Figure 1. Connection of an LED to a Microcontroller. (a) Forward Bias With Light Emission; (b) Reverse Bias; (c) Sensing Light.
Light Emitting Diodes (LEDs) are low cost electronic devices widely used for displays. One of LED properties, apart from the ability to emit light, is to operate as a photo sensor. Thus, a LED may be simultaneously used as a display element and as a sensing element. This characteristic may implement electronic displays with touch sensing capability. In this paper, the author describe the development of a display prototype that senses a finger touch and alters its operation. This operation may be extended to multi-touch sensing, boosting displays performance while keeping hardware requirements to the same level. It is worth mentioning that the sensing ability is provided by software, exploiting LED sensing capability, with the same hardware as this used for displaying purposes. The implication of this project is the ability to construct displays that their operation can be dynamically configured without any other hardware overhead or any additional cost. Also, the sensing capability of LEDs may be used to develop optical serial communication between two devices. Thus, LEDs may implement communication functions at a minimal cost and in cases where other technologies are expensive (infrared, bluetooth, etc). For example, the power light of a device may be used for maintenance reports or firmware update with the existing hardware. This experimentation has shown that LEDs are inexpensive elements that may offer interested advantages both for displaying and sensing purposes.
Light Emitting Diodes (LEDs) are electronic light sources that are widely used in display systems because they are inexpensive, long lasting, bright and require simple driving hardware. Since their invention in 1920, the development of LED technology has caused their efficiency and light output to increase exponentially, in parallel to the development of semiconductor technologies and the advances in optics and material science. All early devices emitted low-intensity red light, but modern LEDs are available across the visible, ultraviolet and infra red wavelengths, with very high brightness. The emergence of Organic LEDs (OLEDs) and polymer LEDs (PLEDs or FLEDs) will expand their usage as elements of display systems. The emitting layer material of an OLED is an organic compound or a polymer. Polymer materials can be flexible and present certain advantages over common LEDs, as brightness and flexibility but their life is still too short. Besides their usage as display elements, infrared LEDs are used for communication (TV remote control), photodiodes or phototransistors provide a signal path with electrical isolation between two circuits (known as optoisolators), common or infrared LEDs are used as sensors (Wii console uses infrared LEDs as a movement sensor), etc. In 1970s, Forrest M. Mims reminds that LEDs can also be used as photodiodes [1]. Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This very important characteristic of the diode could be used for the provision of a touch sensing screen that register reflected light from a finger or stylus.
This paper presents the inter-changeability between solidstate light emission and detection in an explicit way, referring to the theoretical background of the LEDs sensing ability and by developing a prototype matrix LED display that is touch sensitive. The consequences of the LEDs photosensitive nature are the development of touch buttons in a screen and optical data exchange applications [2-4]. Scott Hudson has also developed a microcontroller based single matrix LED display that is capable of sensing the finger touch and surveys the sensor response under various ambient light conditions [5].
Our prototype is a microcontroller monitored display consisting of three 8×8 dot matrix LEDs where touch sensing capability is evaluated. The sensing ability is examined under various driving and ambient light conditions. Also, the optical communication capability is examined by exchanging data between the display device and a battery handheld single LED tiny device.
Figure 1 illustrates the operation of a light emitting diode as an emitter and at the same time as a light detector. The LED is connected to a microcontroller together with a limiting resistor. When port pin PC is high and port pin PB is low, the device operates as a light emitter because it is forward biased. On the other hand, the device is reversed biased by applying low voltage to PC and high voltage to PB. This charges the small intrinsic diode capacitance which may be discharged through PB, if this pin is turned to a high impedance input mode. The discharging time depends on the incident light and is less as the amount of ambient light is larger.
From the logic 'high' to 'low', we can evaluate the incident light. Shorter times mean higher incident light whereas longer times mean lower incident light, which may be originated from a finger touch. Therefore, a threshold value of the discharging time is set as long as the sensing LED is working with the ambient light. If, at any instant, this time becomes larger than the threshold value, this means that an obstacle (like a finger touch) is in front of the sensing LED confining the incident light.
Figure 1. Connection of an LED to a Microcontroller. (a) Forward Bias With Light Emission; (b) Reverse Bias; (c) Sensing Light.
Figure 2. shows the flow chart of the above described procedure, which is the sensing routine for a finger touch.
The assembly language routine used for materializing the above flowchart, for a PIC microcontroller, is shown in Figure 3. Port pins PC2 and PB0 are connected to the anode and cathode of the sensing LED respectively. Timer 0 is used to measure the discharge time and flag touch_f signals the sensing routine output. If the flag is set, a finger touch has been sensed whereas if it is reset there is no finger touch. The cited instructions are those that correspond to the above mentioned flowchart and they should be completed with a number of supplemental instructions for a real time application.
It is evident that the threshold value is ambient light dependent, which means that in a dark environment a finger touch cannot be detected. To avoid this situation and the operation to be independent of ambient conditions, the sensing LED is set among other neighboring illuminated LEDs, like Figure 4. This configuration simulates a key and prompts the user to touch the LED at the center. In this case, touching the sensing LED, which lies in the centre of this setup, will reflect some light to it and the discharge time will be shorter or the timer count will be less than the preset threshold value. Sensing is difficult in very bright environments because the reflected light with touch will be slightly greater than the ambient light and the measured time will be near the threshold value.
This set up may lead to false outputs if the ambient conditions vary significantly during the sensing process. If the incident light is suddenly increased, between the threshold setting phase and the sensing phase, a false finger touch will be detected. This effect can be avoided by repeating the two phases after a short time interval and then decide about the kind of output. The same procedure is followed to normal keyboards where “debouncing” is employed for deciding if a key press is true or false due to noise.
Figure 2. Flowchart of the Procedure for Sensing a Finger Touch.
Figure 3. Pic Assembly Language Routine for Finger Touch Sensing
Figure 4.Simulation of a Touching Key
Figure 5 illustrates the circuitry of a one 8×8 matrix LED display. The microcontroller used is PIC16F877A while the LED matrix is the common anode TA23-11BWA of King Bright. Port RD drives the LED common anodes through a selected latch (74HC573) and a non-inverting high current driver (UDN2981). The display may be expanded up to 8 digits by driving each one with a pair of latch and driver which are selected by the pins of port RC. The cathodes are connected to the port pins AN0 up to AN7 supplying the segment information of the display. At the same time, these port pins are used as sensing input pins as explained above.
Figure 5. Circuit of one Digit Matrix Display for the Evaluation of LEDs Sensing Capability.
In this work, a prototype device has been developed that consists of a three digit matrix LED display and is capable to sense a finger touch. The sensing algorithm is based on the LEDs property not only to emit but also to sense light. The sensing element is reverse biased and its tiny internal capacitance is charged. Then, the capacitance is discharged through the microcontroller port pin which was in the high state and it has been turned to an input high impedance state. The discharge time depends on the amount of incident light and is measured by a timer. If the measured time is greater than a threshold value, it notices a finger touch. The consequence of the LEDs sensing property is that every LED display can sense a finger touch with a minimum hardware overhead and thus, it can perform other desirable operations (like system software updates, display messages manipulation without a keyboard, etc). Generally speaking, LED can be used as a bidirectional device with a numerous of applications like touch sensing or optical communication.