INTRODUCTION 

Basic Idea is to send infrared light through IR LEDs which is then reflected by any object in front of sensor. One of the biggest problems that can cause the malfunctioning of an IR proximity sensor, is the ambient light and surrounding sources of IR like the sun and halogen lamps that can cause false triggering of the sensor due to emission of infrared light. To avoid getting false detection the solution is to send pulses of IR light at a certain frequency instead of a constant beam, and build a receiver that would only detect IR pulses of the same exact frequency, cutting of all pulses of higher or lower frequency. The kind of device capable of filtering signals this way is called a band pass filter. 

There are a lot of types of band pass filters, a whole branch of electricity is dedicated to this subject. The central frequency is fixed by the constructor usually at 1 khz. IR receiver filters all the source of light except the 1Khz IR signal. It all starts by generating the 1 Khz pulses of electricity that are fed to an IR LED, emitting 1Khz pulses of Infra-Red light. A weaker signal but with the same frequency is reflected from an eventual obstacle to the IR receiver, it passes through the IR-PASS filter, which will eliminate other sources of light which are not IR (visible light). The IR-PASS filter still detects a lot of noise due to other sources of IR light like the sun for example, so the signals received by the diode are fed to another stage composed of an active filter to select the 1Khz IR signals among all others, amplify it and demodulate it, providing a clean logic output (5 or 0 volts).