rotating or oscillating piece of equipment (eg. a spinning shaft, vibrating rod), and you want to know how fast it is moving, this app can help you. If the strobe light that flashes at the same speed as the vibration or rotation, the item will appear to ‘freeze’ under the strobe light. You know the frequency of the flashing light, so you can tell the frequency of the vibration or the speed of the shaft.
To Use
Dim the ambient lights until the strobe is the dominant light source. Shine the light at the item spinning / vibrating and adjust the frequency until the item appears to be still. The frequency of the light is the same as that of the object and you can read off the frequency (HZ) or speed (RPM). Start high and work your way down.
Caution
If your light frequency is 1/2 or 1/3 or 1/4 x or any exact proportion of the shaft speed, the shaft will also appear stationary. This is called a harmonic. To ensure you don’t have a harmonic, double the frequency and check again. The highest frequency with effectively stops the motion of the shaft is the true frequency.
Health Caution
Flashing lights occasionally cause epilepsy or other troubles in some people. If you feel unwell using this app then stop. This app is a tool, not a toy, please be careful.
Here is a good video about how to use this tool: http://www.youtube.com/watch?v=MqLwgisyjjw