A movement detecting device having a light transmitter 44 and a spaced light receiver 46, with a moving plate 32 containing slit apertures therein being positioned between the transmitter and receiver for interrupting the light path between the transmitter and the receiver in dependence upon the motion of the moving plate. So as to improve the clarity of light detection, stationary apertured plates 56U, 56D are unitarily formed and are positioned on respective sides of the moving plate and adjacent a respective one of the transmitter or receiver. The stationary apertured plates 56U, 56D may be formed from a single sheet of material, or from two separate sheets connectingly joined by a flanged pin, or a combination thereof. The unitarily formed stationary apertured plates 56U, 56D may be directly moulded into a support housing with locating holes 64, 66 for the light transmitter 44 and light receiver 46, or may be formed in a moulded sub assembly prior to moulding into a support housing.