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The Sensors
Light Sensors
These comprise of two Light Dependent Resistors (LDR)
Everything has an electrical resistance, some more than others.
An LDR will have a resistance that varies according to the amount of
visible light that falls on it. A close up of an LDR is shown below:
The light falling on the brown zigzag lines on the sensor, causes the
resistance of the device to fall. This is known as a negative
co-efficient. There are some LDRs that work in the opposite way i.e.
their resistance increases with light (called positive co-efficient). I
won't go into the physics of how the device changes its resistance, so
just take it as read.
Now, in order to use this device in a simple circuit, all we need to do
is put a voltage across it and measure the current flowing through it.
However, measuring current can be a little tricky. So, we put
another resistor in series, and measure the voltage across the LDR.
This makes us a potential divider, and the voltage across the LDR is
proportional to the current. The diagrams below show the concept.
Here, the current is directly proportional to
the resistance of the LDR
A much easier way is to put a second resistor
in series with the LDR and measure the voltage across the LDR
To take the extreme cases, we can therefore detect if light is
present or not, just by simply detecting if we have a voltage or not.
If we use two sensors like those on Cybot, then we can also detect which
direction the light is coming from simply by seeing which sensor has the
stronger voltage.
Ultrasonics
These sensors emit a very high frequency sound. In fact it is so
high, that we can't hear them. A picture of two ultrasonic sensors is
shown below:
Two sensors work in unison, one as the transmitter and one as the
receiver (this is why Cybot has four of them). The transmitter
typically sends out a constant beam of sound at a frequency of 40KHz (note
that the human hearing barely goes above 17KHz). The receiver
detects any sounds coming in and gives us a voltage out. So, what
happens is the transmitter sends out a signal. If there isn't an
object in front of it, then the sound wave will carry on (note there is a
limit to the distance here!). If, and only if, there is an object in
the way, the sound waves will bounce back along the same path, and so be
picked up by our receiver. Simple, huh?
Now, and this is the clever bit, if we can note the time between
transmitting the sound, and when we receive it, then we can work out the
distance of the object from our sensors. This, by the way is how
parking detectors work on some of the newer cars.
Line Sensor
Now this may seem to be a very special kind of
sensor, after all what kind of sensor can see a line?
Well, the principles are very simple. It
consists of just two components. The first is an Infra-Red (IR)
transmitter (usually an LED), while the second is an Infra-Red receiver
(usually a transistor). IR is transmitted out of the sensor unit.
If the IR is reflected back, it is picked up by the IR receiver
transistor.
But how does it follow a line, you ask?
Well, IR is basically heat (the heat from the sun is predominantly in the
IR part of the spectrum). Black, as you probably know, absorbs heat,
which is why it is best not to wear black in the summer months. If
black absorbs heat, then it also absorbs IR. And this is the
principle. While the sensor is over a black line, no IR is reflected
back to the receiver. If the sensor strays away from the line, then
IR is reflected back. This is why Cybot 'follows a black line'.
For best results the black line is placed on a white background, which will
give the extreme two cases - white reflects IR.
The diagram below illustrates this, but as you
can see it is quite a simple concept.
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IR is
reflected IR is not reflected
here in the black region |
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This completes the section on Sensors.
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