Abstract: An adjustable encoder includes a source generating a first signal and transmitting it to a transducer element for generating an electrical signal responsive to the first signal. An object is rotated between the source and the transducer element so as alternately to block and to permit or unblock the path of the first signal from its source to the transducer element. The transducer element and the source are simultaneously movable parallel to the plane of rotation of the rotatable object, so that the exact timing of interruption of the first signal can be adjusted while the encoder is in operation.