Light emitting diodes (LEDs) are inexpensive and widely used as light sources. Their diverse applications include numeric displays, flashlights, liquid crystal backlights, vehicle brake lights, traffic signals, backlights, and the ubiquitous power-on indicator light on almost every electronic device, and modern electrical appliances.
Because LEDs are most often used as light emitters, it is often forgotten that they can also operate as photodiodes, i.e., light detectors. Although most LEDs are designed as light emitters, and not light detectors, all LEDs can effectively operate in either mode.
This interchangeability between solid-state light emission and light detection with common LEDs was first described in the 1970's, see Mims, “Siliconnections: Coming of Age in the Electronic Era,” 139-149, McGraw-Hill, New York, N.Y., 1986, and Mims, “LED Circuits and Projects,” Howard W. Sams and Co., Inc., New York, N.Y., 1973.
Light emitting diodes emit light in a fairly narrow frequency band when a small current is applied in the correct direction through the diode, i.e., with a forward bias. Because the current-voltage characteristic is exponential, it is difficult to control a voltage applied directly across an LED accurately enough to attain a desired current. Therefore, some means must be provided to limit the current. In discrete electronic systems, this is typically done by placing a resistor in series with the LED.
One important application that uses LEDs is optical signal communications. In most prior art optical communications applications, an LED is used in the transmitter, and a photodiode is used in the receiver. In addition, each component is typically driven separately by a specially designed circuit. The photodiodes are most often specifically designed to receive optical signals in a specific narrow frequency range. Most photodiodes cannot emit light. Consequently, there is one circuit that drives the transmitter, and another circuit for driving the receiver. This increases the cost and complexity of the communications system.
In the prior art, a direct and an indirect method are known for using an LED as a photo sensor. In the direct method, the output current or voltage of the LED junction is measured directly. This method requires expensive, low-noise A/D converters when the LED is interfaced with digital circuits.
In the indirect method, the LED junction is first pre-charged, and then the time it takes for photon-induced leakage to discharge the capacitance of the junction to below a fixed threshold is measured. This method is described in U.S. patent application Ser. No. 10/126,761 “Communications Using Bi-Directional LEDs” Filed Apr. 19, 2002 by Dietz et al, which is a continuation-in-part application of U.S. patent application Ser. No. 10/115,299 “Automatic Backlight for Handheld Devices,” filed by Dietz on Apr. 3, 2002.
In the indirect method, the LED junction is reverse-biased by suitably setting micro-controller pins coupled to the LED. The micro-controller pins are then set with the anode held at zero volts, and the signal on the cathode pin is set to be read in as a logic input. The signal on the cathode pin reads as a logic one state. As soon as the micro-controller sets the cathode pin as an input, a timer is started. As the LED is exposed to light, a photocurrent discharges the junction capacitance and causes the voltage sensed by the micro-controller's cathode pin to decrease. When the voltage crosses a threshold, the signal value changes from logical one to logical zero, which is used to stop the timer. The length of time is an inverse function of the incident light flux. If the light flux is high, then the photocurrent is correspondingly high, and the junction capacitance discharges quickly. If the light flux is low, then the photocurrent is small, and the junction capacitance discharges slowly.
As an advantage, the second method does not require the A/D converter as in the first method. However, the time it takes to discharge the junction can be quite long when the photon flux is low. This makes the second method unsuitable for real-time sensing applications or data communication at high speed, when the light flux is low.
Therefore, there is a need for an LED circuit, which can rapidly sense low-level light.