Week 4: 1-5, February 2016

  1.     (Table and graph) Use the transistor by itself. The goal is to create the graph for I_C (y axis) versus V_BE (x axis). Connect base and collector. Use 10K potentiometer to generate the voltage. Use 5 V but DO NOT EXCEED 1 V for V_BE. Make sure you have the required voltage value set before applying it to the base. Transistor might get really hot. Do not TOUCH THE TRANSISTOR! Make sure to get enough data points to graph. (Suggestion: measure for V_BE = 0V, 0.5V, and 1V and fill the gaps if necessary by taking extra measurements). The circuit should look like below:

Circuit 1 Schematics

With the circuit setup in accordance with the schematic above, we were able to use the 10k potentiometer to apply various voltages to the base of the transmitter.  As the voltage increases from 0-0.6V there is only a slight increase in current at the Collector Node.  When the voltage increases to 0.65V the current begins to increase exponentially reaching a peak of 4.18 mA when we stopped the experiment at 0.66V to prevent potential damage to the transistor.

This table shows the values we recorded for VBE and IC

Chart showing the Exponential increase in Current at the Transistor's Collector Node as Voltage at the Base increases past its trigger point.

  2.     (Table and graph) Create the graph for I_C (y axis) versus V_CE (x axis). Vary VCE from 0 V to 5 V. Do this measurement for 3 different VBE values: 0V, 0.7V, and 0.8V. The circuit should look like below:

Circuit 2 Schematic

This table above displays the values we found for this particular experiment.

With 0 Volts present at the Transistor Base only 1 mA of Current was present at the collector base

With 1 Volt present at the Transistor Collector Current increased gradually from 0 to 14.8 mA as the Base voltage increased from 0 to 0.7v between 0.7 to 0.8v the current began increase exponentially peaking at 79.2.

With 2 Volt present at the Transistor Collector Current increased gradually from 0 to 29.3 mA as the Base voltage increased from 0 to 0.7v between 0.7 to 0.8v the current began increase exponentially peaking at 142.

With 4 Volt present at the Transistor Collector Current increased gradually from 0 to 23.8 mA as the Base voltage increased from 0 to 0.7v between 0.7 to 0.8v the current began increase exponentially peaking at 272.

  
  3.     (Table) Apply the following bias voltages and fill out the table. How is I_C and I_B related? Does your data support your theory?

I_C and I_B are related by beta, which is equal to I_C divided by I_B . For example, if beta was equal to 100, that would mean that whatever the current for I_B was, the the current for I_C would be 100 times greater. Our data did not support this. We believe that our transistor was damaged in the previous circuit, yielding us inaccurate results. 

  4.     (Table) Explain photocell outputs with different light settings. Create a table for the light conditions and photocell resistance.
   
    A photocell provides different resistance values depending on the amount of light that it receives. The more light it receives, the less resistance it supplies. Our data we collected supports this.

This table illustrates the resistance value measured across the Photo Sensor under various lighting conditions. 

  5.     (Table) Apply voltage (0 to 5 V in 1 V increments) to DC motor directly and measure the current using the DMM.

This table illustrates the current drawn by the motor as the voltage increases from 0-5V in 1 volt increments. 

  6.     Apply 2 V to the DC motor and measure the current. Repeat this by increasing the load on the DC motor. Slightly pinching the shaft would do the trick.

    With 2 volts applied increasing pressure was applied to the shaft of the motor.  The amount of current being drawn increased from 36.1 mA with no load applied, to 97.3 mA at the point sufficient pressure was applied to stop the motor. 

The table above shows the current being drawn by the electric motor as various amounts of pressure are applied to the spinning shaft.

  7.     (Video) Create the circuit below (same circuit from week 1). Explain the operation in detail.

Rube Goldberg Circuit Schematic

The Circuit above operated by using a photo sensor in conjunction with a 1k W resistor to control the voltage present at the base of the transistor.  A second power supply provides power to the transistor's collector node.  As the light intensity increase the photo sensor's resistance decreases, this increases the voltage present at the transistor base, allowing voltage to flow from the Collector to the Emitter providing power to the motor.

Video of the rebuilt Rube Goldberg Circuit in action.

 
  8.     Explain the 47 Ohm Resistor's role by changing its value to a smaller and bigger resistors and observing the voltage and the current at the collector of the transistor.

  The 47 Ohm resistor in the Rube Goldberg circuit serves as a buffer that prevents excessive current from the power supply from damaging the transistor.  Ohm's Law dictates the if the buffer's resistance is doubled the current value is halved, and if the resistance is halved the current will double.  This prediction is confirmed by the data we collected which can be seen on the table below.

Chart of Current and Voltage present at the Transistor's Collector Node.  

  9.     (Video) Create your own Rube Goldberg Setup.

  Our team's approach to the Rube Goldberg circuit is to utilize the a paddle attached to the motor to knock over a Formation of Dominoes that can then be used to trigger the next portion of the circuit. 

A successful test of our Rube Goldberg Circuit knocking over a Formation of Dominoes

* Video of our previous "failed" tests are posted on our Lab Team's You Tube Channel at

Lab Team 4 on YouTube