Infra-Red Proximity Detector

Description: The detector is designed to recognize obstructions at distances ranging from a few millimeters to several centimeters. Similar devices are utilized in industrial and healthcare applications, such as activating a water tap through a magnetic valve. The sensor, IC2, is a Type SFH900 optoisolator manufactured by Siemens or a comparable component. A phase-locked loop (PLL) within decoder IC1 compares the frequency of the input signal from IC2 with that of an internally generated signal. When both signals fall within the same frequency band, the output at pin 8 of IC1 changes state from high to low. The internal oscillator produces a signal at approximately 4.5 kHz, determined by the time constant of resistor R1 and capacitor C1. This rectangular signal at pin 5 activates the light-emitting diode in IC2 via transistor T1. The diode emits an infra-red light signal pulsed at 4.5 kHz. When this infra-red light reflects off a nearby object, the phototransistor in IC2 sends a signal to pin 3 of IC1. If the frequency of this signal matches that of the internal oscillator, pin 8 connects to ground, causing diode D1 to illuminate. The PLL comparison prevents the circuit from responding to stray light. The sensitivity of the detector can be adjusted with potentiometer P1. The detector, with the specified components, draws a current between 10 and 30 mA. As mentioned, the optoisolator may be replaced with various types, including a discrete LED and phototransistor configuration, although special care is necessary to prevent the phototransistor from receiving light from the LED. A suitable solid-state relay at the output allows for the switching of larger loads. Circuit IC1 can switch currents up to 100 mA to ground, in which case diode D1 should be omitted.

The described obstruction detector circuit utilizes an optoisolator (IC2) to emit and detect infrared light, facilitating the recognition of nearby objects. The architecture includes a phase-locked loop (PLL) mechanism in decoder IC1, which serves to ensure precise frequency comparison between the emitted signal and the reflected signal from the detected object. This design is critical for applications where false triggers from ambient light need to be minimized.

The internal oscillator's frequency is set at approximately 4.5 kHz, which is defined by the values of resistor R1 and capacitor C1. The output from the oscillator is a square wave that drives the LED in the optoisolator. The choice of an SFH900 optoisolator is significant due to its sensitivity and fast response times, which are essential for detecting minute changes in distance.

When the infrared light emitted by the LED reflects off an object, the phototransistor within the optoisolator detects this reflected light. The signal generated by the phototransistor is then fed into pin 3 of IC1. The PLL within IC1 continuously compares this signal with its internal reference. If both signals are within the same frequency band, the PLL changes the state of pin 8, which can be used to drive additional circuitry or an output device.

The circuit allows for sensitivity adjustments through potentiometer P1, enabling it to be fine-tuned for various applications and environmental conditions. The overall current consumption of the detector is between 10 mA and 30 mA, which is relatively low, making it suitable for battery-operated devices.

For applications requiring the control of larger loads, a solid-state relay can be integrated at the output, utilizing the switching capability of IC1, which can handle currents up to 100 mA. In scenarios where the relay is employed, it is advisable to omit diode D1 to prevent interference in the output signal.

This circuit design is versatile and can be adapted for various industrial and healthcare applications, ensuring reliable operation in detecting obstructions and enhancing automation processes.The detector is intended for the recognition of obstructions at distances of a few millimeters to a few centimeters. Similar detectors are used in the industry and health services, for instance, to open a water tap via a magnetic valve.

The sensor, IC2, is a Type SFH900 optoisolator from Siemens or similar. A phase-locked loop (PLL) in decoder IC1 compares the frequency of the input signal from IC2 with that of an internally generated signal. When the two signals fall within the same band, the output, pin 8, of IC1 changes state (from high to low). The internal oscillator generates a signal at a frequency of about 4. 5 kHz (determined by time constant R1-C1). Its rectangular signal at pin 5 switches on the light-emitting diode in IC2 via T1. The diode then transmits an infra-red light signal pulsed at 4. 5 kHz. When the infra-red light is reflected by a nearby object, the photo transistor in IC2 provides a signal to pin 3 of IC1 If the frequency of this signal lies within the same band as that of the internal generator, pin 8 is connected to earth, whereupon diode D1 lights.

The comparison by the PLL prevents the circuit reacting to stray light. The sensitivity of the detector may be varied with P1. The detector with components as specified draws a current of 10 30 mA. As stated earlier, the optoisolator may be one of several types. It may also be built from a discrete LED and photo-transistor, but great care should then be taken to ensure that the photo transistor cannot receive light transmitted by the LED. A suitable solid-state relay at the output enables larger loads to be switched. Circuit IC1 can switch currents of up to 100mA to earth. Diode D1 should then be omitted.