Infrared Proximity

Introduction

Infrared sensors are light sensors that are active and function in the infrared part of spectrum. They consist mainly of an infrared emitter and an infrared receiver. They are found in many applications such as object detection and automatic doors, burglar alarms, detection of object surface, the axis of rotation encoding, and decoding of barcodes. Some applications that were recently used in mobile devices include auto-answer calls, enabling automatic speaker detection and flip phones to dump. This application note comes with the firmware and hardware for the demonstration interface and analog signal processing of infrared proximity sensors using PSoC. It alsoexplains infrared light detection and the benefits of using PSoC in this solution.

Infrared proximity sensor

The infrared sensor consists of transmitter (infrared LED) and detector (photodiode). The transmitter emits infrared light pulse and the receiver detects the light pulses returned. The sensor is classified into transmissive and reflective sensors. As shown in Figure 1, in a transmissive sensor, the transmitter and the detector face each other. Objects are detected if they interrupt the light beam between the emitter and detector.

For a sensor of reflection, the emitter and detector are next to each other and are separated by a barrier. Objects are detected when light reflects on them and return to the detector. The sensor concept can be used in proximity. Some suppliers provide an integrated sensor which 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 higher current (perhaps more than 100 mA peak). The intensity of the infrared emitter affects the detection range. Considering the critical requirement of the size of PCBs into the mobile phone application, the demonstrator circuit uses two GPIO pins to drive the LEDs in parallel and supply power to the transmitter. On the side of the IR receiver, the desired output voltage depends on the distance of detection and the value of the load resistance (RLOAD). This voltage signal output can be connected to the next step, as an analog amplifier, comparator, Schmitt-trigger or to control various functions.

The selection of the load resistance, RLOAD, plays an important role in the functioning of the circuit. If RLOAD is too large, the RC time constant increase, which increases the response time. However, if the RLOAD is too small, it contributes more than

thermal noise on the circuit as the highest. Therefore, it is important to note the characteristics of current-voltage transfer this part of the circuit. According to applications, RLOAD can be selected from a range of 50 Kohm several mega ohms. In this circuit demonstration, we use a resistance of 1 Mohm evaluation.

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