Note the prototype has developed instability perhaps caused by dust collection.

Do not attempt to reproduce this device if you are not able to solve any problems with its operation.

AC fans are popular because of their quiet operation and long life however they do not come with a rotation signal that can be monitored by a computer. The following circuit should do the job. A prototype has been made and works but component differences may cause problems. Only build this if you are prepared to solve any problems that do show up.

The layout below can transfered to 1/10 inch graph paper then the locations for holes transfered to copper laminate board with a punch or scribe.

The mounting hole is drilled and the tracks drawn with an etch resistant pen. I use tar thinned with turps and a mapping pen.

The board is then etched in ferric chloride solution, the holes made with .8 and 1mm drills as necessary

and the components soldered on. MAKE SURE THAT THE SOCKET IS ORIENTED AS SHOWN IN THE PICTURE. Angle the light source slightly twards the photo transistor as shown.

The resistor in the lead is given some support with a couple of knots and heatshrink tubing. Make sure the lead you make connects to the correct motherboard pins. When correctly made it should not matter which end is which.

Just as an exercise I made a rough user manual that should help to explain things.