Reliable Filament Runout Sensor

[ Edit Feb 27 2018: See also the recent (and still more thorough)
"Even More Reliable Filament Runout Sensor and Spool Holder" --
https://www.thingiverse.com/thing:2809260 ]

Here is my design for a more reliable filament runout sensor, designed for mounting on and below a 2020 bar. I'm using it on a Folgertech FT-5, with the filament spool above it, but it should be usable with other printers with similar bars for mounting.

**Background**
I've been trying to install a reliable sensor for some time (tried like about six different designs including several variations of some of those!) but always seemed to run into problems. My issues might be from the type of printing I've been doing -- large objects printed with a wide nozzle so that filament gets used quickly (which is why there is urgent need for a sensor!) and the filament usually moves through the system quickly causing vibration and significant tension changes.

Sensors that depend on filament directly pushing a m