In this video i take a quick look at a mechanical timer from the Grass Instruments Polygraph chart recorder.
We believe this opto-mechanical device is used to provide a two level pulse to put markings on the chart paper as it records.
The wheel is driven by a synchronous motor, it rotates at 12 RPM meaning the notches pass over the optical sensor every second with the bigger notch every complete rotation of 5 seconds.
The larger notch allows more light into the sensor so provides a bigger output and a bigger mark on the paper.
