Transistor Labs

Force sensitive resistor and LED

1. A. Force sensitive resistors at doorways, so that lights turn on if the resistors activate in a certain direction (have multiple resistors), and off in the opposite.
B. Force sensitive resistors to determine which side of the elevator you are on. Eg for staircases with the machines that act as elevators, whichever side has an activated resistor is the side that the person is coming from.

Temperature sensor and LED

1. You technically are still using a voltage divider circuit. For the others, the other resistors had the ability to have a voltage drop of 0 (meaning resistor not activated). Here, in real-time terms, we're never going to reach temperatures close to absolute zero, so there is always some resistance in the temperature. The 3 pin nature of the temperature resistor alludes to a voltage divider circuit still being used, just internally.
2. Whatever interactions rely on this sensor need to be based on a scales of large amounts of time. Eg using a temperature sensor to detect if someone walked into a room or not wouldn't work. However, it'd work decently knowing whether to turn on the heater or not.
3. A. Turning on the heater or A/C in a room.
B. Tuning shower water temperature to a comfortable one (user has a specified preferable temperature, shower automatically attempts to keep water around that temperature).

Transistor as switch

1. The circuit involving the switch has a 10k ohm resistor. The circuit involving the transistor has a 560 ohm resistor. By applying Ohm's law where the voltage is 5 V, the current in the transistor circuit must be higher than the current in the switch circuit, which explains the brightness difference (more current implies brighter LED).
2. According to http://electronics.stackexchange.com/questions/83685/need-help-calculating-resistance-for-transistor-base, the transistor has a resistance of about 1k ohms. However, given the question asks to ignore the effect the transistor might have, we can say it has a resistance of 0 ohms. Then the circuit containing the transistor has 5/560 A, or about 9 mA, and the circuit containing the switch has 5/10000 A, or about 0.5 mA.

Transistor as amplifier

1. When the potentiometer is at minimum resistance, the LED is very sensitive to changes in the photoresistor. When the potentiometer is at maximum resistance, the LED is not very sensitive to changes in the photoresistor. Maybe I have that backwards because when looking at the circuit, I didn't know which end of the potentiometer implied minimum (or maximum) resistance.

Transistor controlled by Arduino

switch

amplifier

(no questions)