Optimizing Temperature Alarm Circuit with Thermistor and Resistor

[Stripe]

Homework Statement

Ok so for Diagram A:
It is desired to set up an alarm to indicate when it is too cold. For this purpose the circuit shown is to be used and it is intended that the globe will go on only when the temperature falls below 18C. The positions A and B in the circuit are to be occupied by a resistor and a thermistor (20 ohms resistance at 100C and 500 ohms resistance 0C.)
Which of the following combinations would be successful?

A is thermistor B is 1 ohm resistor

A is thermistor B is 100 ohm resistor

A is a 1 ohm resistor B is a thermistor

A is a 100 ohm resistor B is a thermistor

AND then diagram B:

It was imagined that the globe should glow but did not. If all components were correctly soldered, which of the following could best explain the fault?

The thermistor is too hot.

The base voltage needs to be lowered

Resistance of the transistor is too low

The 100 Ohm resistance should be bigger.

Homework Equations

http://img35.imageshack.us/img35/3010/mathhh.png

The Attempt at a Solution

Ok so part a, i thought that the answer was one of the first 2, because i thought that because the electricity would reach a first, it would have to be changed there before it reaches the base.

I thought that if A was a normal resistor, say 100 ohm, regardless of what happens at B, the base will always receive the same voltage because if B was the thermistor in this case, it would only affect the voltage after it?

Along with this could someone explain the importance of A and B? i thought the base would still work if b was not present? Then i thought maybe B was there to create a parallel circuit so A could work?

For part B, the answer was that the 100 ohm resistor should be bigger. BUT WHY? isn't the 100 ohm resistor located AFTER the transistor so it makes no difference?

and i was told that voltage never changes but current does? GOD I AM CONFUSED

:| please help

 

[Delphi51]

Well, the circuit is too complicated to understand at a glance, but if you realize that the current through the base of a transistor is "very small", and ignore it - then you can understand it. Just ignore the transistor for a moment. Then the resistors a and b form a voltage divider; they pick out part of the battery voltage. If a and b have equal resistance, then the voltage where they join will be half the battery V. If b is bigger than a, it picks off more of the battery V. In fact the voltage is Rb/(Ra +Rb)*battery V.

V must increase to turn the transistor on so it will turn on the light. So b has to increase (or a has to decrease) when it gets cold. This determines whether you put the thermistor at a or at b.

What base voltage do you need to turn on a transistor? I think it is 0.7 but I'm not sure (it has been a while for me). Anyway, if you know the turn-on V, and can estimate the resistance of the thermistor at 18 degrees, you can calculate the R for the fixed resistor.