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should be quoted because noise power is
frequency dependent and noise may be present well outside the range of signal frequencies of interest.
Next month we will continue to look at
random noise, meanwhile let us turn our
attention to magnetic sensors.
REED SWITCHES
The simplest magnetic sensor is the reed
switch, which usually consists of two contacts made of ferrous metal housed in a
sealed glass tube. For most reed switches
the contacts are usually open but, when a
magnet is brought sufficiently close the two
strips of metal become magnetized and will
attract each other, closing the switch.
Typical example of a reed switch.
Fig.6.9. Guard signal for instrumentation amplifier.
Batteries create electric current through
A typical random noise signal is shown
electrochemical effects. Similar processes
in Fig.6.10. A sine wave suffering from a
can occur if contaminants are present on
significant amount of random noise is
circuit boards and terminals. Variations in
shown in Fig.6.11.
humidity can affect sensor systems with
The fact that the components in any elecvery high impedances. Contaminants (e.g.
tronic circuit or system generate random
from handling cables and connectors) can
noise means that there is always a certain
also reduce insulation resistance leading to
level of noise, even with no signal present.
errors. High precision circuits and sensors
This is known as the noise floor, which is
must be kept clean!
important because the circuit cannot meaningfully process sensor signals that are
RANDOM NOISE
smaller than the noise floor.
The electronic components and sensors
As the noise floor relates to noise within
used to build circuits generate noise from
the circuit, this is different from noise withwithin themselves that is random in nature.
in the input signal. If the properties of the
This means that we can never predict the
required signal are known, there are techactual instantaneous value of the noise voltniques which can extract signals that are
age (or current), although the overall statistics
smaller than noise present within the sigof how the noise behaves is known, however.
nal, for example, due to interference.
The random noise generated within electronic components and sensors is fundamentally due to the discrete nature of electricity
at the atomic level, i.e. electric charge in circuits is carried in packets of fixed size on
electrons. Random noise may be classed
according to the frequency, or range of frequencies present. White noise has the same
power throughout the frequency spectrum,
whereas 1/f noise (or pink noise) decreases
in proportion to frequency.
For 1/f noise there is the same amount of
noise power in the bandwidth of say 100Hz
to 1kHz are there is in 1kHz to 10kHz,
whereas for white noise there would be 10
Fig.6.10. Example of random noise,
times as much power in the bandwidth
illustrated via the Picoscope.
1kHz to 10kHz as 100Hz to 1kHz because
it is 10 times larger.
The difference between the signal and
the noise is often of great importance. This
is expressed as the signal-to-noise ratio
(SNR), usually in decibels (dB) and based
on the ratio of noise power (hence the v2
terms in the equation).
SNR = 10log10
Fig.6.11. Picoscope display of a
sinewave heavily contaminated by noise.
BOUNCY SWITCHES
One major concern with reed switches
(and all switches) is switch bounce that
occurs when a switch is closed – because
the switch contacts are springy, they will
bounce open and closed for several milliseconds before remaining closed. This may
be OK for turning lights on and off but
causes problems if we wish to connect a
switch to a logic circuit.
In Fig.6.12 is shown a screenshot of a
reed switch bouncing, captured using the
Picoscope. You can imagine a circuit
designed to count the number of times a
door is opened. The circuit is straightforward – a reed switch connected to the input
of a counter device which may display the
count value on l.e.d.s. Every time the
switch is activated, it will bounce three or
possible four times, causing the counter to
count three or four times.
vs2
vn2
()
Where vs is the r.m.s. signal voltage and
vn is the r.m.s. noise voltage. When using or
quoting SNR values the bandwidth (range
of signal and noise frequencies considered)
Everyday Practical Electronics, April 2002
Such a simple device has many applications, ranging from detecting when doors
open and close to detecting the rate of rotation of an object such as an axle. In the first
case, the switch is placed on the wall close
to the door and the magnet is attached to
the door, while in the second case the magnet is placed on the axle and causes the reed
switch to periodically close as the axle
rotates. We will be using this approach to
measure wind speed in Lab Work, where
the sensor will be a Hall effect sensor
instead of a reed switch.
Fig.6.12. Picoscope display of switch
bounce.
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