Stroboscopic torsion pendulum

We report on a simple, inexpensive readout for torsion pendulums, suitable for robust applications like teaching-lab equipment or monitoring of large ampl- itude oscillations. A short light pulse is recorded every time a reflective band on the pendulum inertial member passes in front of the sensor, an infrared LED pair (emitter–receiver). Simple algebraic manipulations on the time series of these pulses arrival times allow one to extract the resonant frequency as well as the decay time of the pendulum. This readout is insensitive to the amplitude of oscillation and is therefore suitable for monitoring torsional oscillations of large amplitude, where traditonal readouts like optical levers or auto- collimators encounter dynamic range limitations.