IR Short Distance Beam Cut Detector

Posted on Feb 5, 2014

A circuit that can detect a light beam cut. The first circuit i made was a light/dark detector with photoresistor. Although it could sense the light cut, it had a major disadvantage: extreme interferences from ambient light. That is why i designed this simple circuit. I tried to keep it as simple as possible. The maximum distance of the IR LED pair that i used is up to about 2. 5cm (close to 1 inch). That explains the

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title "Short Distance". It has a very good sensitivity. With a potentiometer you can adjust it to sense completely opaque objects, or it can sense semi-transparent films as well! It is also suitable for rotary encoders. It is not suitable for making security systems which require a beam that can go up to 3 or 4 meters long. This is the first part of this IR pair. The transmitter circuit is a pulse generator that drives an infrared LED. I have use the good old 555 chip. The pulses are about 1. 5KHz. You can use of course any kind of pulse generator like an Astable Multivibrator transistor circuit. That is up to you. Here is the schematic for the 555 connected as an astable multivibrator, oscillating at around 1. 5KHz: I began designing a complicated interference-proof circuit, with an active filter and other stages of filtering. Then, i realized that i was going the long way. There is no need for such filtering. This is an IR pair that will be most likely inside a housing, with the transmitter close to the receiver, away from parasitic lights. So, i start removing stages until i got down to minimum. The circuit that follows can be powered with a single 5 volts supply (unlike the other circuits that i designed that required double power supply). It has a simple capacitors filter that drops all constant and low frequency IR light sources such as the sun, the household lighting. The IR detector itself will reject any other...

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