Night Light Circuit Lab

Objective: 
We reviewed the representation of a current controlled current sources (CCCS) and other dependent sources. Furthermore, we reviewed how to add resistors in series where Req = R1+R2+.. and adding resistors in parallel as 1/Req = 1/R1+1/R2 = R1+R2/(R1*R2) that can be rearrange into Req = R1*R2/R1+R2. The main objective is to understand the idea of voltage division and using the Req to find certain values.

Hot Dog Experiment:
1. A hot is connected to an alternating voltage source as seen in Figure 1. The focus of the experiment is to see what the result would be for the hot dog. We hypothesized that the hot dog may represent a human being and concluded that the hot dog would slowly cook and eventually burn. The result was exactly what we predicted.

2. The next step is to predict what would happen when led's are placed in a parallel and in perpendicular. The result was that the LED's that were in series turned on while the middle LEDs which were perpendicular were turned off. However, we noticed that the LED's suddenly turned off as the hot dog was burning. 

Figure 1. Hot dog experiment with an alternating voltage and LED's lights attached. 

Group Practice Problems:

1.  The circuit in Figure 2 shows that there are dependent sources of a current controlled current source (CCCS). The objective is to find the unknown voltage across the resistor of 8 ohms. We calculated the Vo by using Kirchoff's current law. 

Figure 2. A circuit with a current controlled current source and dependent source.

2.  The problem in Figure 3 tells us to to find the voltage across R1 and R2. In order to this, we use voltage division. This means that we will need to the total Current which is I =Vin/R1+R2. So, the voltage at R1 is V1 = R1*(Vin/R1+R2) and V2 which is also the Vout is V2=Vout= R2*(Vin/R1+R2)

Figure 3. Circuit where Resistors are in series. 

3. We created a circuit where we ran a 12V output and 3.3V ADC and can sink at a max of .01A. The question is to find the necessary resistance for the appropriate circuit. Using the idea of voltage division, we figured that the voltage at R1 should 8.70 and at R2 should be 3.3V. Using the equation R = V/I, we can acquire the the resistance at each Resistor. We found that R1 = 870 ohms and R2 = 330 ohms as seen in Figure 4. 

Figure 4. Creating a circuit with the appropriate resistor.

4. The problem below in figure 5 shows an LED with running voltage of 4V. The problems asks to find the appropriate resistance for the circuit. We found that the needed resistor according the range of current gives us 3.3 ohms<R<10 ohms. However, this is not out true answer as we must find the appropriate resistor in hand. We found a 7.5 resistor in class that falls within the range. 

Figure 5. A circuit with an LED and resistor with a running voltage of 4V. 

5.  In the end, we followed the initial instructions for the lab where we needed to find the voltages at the photocell. As seen in Figure 5. We acquired a voltage at a resistance of 5K of Vb = 1.7V and Vb = 3.33V at 20K. We found the current by adding the resistors in series and finally calculating the above voltage at each resistor using ohms law V=IR. 

Figure 5. Pre Lab calculations. 

Dusk to Dawn Lab Procedures:

1. We create a circuit as seen in Figure 6  and Figure 7. where there is a photocell, LED, a resistor of 10000 ohms, and a Bipolar Junction Transistor (BJT). The BJT can be considered as a current controlled current source.

2. We use the Analog Discover to measure the base voltage of the BJT and the voltage difference across the LED when the photocell is exposed to light. We acquired Vb =.91V+/-.05 and
 Vd =.54V+/-.05
3. In order to see the circuit behaved like a nightlight, we made sure that when the photocell is exposed to light, the LED does not light up. Yet, when the photocell is covered, the LED is turned on. We record the voltage when the photocell is covered. Vb = 2.45V+/-.05   and Vd= 1.93V+/-.05
4. A video can be seen here: https://youtu.be/EzZvij3H2_o

Figure 6. Circuit representation.

Figure 7. Actual circuit. However, the BJT was not correct which is why the LED is turned on. Though it had been fixed and can be seen at the link https://youtu.be/EzZvij3H2_o

Summary of the Lab and Overall Learning Outcome:

The goal of the lab is to connect the circuit according to Figure 6. We made sure it worked according to the instructions by checking that when the photocell is exposed to light, the LED should not turn on. Conversely, when the light is not being exposed to light by covering the photocell, then the light should turn which means that the circuit with the LED acts as a night light. The reason for which this occurs is because the photocell is a variable resistor where R~1/L. This means that when the luminosity is lighter, than the resistance would be lower. Furthermore, when the luminosity is lower,  the resistance is higher. Our final measurement results, respectively, for the voltage at Vd and Vb when light is off or when exposed to light are .54V+/-.05 and .91V+/-.05 When the light is on and not exposed to light, then the Vd= 1.93V+/-.05 and Vb =2.45V+/-.05. Our results for the pre lab differed since our theoretical results was Vb=1.7V at a resistance of 5k and Vb=3.33V at a resistance of 20K. Since our experimental resistance of R = 10K was between the theoretical resistance between 5K and 20K. Then it make sense that our experimental result of Vb = 2.45V falls between our theoretical voltage range of 1.7V and 3.33V