Optical keypad for explosive locations
AssigneePR Electronics A/S
InventorBjarne Funch SKIPPER
Life of this patent
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Abstract
An optical keypad for use in hazardous areas and which can ensure safe and reliable detection of keystrokes through an at least 8 mm thick glass window, as is required for electronics equipment located in hazardous explosive areas (In accordance with ATEX Directive 94/9/EC and similar requirements) is obtained by providing a system in which at least 6 LEDs are lit in a pseudo-random sequence, modulated by a pseudo-random frequency and the interval between the lighting of each of the LEDs is change randomly to avoid interference between collocated keypads and light from other sources, such as daylight and artificial light.
Claims7
An optical keypad, comprising:
- at least one transparent panel having a plurality of apertures surrounded by closed areas, which optical keypad comprises a plurality of light transmitting diodes (LEDs) emitting modulated light and at least one photo detector diode placed in a relation to the apertures that enables light from the LEDs to be transmitted through the transparent panel and reflected by scattering from a tool or fingertip producing a keystroke in relation to one of the apertures in the transparent panel, back through the transparent panel to the at least one photo detector diode for demodulation,
- wherein the emitted modulated light from each LED (4) is controlled by a combination of at least one of the following parameters:
- a:
- modulation frequency of the emitted light from the transmitting LEDs (4)
- b:
- the sequence in which the transmitting LEDs (4) are lit,
- c:
- the time interval between lighting of the individual transmitting LEDs (4)
- where at least one of the parameters is varied randomly, and
- where detection of the reflected light is able to be performed by a synchronous detection of current flowing through the at least one photo detector diode (5), and a randomly received signal compared with a randomly coded signal transmitted from the LEDs (4).
Method to operate an optical keypad having at least one transparent panel having a plurality of apertures surrounded by closed areas, a plurality of light transmitting diodes (LEDs) emitting modulated light and at least one photo detector diode placed in a relation to the apertures that enables light from the LEDs to be transmitted through the transparent panel and reflected by scattering from a tool or fingertip producing a keystroke in relation to one of the apertures in the transparent panel, back through the transparent panel to the at least one photo detector diode for demodulation, comprising the following sequence of steps:
- a:
- prepare a train of light pulses from one or more of the individual transmitting LEDs,
- b:
- prepare random modulation of the train of light pulses,
- c:
- select one or more of the following parameters for random modulation,
- modulation frequency of light emitted from transmitting LEDs
- a sequence in which transmitting LEDs are lit, and
- a time interval between lighting of transmitting LEDs,
- d:
- perform transmission of the randomly modulated light by one or more of the individual transmitting LEDs,
- e:
- let the optical receiver photo diode receive the reflected signal,
- f:
- perform signal conditioning, amplification and synchronous detection of the received optical signal,
- g:
- by comparison of the detected signal and the adaptive adjusted threshold level evaluate whether a reflection or no reflection has been present for the individual LEDs and
- h:
- on the basis of the evaluation and a truth-table decide whether a keystroke has been performed or not.
Dependencies are inferred from the claim text.