Control circuit for array of light-emitting diodes
AssigneeOki Electric Industry Co., Ltd.
InventorsYukio Nakamura, Kazuo Tokura, Shigemitsu Horikawa, Tokio Sato
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Abstract
An array of LEDs is controlled by a control circuit so as to emit light in a write mode and sense light in a read mode. In the read mode, each LED is alternately charged for a first interval, then allowed to discharge by flow of photocurrent for a second interval. At the end of the second interval, just before charging of the LED begins again, the anode voltage of the LED is read by coupling the anode of the LED to an output terminal for a third interval. The third interval of each LED may coincide with the first interval of the preceding LED in tile array, so that each LED is read while the preceding LED is being charged.
Claims26
A method of reading stored charges from an array of light-emitting diodes used for sensing as well as emitting light, comprising the steps of:
- charging the light-emitting diodes in said array for mutually non-overlapping first intervals of time of equal first lengths;
- allowing said light-emitting diodes to discharge by flow of photocurrent for respective second intervals of time of equal second lengths;
- and
- detecting anode voltages of said light-emitting diodes during mutually non-overlapping third intervals of time, said third intervals being contained in respective second intervals and comprising final portions of respective second intervals.
The method of claim wherein, if a first light-emitting diode is followed by a second light-emitting diode in said array, the third interval in which the anode voltage of said second light-emitting diode is detected coincides with the first interval in which said first light-emitting diode is charged.
A control circuit for controlling an array of light-emitting diodes in a write mode in which said light-emitting diodes emit light responsive to data, and a read mode in which charges stored in the light-emitting diodes are read to detect illumination received by said light-emitting diodes, comprising:
- a first shift register for serially receiving and shifting said data;
- a latch coupled to receive said data in parallel from said first shift register, latch said data, and generate corresponding data signals;
- an output terminal;
- and
- a switch-and-driver circuit coupled to control said light-emitting diodes responsive to said data signals;
- wherein
- each of said light-emitting diodes has a cathode and an anode, said cathode being grounded in said write mode and biased at positive potential in said read mode, so that an electrical capacitance exists between said anode and said cathode in said read mode;
- and wherein
- said switch-and driver circuit separately comprises, for each light-emitting diode in said array:
- a first switching element coupled to supply a driving potential to the anode of said light-emitting diode, said first switching element being switched on to light said light-emitting diode in said write mode, switched off to extinguish said light-emitting diode in said write mode, and switched off in said read mode;
- a second switching element coupled to supply a ground potential to the anode of said light-emitting diode, said second switching element being switched off to light said light-emitting diode in said write mode, switched on to extinguish said light-emitting diode in said write mode, switched on to charge the capacitance of said light-emitting diode in said read mode, and switched off in said read mode when not charging the capacitance of said light-emitting diode;
- and
- a third switching element for coupling the anode of said light-emitting diode to said output terminal, said third switching element being switched off in said write mode, switched off when said light-emitting diode is being charged by said second switching element in said read mode, and switched on to read an anode voltage of said light-emitting diode just before said light-emitting diode is charged in said read mode.
Dependencies are inferred from the claim text.