3.4. Analogue devices.   

The device sends an analogue or continuously variable signal to the computer which is changed by an Analogue to Digital converter into a digital value that the machine can use. 


     
Variable resistor

    
                  Fig. 3.6 : Variable resistor
Symbol is Variable Resistor, third connection can be changed to tap off different voltages like volume control. This Vout is used to indicate different coordinates, rates of change, etc.

Variable resistor is used in most analogue devices.
Dials: Simple variable resistor or potentiometer used to give digital information 


     
Dial

    
                         Fig. 3.7 : Dial



Vout changes as the tap is moved.

The Voltage Follower is used to isolate the potentiometer

from the A to D. This and any other components between pot

and the A/D must not change the voltage in any way

(must not change as temperature changes, as supply voltage changes).

 The A/D is an analogue to digital converter and takes

analogue signal (voltage 0 to Vin) and converts to N binary bits.

If we choose N=8 we get 8 bits of information. That is a digital value from 00000000 to 11111111 : 0 -> 255. 256 levels. Resolution is only 1 in 256. Not very good if screen resolution is 1024 x 1024! Therefore may have to use 10 or more bits -> Hardware becomes more sophisticated and sensitive. Noise may become a problem since 1 bit may now represent 5 millivolts.

Note T4010 uses this approach with two potentiometers to control X and Y coordinate of cross hair. A joystick will also use this mechanism.