Infrared sensors are light sensors, which are active and function in the infrared part of the frequency spectrum. They
primarily consist of an infrared emitter and an infrared receiver. They are found in many object detection
applications such as automatic door openers and burglar alarms, surface feature detection, rotation shaft encoding,
and barcode decoding. Some recently used applications in mobile devices include auto call-answering, auto speaker
phone activation, and detection of flip-in-clamshell phones. This application note is bundled with the firmware and
hardware for demonstrating interface and analog signal processing of infrared proximity sensors using PSoC. It also
explains infrared light sensing and the advantages of using PSoC in this solution.
Infrared Proximity Sensor
The infrared sensor is made up of the emitter (infrared LED) and detector (photodiode). The emitter emits IR light pulse and the receiver detects the corresponding light pulse. The sensor is classified into transmissive and reflective sensors. As shown in Figure 1, in a transmissive sensor, the emitter and the detector face each other. Objects are detected if they interrupt the beam of light between the emitter and the detector.
For a reflective sensor, the emitter and the detector are next to each other, and are separated by a barrier. Objects are detected when light is reflected off them and back into the detector. The reflective sensor can be used in proximity detection. Some vendors provide a built-in sensor that includes the receiver and transmitter in a small package. In this application note, Avago HDSL-9100 is used in the demonstration circuit. to drive the LED at a higher current (possibly higher than 100 mA peak). The infrared intensity of the emitter influences the detection range. Considering the critical requirement of the PCB size in mobile phone application, the demonstration circuit uses two GPIO pins to drive the LED in parallel and provide current to the emitter. On the IR receiver side, the desired output voltage depends on the detection distance and the value of the load resistor (RLOAD). This output voltage signal can be connected to the next stage, such as an analog amplifier, comparator, or Schmitt-Trigger, to control various functions.
The selection of the load resistor, RLOAD, plays a significant role in circuit operation. If the RLOAD is too large, the RC time constant increases, thereby increasing the response time. However, if the RLOAD is too small, it contributes more
thermal noise to the circuit than higher ones. Therefore, it is important to note the current-to-voltage transfer
characteristics for this part of the circuit. According to the applications, the RLOAD can be selected in a range of
50 Kohm to several mega ohms. In this demonstration circuit, we use a 1 Mohm resistor for evaluation.
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