25-07-2012, 12:03 PM
Light Emitting Diode (later LED)
Light Emitting Diode (later LED).pdf (Size: 347.19 KB / Downloads: 114)
In this tutorial, I want to describe simple Infrared (later IR) obstacle avoiding
scheme. It might be interesting to those who are just entering amazing world of
robotics and wants to know more obstacle sensors.
There are many ways to realize remote object detection. The simplest is just IR
Light Emitting Diode (later LED) and phototransistor. IR LED emits light with
wavelength approximately 850 nanometers. Light reaches obstacle and reflects
back. There it is picked up with phototransistor. Voltage in measurement point
changes (fig. 1), and this change is proportional to picked up light intensity.
Comparator or ADC might be used to convert it to digital form and decide, if
there is an obstacle in front of us. For example if 5 Volts supply is used: 4.5
Volts mean obstacle is far, and 1 Volt means obstacle is close (as phototransistor
current flows, voltage dropout on resistor grows according to Ohm’s law, and
voltage in point of measurement falls).
This schematic has big drawback. First, the Sun is powerful IR source, so there
will be false readings on sunlight . Also any light bulb emits IR light
intensely, so false readings again.
There is one way to partially cut-off ambient light with color filter and let only
LED light to reach phototransistor. The simplest filter it a piece of color photo
film, exposed to fluorescent light, and then developed. Film would look black,
but it let light with wavelength 820-1050 nm to pass through. However, even
with such improvement this schematic lacks of stability.
There is better approach to improve the design. We can modulate IR LED like
shown (fig. 3). Just turn LED on and off to do this. NPN transistor is used not to
draw too much current from microcontroller (later MCU) pin, as it can be above
50 milliamps during impulse.
Fig (3). Modulation of light intensity using NPN transistor
Light intensity will be modulated, and voltage at the output of receiver will also
be modulated. Now, ambient noise is directional current (later DC) signal, but
transmitter signal is alternating current (later AC). Capacitor is used to block DC
and get voltage proportional to AC only.