A position sensor comprises two curved surfaces concentrically aligned to form a container which is filled with a viscous fluid and a bubble of a lighter-weight fluid and placed between a radiation source and a radiation detector. The bubble changes position within the container when the sensor is moved, transmitting a beam of radiation from the radiation source through the bubble to activate a section of the radiation detector while the remainder of the radiation is blocked by the fluid. Position sensing circuitry translates a signal from the activated section of the radiation detector into position coordinates for point in space corresponding to the position of the bubble in the container. The position sensor is suitable for use in a digital controller that generates display attributes to define a locator symbol on an output device. The digital controller can include an optional button to permit a user to change the size of the locator symbol, making it appear to approach or recede, to provide a simulated three-dimensional display.