Design of transceiver based on IR proximity detection sensor

Abstract: This article introduces a simple IR transceiver design. The generated IR signal is modulated at a 10kHz carrier frequency. The reflected signal is then amplified using a single op amp (MAX4230), which is also configured as a second-order bandpass filter to demodulate the 10kHz IR receive signal.

IR proximity detection sensors are widely used to detect the presence of an object, the distance to a reference object, or both. Specific applications include: speed measurement, automatic faucets, automatic counters or conveyor belt object detection, printer paper edge detection, and other applications. The new generation of smartphones also uses proximity detection technology. When the phone is pressed under a person's chin or ear, it can automatically turn off the LCD touch screen to avoid accidental operation of the touch screen.

When detecting an object, the proximity sensor first transmits an IR (infrared) pulse toward the target and then "listens" for the reflected signal to detect if any is present (Figure 1). The IR LED transmits the IR signal and any reflected signal can be captured by the IR photodetector. The strength of the reflected signal is inversely proportional to the distance between the target and the IR transceiver. The closer the distance, the stronger the IR reflected signal. This can be used to calibrate the output of the photodetector to determine the exact distance of the target (a distance detection threshold can be set to determine if the target is present).

Figure 1. Basic principle of IR proximity detection sensor.

The photodiode detects the IR signal reflected from the target, and can also detect the IR signal generated by the surrounding environment. Designers need to filter out these IR noises to avoid false detection. The general solution is to modulate the IR signal emitted by the LED to an appropriate frequency, and the receiver only detects the modulated IR signal, ensuring that the actual detected signal only comes from the reflection of the target object.

The IR proximity sensor shown in Figure 2 has a simple transmitting and receiving circuit. The transmitting circuit includes an IR LED (IR11-21C) with a wavelength of 940nm, which is turned on and off at an oscillation frequency of 10kHz. The transmitting power is controlled by adjusting the LED current, thereby controlling the detection range. To reduce power consumption, the transmitting circuit usually uses a small duty cycle (typically 10%) transmitting pulse.

Figure 2. A simple IR transceiver detects the presence and distance of an object from the transceiver.

The receiver circuit demodulates and amplifies the IR signal detected by the photodiode (PD15-22C), and the peak of the detection signal appears at a wavelength of 940nm. The photodiode output is AC-coupled to the non-inverting input of the operational amplifier. The AC coupling allows the passage of 10kHz signals, while the 300Hz cutoff frequency established by the coupling capacitor will suppress DC noise and IR background noise at the input of the operational amplifier.

In this type of circuit design, it is best to choose a low-noise, wide-bandwidth, full-rail input/output operational amplifier (MAX4230) for signal demodulation and amplification. In addition, the excellent RF rejection of the operational amplifier helps to avoid the 217Hz buzzing noise common in GSM mobile phones. In the IR receiver, the op amp circuit is configured as a second-order bandpass filter with a gain of 100 and a center frequency of 10kHz, so that the operational amplifier can demodulate the input IR signal using its bandpass filter while amplifying the input IR signal.

The op amp is biased at 2.5V when no IR signal is present. When a 10kHz IR signal is present at the input, the output varies around 2.5V, giving a dynamic range of 5V. The output of the op amp drives a simple diode detector, which rectifies the 10kHz signal and provides a DC signal proportional to the signal amplitude. The amplitude of the analog output signal (OUT) is proportional to the distance of the target from the IR transmitter. This output signal can be used directly for detection or it can be fed into an ADC for further processing. Measuring the signal waveforms at different nodes of the circuit can clearly describe the circuit's operation. Figure 3 shows the signal waveforms when the distance is 1.2 inches and 1.4 inches from the IR transceiver. Note that these waveforms are labeled with specific numbers corresponding to different nodes of the circuit.

Figure 3. The waveforms shown here, generated by the circuit in Figure 2, represent reflections from objects 1.2 inches and 1.4 inches from the IR transceiver.