4 - Resistive Input Transducers

2016-04-20 22:52:00 +0000, 2 years and 4 months ago

4.01 - light dependant resistor

As light level increases, the resistance of an LDR decreases. This behaviour is shown in the following graph:

10^7 +\.
     |  \.     x = illumination / lux
10^6 +    \.   y = resistance / ohms
     |      \.
10^5 +        \.
     |          \.
10^4 +            \.
     |              \.
10^3 +                \.
     |                  \.
10^2 +---+--+---+---+-----+   
     0.1 1 10 10^2 10^3 10^4

As shown, light level is directly proportional with resistance.
The symbol for an LDR is:

4.02 - thermistor (negative temperature coeffcient, NTC)

As temperature on around the thermistor increases, the resistance of the thermistor decreases. Therefore it can be said that the thermistor has a negative temperature coeffcient. This behaviour is shown in the following grap:

10^6 +\.
     |  \.     x = temperature / celcius
10^5 +    \.   y = resistance / ohms
     |      \.
10^4 +        \.
     |          \.
10^3 +            \.
     |              \.
10^2 +                \.
     |                  \
10   +---+---+---+---+---+   
     0  20  40  60  80 100

As shown, temperature is directly proportional with resistance.
The symbol for an thermistor is:

4.03 - switches

All switches have a common feature - they all rely on the mechanical movement of piece of metal to come into contact, to either; close, separate or break an electrical circuit.

Contacts that close to close the circuit are known as normally open, or NO. Contacts that open to open a circuit are known as normally closed, or NC.

NO resistance is infinite when open

   /
--*  *--  contacts open

NC resistance is 0 when closed

--*--*-- contacts closed

The most common type of switch is a toggle switch. This switch can only open one circuit and so is called SPST (single pole, single throw)

   /
--*  *-- single pole, single throw (SPST)

   / *-- 
--*
     *-- single pole, double throw (SPDT)

   /
--*   *-- } 
          } double pole, single throw (DPST)
   /      }
--*   *-- }

The ‘pole’ is the arm of the switch, and the ‘throw’, is the receiving end.

Tilt switches are sensitive to gyroscopic changes, and so the output changes accordingly to its physical orientation.

    +----------+             +-------+ container
----+-         | container   |       | NC
    |          | NC          |       |
----+-  /----\ |             | /---\ | mercury
    +---+----+-+             +-+---+-+
contacts   mercury             |   |  contacts

In respect to the diagram, when the switch turns 90 degrees anti-clockwise the mercury moves, and covers the two contacts, since mercury (or perhaps some other newtonian fluid) is conductive, it allows current to pass through it, in essence creating a simple toggle switch.

  +--------------------------+
  |           ..___________,,| 
--|.,_________,,           ''|--
  +--------------------------+
            contacts 

Reed switches are sensitive to magnetic fields, when a magnetic field is present, it induces a opposite magnetic field which causes the contacts to close via repulsion or attraction (dependant on the type of switch). Most reed switches are SPST and are only suitable for small currents (< 200mA) due to their intrinsic design. Because of their design, they also only require a small amount of movement to switch, and therefore can switch very quickly (a few hundred 100Hz).

4.04 - variable resistors

A variable resistor (potentiometer) is a convenient way of altering resistance. Variable resistors are available with linear or logarithmic tracks. There are two main types; slide and rotary.

       +++  slider              
low +--+ +------------+ high    
    |  +++            |            
         resistor 

Resistance of the variable resistor increases the further right the slider moves.

4.05 - voltage divider

Two fixed series resistors can be used to obtain a lower voltage from a fixed voltage supply.

+------+
+Supply+-->--+
+------+     |     Voltage 
            +++    Divider
            | | R1
            | |
            +++
             |
             *-->----
             |        ^
            +++       |
            | | R2    |
            | |       |
            +++       |
             |        V
OV -------<--*--<----

The voltage across each resistor is in the same ratio of their resistances, therefore Vout can be determined by a simple proportion calculation.

R2 and R1 are in series with Vin.

Assuming Vout is not connected, then the total current will be passing through both resistors:

The output voltage, Vout is given by:

Where Rtotal = R1 + R2

4.06 - using resistive input transducers with voltage dividers

+------+
+Supply+-->--+
+------+     |     Voltage 
           +++++   Divider w/
           |LDR|   LDR
           +++++
             |
             *-->----
             |        ^
            +++       |
            | | R2    |
            | |       |
            +++       |
             |        V
OV -------<--*--<----

R1 has been replaced with an LDR which has a high resistance in the dark, and low resistance when in a bright light. This means that current is allowed to flow when the LDR is in bright light, since there is little resistance (0 o), vice versa when the LDR is in the dark, therefore:

If the LDR were to be placed such that R2’s position was switched, then Vout would be logic 1 when the LDR was in the dark, this is because the resistance would be very high, and so the current would pass through Vout, when light however; the current would pass through the LDR, since its resistance is low.

This behaviour is expressed in the following diagram:

+------+
+Supply+-->--+
+------+     |    
           +++++   
           |LDR| BRIGHT  
           +++++           
             |             
             *-->----
             |        ^
            +++       |
            | | R2    |
            | |       |
            +++       |
             |        V
OV -------<--*--<----

Low resistance, current passes through the LDR. This is explained through:

+------+
+Supply+-->--+
+------+     |     
            +++   
            | |
            | | R1
            +++
             |
             *-->----
             |        ^
           +++++      |
           |LDR| DARK |  
           +++++      |
             |        V
OV -------<--*--<----

Current still passes to Vout, since when dark, the resistance of the LDR is higher than that of Vout, so current goes through the path of least resistance.

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