05-12-2012, 12:24 PM
Smart Home Technologies
[attachment=42570]
Sensor and Actuator Hardware
To build sensors we have to understand the basic principles of electronic circuits
Voltage, Current, Resistance, Capacitance
Electronic components, ICs
Definition: Current
How is current (I) defined?
Pick any point in an electrical circuit
Define a unit of charge (Q)
The charge of one electron = -1.6x10-19 coulombs
Measure the change in charge with respect to time at this point
dQ/dt I
What is the unit for I?
1 ampere = 1 coulomb / 1 second
Most of the time this is just referred to as an amp
Definition: Voltage
At any point in a circuit, a positive charge (Q) has some level of potential energy (W)
Caused by its attraction to any build-up of negative charges in the circuit
Voltage (V) is defined as the normalized value of this PE
I.e., V W/Q
Units
1 volt = 1 joule / 1 coulomb
So a voltage drop between two points in a circuit is really a relative measurement of the change in PE for a given charge
Definition: Resistance
Static resistance is something that blocks the flow of Direct Current (DC)
A resistance between point A and point B will cause a difference in the PE between the points, and thus a voltage difference
This difference is also dependent on how much current is flowing from A to B
I.e., R = (V2 – V1)/I
Units
1 ohm = 1 volt /1 amp
Definition: Power
Now that we have a definition for current and voltage, we can get a definition for power
P dW/dt or more simply, P = V x I
Power is the amount of work that can result from the circuit
For example: lighting a light bulb
If no useful work can be done, the power is lost as heat
Definition: Capacitance
How is capacitance © defined?
C Q/V
OK, what does that mean?
Capacitance occurs when two conducting surfaces are separated by a dielectric
OK, what’s a dielectric?
a substance that
is a poor conductor
but a good medium for an electromagnetic field
What is the units for C?
1 farad = 1 coulomb / 1 volt
What Is a Semiconductor?
A substance that has a natural property that
allows it to act like either a conductor or an insulator
By adjusting this natural property by adding impurities to the substance
we can use two or more of these substances to control the way current flows through a circuit in very interesting ways
To understand why this is important to the study of sensor and actuators
we need to introduce something called Band Theory
Sensor Components for the Lab
Now that we have the necessary foundation
we can briefly address the use of the electronic components you will be seeing in the lab
then, start designing some simple sensors and actuators
and finally, use this knowledge to talk about some sensors and actuators that are a little too complex to play with in this class
The Resistor
A resistor provides a known resistance
It has three values:
Resistance, measured in ohms
Tolerance, measured in +/- percent error
Power dissipation, measured in watts
Using Ohm’s law (V=IR), it can be used to create a desired voltage or current
but a voltage drop across a resistor is converted to waste heat, so this is not always the best way to do that
A Light Bulb
It is a resistor encased in a vacuum in a clear or translucent container
It has two values
Rated voltage (either AC or DC)
Lumen (how much light it puts out at its rated voltage)
It obeys Ohm’s law (V=IR)
but a voltage drop across a resistor is converted to both to light and to waste heat
Even small light bulbs use a lot of current
so never try to drive them directly off an I/O line !
The Capacitor – A bit more complex
First, some relative definitions
Let us assume that a capacitor already has some positive charge on one plate and some negative charge on the other, then
a positive voltage difference (between the plates) is one that supports (i.e, is in the same direction) as this existing charge
a negative voltage difference is one that counters (i.e, is in the opposite direction) as this existing charge
OK, now we can start to talk about what it does
When there is a positive voltage increase between the two plates, more charge will build up on both plates
When there is an negative voltage increase, there will be a reduction in the charge built up on the plates
Capacitors in a DC Circuit
When DC is first applied to a capacitor
current will ‘pass’ through the capacitor for a very short time while its plates charge to match the voltage difference seen by the capacitor
then, no DC will pass
So, a capacitor will
once charged, look like an infinite resistance to any DC trying to pass through it
act like a very short term battery when the DC current in the circuit is turned off or reduced
Capacitors in a AC circuit
Alternating Current (AC) can pass through a capacitor
How ‘well’ it passes depends on
the frequency of the AC
the relative charge capacity of the capacitor for the given AC voltage (measured in farads)
the way the capacitor is wired to the circuit
So, the impedance (or AC resistance) of a capacitor can be used to filter out AC at frequencies you do not want
[attachment=42570]
Sensor and Actuator Hardware
To build sensors we have to understand the basic principles of electronic circuits
Voltage, Current, Resistance, Capacitance
Electronic components, ICs
Definition: Current
How is current (I) defined?
