Speed controller problem making no sense

[daliad100]

[attachment=0:390if79c]circuits.png[/attachment:390if79c]
Top left: 0 crossing detector
Top right: Inverted 0 crossing detector
Bottom left: Schmitt trigger (set R2 to 0 to set thresholds to 0v)
Bottom right: Inverted Schmitt trigger (ditto)

Both inverted circuits will do what you want them to do (from what I can tell, although I don't know why you'd want an inverted signal)

The Schmitt trigger ones will be better as they are far less susceptible to noise. You will get a little bit of circuit loading but can fix this with a normal voltage follower in the input.