Photocells, Voltage&Current divider review, Equivalent resistance

Today in class we did an experiment with a moldy, smelly hot dog, involving plugging each end of the dog into a prong connected to 120V AC from a wall socket.  The acrid scent of burning hot-dog is now unfortunately seared into our collective consciousness.  The purpose of the exercise was to predict current behavior in the putrid log of moldy meat.  We found that current decreases over time, as the sac of trans fats and cancer cooked.  We confirmed this when Mason plugged LEDs into the hot pseudo-meat logs.  We found that the LEDs that were parallel to the axis of the once-vibrant-animal-turned-filth turned on before flickering out as current decreased.  Professor Mason seemed to take special pleasure in poking the meat and overstating its size.

We then went over the MOSFET lab.  We noted that a MOSFET can serve two purposes:

• To act as an on-off transistor between high and low voltages
• To (roughly) linearly increase current passed within a range of voltages on its gate.

We then did a few current and voltage divider example problems in class. They were simple enough to skip over. The homework problems were like these, but harder.

I suppose I should mention parallel resistors were brought up.  Post-break we did a simple parallel resistor problem. Onto more interesting things!

LAB

Our lab involved a potentiometer photocell to measure the light in the room.  We can qualitatively observe an LED get lighter or darker proportional to the amount of light in the room. 

We had a little discussion of anodes and cathodes of diodes and how they block current in one direction.

Reverse bias - no current flow. Two terminal semiconductor devices conduct current in only one direction.  

BIPOLAR JUNCTION TRANSISTORs

ie. MOSFETs, are our voltage controlled current source.  BJTs are conveniently modeled current dependent sources.

in the context of our lab, a low resistance across the photocell will turn the transistor off, and a high resistance across the photocell will turn the transistor on.

Below are our calculations for a hypothetical photocell with min/max resistance of 5k to 20k ohms resistance.

We measured the actual min/max voltages of the potentiometer, and found resistance of 100 in a light setting to 50k Ohms in a dark setting.
We calculated the minimum and maximum voltage on the MOSFET gate given the photocell potentiometer resistances.
We had predicted that our maximum voltage would get up to 4.3V.
1. calculated values for Rmin and max, measured with a potentiometer.

2. Measured values: However, in our measurements, the range of voltages was only .32V at the lowest, and .72V at the highest resistances for the potentiometer.  This was confusing, but we hypothesize that some current travels from the v_out point through the potentiometer, and some travels through the gate to the LED. 

3. Video

This would explain the unexpectedly low voltage, .72V, dropped across the potentiometer, as the LED and potentiometer in parallel in parallel would be, according to a quick calculation using the voltage divider and parallel resistances, 1.6k Ohms total, and the LED at 1.66kOhms; much less resistive than the potentiometer's 60k ohms.