03/10 - Lab 7: Mesh Analysis 2, and Lab 8: BJT Curve Tracer

Lab 7 - Mesh Analysis 2

In this lab, we analyzed and built a circuit containing multiple power sources. We were instructed to first analyze the circuit found in the lab manual, shown below:

Expected Values: V_1 = 5 V, V_2 = 0.24 V, and I_1 = 0.330 mA

Everycircuit confirms our prediction

After using a little bit of mesh analysis, but mostly everycircuit, we were able to find the values for I_1 and V_1. We then built the circuit using a solderless breadboard and the indicated resistors.

Closeup of the circuit design

The multimeter shows V_1 = 4.95 V, which is -1% from our predicted value of 5 V

Multimeter indicates V_2 = 0.24 V, which is exactly what was predicted (0% error)

The multimeter indicates I_1 = 0.320 mA, which is -3% from our predicted value

As the results of the lab show, our predicted values were very close to the actual values in the circuit. V_1 = 4.95 V (predicted 5 V ; 1% error), V_2 = 0.24 V (predicted 0.24 V ; 0% error), and 

I_1 = 0.321 mA (predicted 0.330 mA ; 3% error).

Lab 8 - BJT Curve Tracer

The purpose of this lab was to understand how transistors operate within a circuit. We were instructed to build the circuit depicted in the lab booklet, with each green box representing the lead to connect to the analog discovery box.

We had to create a custom waveform in order to achieve the "step" cycle going into W2, which was outlined in the lab manual. After setting up the waveform, we constructed the circuit on the wireless bread board:

Closeup of the curve tracer circuit

5 lines we were expected to see as outlined in the lab manual.

Because I suck at taking pictures of graphs, the values for I_C are not shown, however the horizontal markings start at 0 with the bottom line, and increase by 0.025 mA per line. Thus, the blue lines in the photograph land on approximately 0, 10, 20, 30, and 40 µA from bottom to top.

In Class Exercises:

1. Find IB , IC , and vo in the transistor circuit. Assume that the transistor operates in the active mode and that β = 50. Hint: recall that Vbe = 0.7 V in active mode.

Me doing ENGR 44 homework:

03/08 - Lab 6: Nodal Analysis

Lab 6 - Nodal Analysis

In this lab, we built a circuit containing multiple power sources in parallel, and used the node voltage method to predict the voltages across two of the resistors in the circuit. We used the V+ wire on the analog discovery to supply 5V, and we used the V- port to supply -5V to the circuit. To supply a -3V, we used the waveform generator port. The circuit constructed is depicted below:

Using the nodal analysis method to solve for the circuit, we calculated V_2 to be in the range of -0.63V to -0.67V. This range is due to the ± 10% accuracy of the resistor.

After running these numbers, we constructed the actual circuit. Below is a closeup of the circuit, followed by the measured voltages at each node.

In-Class Examples

1. Given the circuit below, solve for Vx.

2. Given the circuit below, solve for the loop currents i1 and i2 indicated using mesh analysis.

3. As a final exercise, we earned how to read the color coding on resistors:

03/03 - Lab 5: Temperature Measurement System

Lab 5 - Temperature Measurement System

In this lab, we designed a circuit that would be able to provide a way to measure temperature using a solder-less bread board and a thermistor (a resistor that varies with temperature). Essentially, the circuit was built to vary the output voltage due to the changing temperature of the thermistor.

Before building the actual circuit, we analyzed the circuit depicted in the lab book. First, we determined V_out as a function of R_th and R. Next, we verified that V_out in the equation increases or decreases with the temperature. Last, we chose a value for R such that V_out changes by at least 0.5V over the specified temperature difference.

According to our analysis, V_out = (R * V_s)/(R_th + R). We tested the hot and cold resistances of the thermistor at 24°C by recording the resistance using a multimeter, then squeezing it between Andrew's fingers to obtain the resistance while at 37°C. At 24°C, the resistance was found to be 7.3 kOhms, and at 37°C, the resistance was found to be 11 kOhms. Plugging these values into the equation showed that there were two possible fixed resistance values that would give us a resolution of 0.1 °C per bit. We needed to choose either a 6.681k or a 12.018k resistor. Since neither of these resistors exist in real life, we had to choose a standard resistor that would be close enough to the calculated values to give us a good result. We had the choice between a 6.8k and 12k resistor, so bwe went with the 12k resistor because it is much closer to the respectie value obtained through calculation. According to the calculations, the voltage was predicted to be 2.6V at 24°C and 3.1V at 37°C.

The photos below depict our actual experiment building the thermistor circuit:

Cold voltage = 2.59V

Warm Voltage = 3.12V

As shown, our predicted values matched the values obtained from the actual circuit. Cold temperature had a percent difference of -0.4%, and the hot temperature had a percent difference of +0.6%. 

In-Class Exercises:

1.  Calculate the Node Voltages in the following circuit:

03/01 - Lab 4: Dusk to Dawn Light

Lab 4 - Dusk-to-Dawn Light

In this lab, we created a light-sensitive LED using a photocell, bipolar junction transistor, and a resistor. If all works out, the LED will be unlit if there is light shining on the photocell. When the photocell is covered up (or the room gets dark), the LED should turn on, similar to a night light for kids. We started by running a few calculations through our circuit diagram in order to determine the correct orientation and values of the resistors.

Calculated values of Vb for photocell resistances of 5 kOhms and 20 kOhms.

We found that the resistance of the photocell under light is 11 kOhms, and it is 62 kOhms when covered up. We reran calculations to find Vb for the actual circuit, which we found to be 2.6V. When we measured the actual voltage across the photocell, it turned out to be 2.5V.

Actual set up on the breadboard.

In this video, you can see the light turn on when the photocell is covered and off again when the finger is lifted.

In-Class Examples:

1) In the beginning of class, we were shown a demonstration involving running electrical current through a hot dog. LEDs were placed in various configurations along the hot dog, and we were asked to guess how the hot dog would react to the applied voltage and which lights would turn on. write our responses on a white board.

2) Find current io and voltage vo in the circuit shown:

5) Find vo and io in the circuit:

4) Find the equivalent Resistance of the following network:

5) To determine the voltage across each resistor substitute the expressions derived for the current into Ohm’s law. Write the voltage across each of the resistors.

6) An automotive pressure sensor has an output between 0-12V and will be interfaced to the TI MSP430 that has a 3.3V ADC and can sink a maximum of 10mA. Model the sensor as an ideal DC source and draw schematic for the appropriate interface circuit.