Circuits

Typical  electronics circuits  are created  out  of a basis  set of primitive elements such  as capacitors, voltage sources,  resistors,  transistors and inductors. This set of elements types  was selected to be nearly  based to primitive principles in our  physical understanding of electronic  circuits in terms  of  current, voltage, resistance, and  the rate of modification  of those  values over time.  So, when  we  buy  a physical resistor, we  go to think  only  of its resistance; and  when  we  buy  a capacitor, we only think  of its ability  to save  charge.   But, that  physical resistor has some capacitance, and  the capacitor has some  resistance.  Still, it gives  us in our  designs to have  systems that are as close to the perfect models as possible;  and it gives that people have been designing with these elements for years,  so that we may adopt the strategies that generations of clever people have designed.

The function of combining circuit  components is to connect  their end terminals with  conductors, a phenomena  that  is usually referred to as wiring.  And  our  function of abstraction is to define the constraints that a circuit components exerts on the voltages  and currents of terminals to which  the component is connected.

So, armed with the standard basis set of analog  circuit element, we may try to create a circuit to control  our robot.  We have a resistance that change with the light level, but we require a voltage  that does  so, as well.  We may achieve  this by using  a voltage  divider, which  is given in diagram. Using  an  abstract, constraint-based method  of the  behaviour of circuit  elements.

we can calculate the given relationship between Vout  and Vin:

So, if RA  = RB, then  Vout  = Vin/2.  Or, in our circuit, if RA actually changes  with  the amount of light shining on it, then Vout  will also modify with the light

Figure 1.3        Voltage  dividers: A. Resistor  divider. B. Connected to a motor,  in a way  that  divides the abstraction from part A. C. Connected to a motor,  with a buffer.

So, now,  we can  imagine that  we may  use this voltage difference that  we have build to operate  the motor  at different velocity, relaying on the light hitting the resistor, using  a circuit something like the one given in diagram B. But, sadly, that will not work.  It creates out that once we connect  the motor  to the circuit,  it actually modifies the voltages, and  we may no longer maintain the voltage  difference used to drive  the motor.

So, although we have created an abstract design  of the behaviour of circuit elements, which lets us analyze the  behaviour of a particular complete circuit  model, it does  not  provide  us modularity.  That is, we cannot  prepare two  parts  of a circuit,  understand each of their  behaviours, and then  obtain the behaviour of the composite machine depend  on the behaviour of the separate components.   Instead, we could have  to reanalyze the joint behaviour of the whole  composite system. Lack of modularity creates  it very difficult to develop large machine, because  two different person, or the same  person at two  different times,  cannot  design parts  and  put  them  together without understanding the entire.