Day 3 Series and Parallel/ Dusk-to-Dawn Light

Series and Parallel

In the early class, we have the conversation with a graduate student on computer engineering about engineering career and experience.

The class lecture begins with the prediction of the hot dog attaching a line cord.

As our group predict, the hot dog is slowly cooked.

Then the professor attach some LED's on the hot dog: few parallel, few perpendicular. And let us predict what will happen. 

As we predict with the parallel attachment the LED will light up, while the perpendicular one will stay off. The voltage change will create the current, and the LED will light on. In the perpendicular connection, there is no voltage drop, so the light is off.

Then we come with the example to measure the voltage in a circuit. In addition, I understand more a about the voltage and current sources' symbols in the circuit, as well as the independent and dependent labels. 

Then we come up with another problem to use the KCL, KVL in order to find the unknown current and voltage

The professor show us another way to solve for the problem. This is more convenient in solving the problem if you choose right node to form the equations.

We learn about voltage divide and its formula. We also learn that the resistor that connect to the negative/ground of the source will be the v(out) in a voltage divider circuit.

Here is many types of resistors that we need to concern when establishing a circuit. As a engineer, we need to concern about how to bring the theoretical problem to the real world.

This is one of the possible situations that satisfies the requirement of the circuit with the use of two obtainable resistors.

We find the resistance of the above circuit.

We solve for an equivalent resistance problem.

DUSK-TO-DAWN LIGHT

We study the fundamentals about the diodes and LEDs, Bipolar Junction Transistors BJTs, and the photocell.

Before the lab, we do some calculation in order to predict what will happen in the lab. As we calculate, the Vb for photocell is 1.65V for 5k Ohm and 4.8V for 20k Ohm. 

Then, we set up the circuit based on the diagram. Tips: 1. diodes only let current passing in one direction, so the anode (longer pin) have to be connected to the positive side.

The set-up of the experiment.

The video with the result of the lighting-on LED.

The record of the Vb when the LED on and off. When the LED is on, Vd(diodes)=2.1V and Vb=2.79V. When the LED is off, Vd=0.15V and Vb=0.452V. The record is really off from our calculation, so we measure the resistance of the photocell directly from DMM. R(off) = 1.6k Ohm, and R(on)=20k Ohm. They are much different from our given values for calculation. With new values we calculate Vb(off)= 0.689V which is 34% different from the measured value , and Vb(on)=3.3V which is 15.5% different form the measured value.

Conclusion: 

Today we learn about the voltage divider and current divider with some practice problems, and perform the lab on the BJT current controlled current source with the photocell that can change its resistance according to the light intensity applied to the sensor. And we did the lab successfully as the above video addresses.