BioNB 440: Lab 1

Introduction to the lab hardware/software.

Introduction.

This assignment will introduce you to the lab environment we will use this semester. The lab will be partly simulation and partly construction of circuits. You will use several instruments to make measurements of the simulated or actual circuits.

Hardware

• PB503: Prototype board, power supplies, controls and signal generator
  • Rows of holes in the prototype board are connected together. There is a diagram of connections. The long rows of connected holes at the top and bottom are intended for power supply connections Using two short wires attached to the DVM leads, probe some of the many holes in the prototype board to discover/verify which holes are connected together. Set the DVM to use the "beep" range.

  • There is one fixed voltage and two variable DC voltages available on thePB503. Use the DVM to verify each voltage and check the range of the variable voltages. Set the DVM to DC voltage.

  • There are several switches and LEDs on the PB503. The switches produce one of two "logic levels", called "high" and "low". The red LEDs light if connected to a "high" level. Connect a wire between one of the toggle switches at the bottom-left of the PB503 and one of the LEDs at the top-rigth . Turn the light off/on with the switch. Use the DVM to measure the voltage applied to the LED.

  • There is a waveform generator which can produce sine, square, and triangle waves of variable frequency and amplitude. The output for the generator is marked as TTL or as sine/triangle/square. Connect the sine/triangle/square ouput to the J1 BNC pin connection and use a BNC cable to connect to the scope. Vary the controls on the signal generator and observe the shapes on the scope.

• Oscilloscope (scope)
  The scope is a device to draw a picture of a voltage vs time. Our scopes can:
  • draw two voltages vs time
  • start (trigger) the drawing at a time based on a third voltage (or one of the two displayed)
  • vary the time scale from microseconds to seconds
  • vary the voltage scale from millivolts to volts
  The scope we will use is a Tektronix TDS-1002 digital scope. Functions include:
  • Setting vertical voltage scale using the two knobs controls (one per channel) in the middle of the scope. In addition each input may be set to zero (GND), DC coupled, or AC coupled using the channel menu buttons. If an input is AC coupled then all steady voltages will be surpressed in the display.
  • Setting horizontal time scale using the knob control thrid from the left.
  • Setting the triggering mode in the right-most panel. This allows you to start the voltage-vs-time display at some interesting time determined by a specific voltage event. We will demo this in lab.
  • Measuring frequency, voltage, time.
  • Displaying the Fourier Transform of a signal.
  The scope features are slightly different from the one described in Introduction to the oscilloscope from U Delaware, but the overall operation is very similar. The TDS1002 users manual is good reading, and is specific to this scope.
  

• Digital voltmeter (DVM)
  The DVM can measure:
  • voltage (AC and DC ranges)
  • current (AC and DC ranges)
  • resistance
  • capacitance (not all of the meters can do this)
  • frequency (not all of the meters can do this)
  A little care is necessary when using the DVM. It is possible to damage the DVM by connecting it incorrectly. For instance, attaching the DVM dirctly to a battery while it is set to measure current will blow a fuse inside the DVM. The reason is that the current-mode has a very low resistance, so the battery attempts to supply a very large current. A general rule: To measure a current you MUST disconnect a wire in your circuit.

Software

• Windows
  WindowsXP is the operating system we will use in this class. I will assume that you have used either Windows or MacOS. We will bring everyone up to speed in the first lab.

• Electronics Workbench
  Electronics workbench is a program which allows you to draw electronic circuits in schematic form, then simulate their operation. You should refer to the assigned reading Introduction to Electronics Workbench for a brief overview. Specific exercises will be given below.

Make yourself a folder in the folder c:\My documents\ .

Download the Kirchhoff's law EWB program by right-clicking over the link and saving it to your directory. You may need to add the ewb extension to the filename in the dialog box. Start EWB from the shortcut on the desktop and open the file you just saved. you should see three circuits: two powered by a DC source, and one by an AC source. The following are schematics of the three circuits:

1. Circuit 1
2. Circuit 2
3. Circuit 3

Note that in this and every lab assignment the verb build means to construct a circuit on the protoype board whith actual parts. The verb simulate means use Electronics Workbench mathematically simulate the circuit.

1. Run the simulation of the three circuits in lab1kirchhoff.ewb. Vary all of the switches and variable resistors. You can do this by typing the key which is indicated next to each variable component. In the example shown below, the "v" key will vary the value of the resistor. Typing "v" alone will lower the value, while "shift-v" will raise the value. As shown, the resistor is set to 50% of its maximum value.

   

   Verify that Kirchhoff's laws are satisfied for all three circuits:

   • for all settings of the switches
   • for several variable resistor values
   • for different frequencies in the third circuit
   How accurate is the simulator? Can you make it fail to be accurate?

2. Build the first two circuits described in the EWB file and verify Kirchhoff's laws with the digital voltmeter.

   Build the third circuit. Vary the frequency of the AC source and show that the voltage across the capacitor drops as the frequency increases.

3. Simlulate a "model axon" as shown below. In real life you might stick a microelectrode into an axon and inject charge. Since the microelectrode has high resistance, it acts as a current source. Thus the current source represents one electrode, and the voltmeter another microelectrode.

   

   Creating all those resistors (and later in the experiment varying them) is tedious, so you should make a subcircuit containing two resistors. Then you can vary the values of all the resistors by changing one value, also, wiring the circuit is simpler. The figure shown below shows how the circuit might look with a subcircuit.

   

   Use a voltmeter to measure the voltage across the "membrane" between each subcircuit unit. Plot the curve of voltage vs distance. Change Rm to 4kOhm and repeat. How does the "length constant" change as you change Rm.