Pick any point in an electrical circuit
Define a unit of charge (Q)
The charge of one electron = -1.6x10-19 coulombs
Measure the change in charge with respect to time at this point
dQ/dt I
What is the unit for I?
1 ampere = 1 coulomb / 1 second
Most of the time this is just referred to as an amp
Definition: Voltage
At any point in a circuit, a positive charge (Q) has some level of potential energy (W)
Caused by its attraction to any build-up of negative charges in the circuit
Voltage (V) is defined as the normalized value of this PE
I.e., V W/Q
Units
1 volt = 1 joule / 1 coulomb
So a voltage drop between two points in a circuit is really a relative measurement of the change in PE for a given charge
Definition: Resistance
Static resistance is something that blocks the flow of Direct Current (DC)
A resistance between point A and point B will cause a difference in the PE between the points, and thus a voltage difference
This difference is also dependent on how much current is flowing from A to B
I.e., R = (V2 – V1)/I
Units
1 ohm = 1 volt /1 amp
Definition: Power
Now that we have a definition for current and voltage, we can get a definition for power
P dW/dt or more simply, P = V x I
Power is the amount of work that can result from the circuit
For example: lighting a light bulb
If no useful work can be done, the power is lost as heat
Definition: Capacitance
How is capacitance © defined?
C Q/V
OK, what does that mean?
Capacitance occurs when two conducting surfaces are separated by a dielectric
OK, what’s a dielectric?
a substance that
is a poor conductor
but a good medium for an electromagnetic field
What is the units for C?
1 farad = 1 coulomb / 1 volt
What Is a Semiconductor?
A substance that has a natural property that
allows it to act like either a conductor or an insulator
By adjusting this natural property by adding impurities to the substance
we can use two or more of these substances to control the way current flows through a circuit in very interesting ways
To understand why this is important to the study of sensor and actuators
we need to introduce something called Band Theory
Sensor Components for the Lab
Now that we have the necessary foundation
we can briefly address the use of the electronic components you will be seeing in the lab
then, start designing some simple sensors and actuators
and finally, use this knowledge to talk about some sensors and actuators that are a little too complex to play with in this class
The Resistor
A resistor provides a known resistance
It has three values:
Resistance, measured in ohms
Tolerance, measured in +/- percent error
Power dissipation, measured in watts
Using Ohm’s law (V=IR), it can be used to create a desired voltage or current
but a voltage drop across a resistor is converted to waste heat, so this is not always the best way to do that
A Light Bulb
It is a resistor encased in a vacuum in a clear or translucent container
It has two values
Rated voltage (either AC or DC)
Lumen (how much light it puts out at its rated voltage)
It obeys Ohm’s law (V=IR)
but a voltage drop across a resistor is converted to both to light and to waste heat
Even small light bulbs use a lot of current
so never try to drive them directly off an I/O line !
The Capacitor – A bit more complex
First, some relative definitions
Let us assume that a capacitor already has some positive charge on one plate and some negative charge on the other, then
a positive voltage difference (between the plates) is one that supports (i.e, is in the same direction) as this existing charge
a negative voltage difference is one that counters (i.e, is in the opposite direction) as this existing charge
OK, now we can start to talk about what it does
When there is a positive voltage increase between the two plates, more charge will build up on both plates
When there is an negative voltage increase, there will be a reduction in the charge built up on the plates
Capacitors in a DC Circuit
When DC is first applied to a capacitor
current will ‘pass’ through the capacitor for a very short time while its plates charge to match the voltage difference seen by the capacitor
then, no DC will pass
So, a capacitor will
once charged, look like an infinite resistance to any DC trying to pass through it
act like a very short term battery when the DC current in the circuit is turned off or reduced
Capacitors in a AC circuit
Alternating Current (AC) can pass through a capacitor
How ‘well’ it passes depends on
the frequency of the AC
the relative charge capacity of the capacitor for the given AC voltage (measured in farads)
the way the capacitor is wired to the circuit
So, the impedance (or AC resistance) of a capacitor can be used to filter out AC at frequencies you do not want