| 摘要 |
<p>A retrofittable device for deriving a pulsed output from a rotating pointer forming bart of the index mechanism of a gas meter comprises a housing which can be secured to the transparent face of the index mechanism. The housing contains a light emitting diode for directing a beam of light, for example infra red light, at a portion of the index dial face in front of which the pointer rotates, for modulation by the pointer and reflection back onto a pair of closely spaced matched photodetectors also disposed in the housing. The photodetectors are positioned such that the modulated beam affects them sequentially, and the output signal of the device is derived from the difference between the respective outputs of the photoconductors. This ensures that ambient lighting changes, as well as the effect on the photodetectors of temperature and aging, tend to cancel out.</p> |
