Lab 1 Optical Communication Link

Lab 1 Optical Communication Link

ECE 363Lab 1Fall 2017

Lab 1 – Optical Communication Link

GOAL

The overall goal is to build an optical communication link based on a LED transmitter and photodiode receiver.

OBJECTIVES

1)Design, simulate, build, and test an LED transmitter (BJT switch).

2)Simulate, build, and test a photodiode receiver (JFET-input op amp).

3)Trim a 741 op amp.

4)Build, test, and simulate an inverting amplifier using a trimmed 741 op amp.

5)Measure the 20 Hz signal from the test LED.

6)Measure the transmitted signal from a TV remote.

GENERAL GUIDELINES

1)Each student builds his/her own circuits.

2)Test stations (e.g. oscilloscope, computer) must be shared.

3)You are allowed (even encouraged) to help each other. Of course Professor Hedrick will be around to provide assistance.

4)Use neat wiring for your circuits! Starting in Lab 2, a messy circuit will cost you 1pt (out of 10 for the demo).

5)Keep your circuit components organized to make it easier to do the subsequent labs!

6)Each student must turn in his/her own lab report. See the course website for the template.

Honor Code Compliance: You must turn in your own work! Blatant duplication of circuit analysis, design, simulations, and/or lab reports will result in ZERO points and possible reporting to the Honor Council.

NOTE: There will be one lab exam near the end of the term. The lab exam will test each student’s ability to use Multisim and prototype a circuit.

Parts and Materials

  • Oscilloscope, scope probes, function generator, BNC+alligator clips, banana cables, digital multimeter
  • Lab kit containing breadboard, wires, wire stripper, and other tools
  • Calculations and simulations of your circuits from PreLab1
  • 2N3904 transistor, TL081 JFET-input op amp, LM741C bipolar op-amp.
  • Red LED, OP906 photodiode
  • Resistors: 10k (brown/black/orange) (three)

100k (brown/black/yellow) (three)

1Meg (brown/black/green) (one)

10k multi-turn potentiometer(one)

  • Capacitors:1 nF (blue)(one)

0.1 uF (yellow)(four)

PART 1: LED Transmitter

In this section you will build and test your LED transmitter from PreLab1. Some comments:

  • Build the circuit on the LEFT side of the board.
  • Make sure the LED leads are bent so that light is transmitted to the RIGHT.
  • Agilent function generator:
  • Configure to “High Z” output.
  • Square wave: 20 Hz, 50% duty cycle, 5 V peak-to-peak, 2.5V offset
  • Use the coaxial cable + alligator clips to apply the square wave to the transmitter
  • Use the coaxial cable to connect “Sync out” to the “External Trig” input of the scope.
  • Scope measurements.
  • Use the scope probe to measure a few cycles of Vinand Vo.
  • Check that your scope measurements have reasonable agreement (e.g. within 10% error) with your simulations.
  • Based on your measurements, compute the LED current and transistor base current when the LED is on.
  • Record scope snapshots of Vinand Vo. You can insert a USB stick into the scope and save a scope snapshot directly to the portable drive.

Demo this circuit to Professor Hedrick (2out of 10 pts of lab demo grade).

PART 2: photodiode receiver

In this section you will build and testa photodiode amplifier (see figure) using a TL081 JFET-input op amp. Some comments:

  • Build the photodiode amplifier on the RIGHT side of the breadboard.
  • The longer pin of the OP906 photodiode is the “positive” side.
  • Make sure the photodiode is facing the LED.
  • See the TL081 datasheet (course website) for the pin diagram.
  • Try to place the .1 uF capacitors fairly close (e.g. within 1 inch) of the op amp. These bypass capacitors are important to minimize voltage spikes in the op amp output.
  • The 1 nF capacitor is important to minimize feedback oscillations in the transimpedance amplifier.
  • It is wise to make a “light shield” over the photodiode -- just use a small piece of folded paper.
  • Scope measurements.
  • You should see that the photodiode is sensitive to the room lights (e.g. waive your hand over the photodiode).
  • Use the scope probe to measure and record a few cycles of V_PD.
  • Based on your measurements, compute the peak-to-peak voltage of V_PD – this is the desired signal. You should get something around 30 mV, but a much higher or lower value is perfectly OK.
  • Remove the 1 nF capacitor and comment on the appearance of the V_PD scope trace.
  • Put the 1 nF capacitor back in – it is important for a clean signal!

Demo this circuit to Professor Hedrick (2 out of 10 pts of lab demo grade).

PART 3: op amp trimming

In this sub-section you will measure and then trim the output voltage offset of a 741 op amp. Some comments:

  • Build the 741 op amp circuit shown on the right.

NOTE: Build it just above or below the photodiode amplifier.

  • Make sure to connect the 10k pot to pins 1 and 5. The wiper goes to -9V.
  • Measure and record VOUT using the scope.
  • Adjust the 10k pot to zero the op amp output (monitor VOUT with the scope).
  • Demo the trimmed op amp to Professor Hedrick (2 out of 10 pts of lab demo grade).

PART 4: testing the optical data link

In this section you will hook attach the inverting amplifier to the output of the photodiode amplifier and demonstrate a working system.

  • The 10k resistor attached to pin 2, which is the (-) input, of the 741 op amp should now be connected to the output of the photodiode amplifier (see below).
  • Do NOT touch the 10k pot! Your 741 op amp is already trimmed (that was the point of the end of Section 3).

  • Record a scope snapshot of the LED signal.
  • Demo your circuit to Professor Hedrick (4 out of 10 pts of lab demo grade).
  • Now disable the LED transmitter (e.g. disconnect the function generator).
  • Aim the TV remote controller at the photodiode. A distance of about 1 foot is reasonable.
  • Press and hold down the “Power” button of the TV remote.
  • Adjust the scope to display a single acquisition of the binary code.
  • Record the snapshot.

(End of Lab 1)

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