Bigfoot needs to know how fast the sewing machine is really running in order to do its job.  For that, it monitors the rotation speed of the sewing machine balance wheel (also called the handwheel.)  

Bigfoot uses a simple photointerrupter and a slotted disc to monitor the turning speed of the handwheel.

I bought some photointerrupters from Amazon that included the slotted discs.

There were some initial difficulties in using the modules.  It turned out that I needed to modify the circuit feeding the comparator on the modules to get a square wave signal from the tachometer.

As delivered, the modules produce an asymmetric  signal at low speed, and no signal at high speed.

Low speed:

If you purchase similar modules, you need to modify them as follows:

Remove C1 entirely and replace R1 with a 750 ohm part to get this much improved output:

On typical sewing machines, one turn of the wheel is one stitch.  The slotted disc has 20 slots, so there are 20 pulses per turn of the disc's shaft.

The handwheel is larger than the pickup wheel on the tachometer, so there are far more than 20 pulses per stitch.  Bigfoot has parameters you set before compiling so that it can calculate the correct number of stitches per minute from the length of the pulses.

Bigfoot works internally with stitches per minute.  It figures RPMs for the pickup wheel, then converts that to stitches per minute to compare with a setting calculated from the foot pedal - also in stitches per minute.

The tachometer is inside the little wooden box that rides the handwheel:

The insides of the tachometer look like this:

There's a bearing to the right there so that the axle runs smoothly.  The axle itself is a length of 8mm brass rod. The tachometer module is screwed to the bottom so that the slotted disc passes through the photointerrupter.  The wheel is an aluminum pulley with a rubber O-ring for traction.

The box is made out of three pieces of 19mm oak board.  I cut them on a bandsaw, then used a wood chisel to make the pocket for the bearing.

I've added some notes on the tachometer to the Bigfoot repository.