ELEC 241 Lab

Experiment 2.3

Optoelectronic Signal Sources and Sinks



Since we will be building an optical communication system, we will need some devices for converting electrical signals to and from light, as well as to and from sound. We will look at two: the photodiode, which converts light into an electric signal, and the light emitting diode (LED), which converts electric current to light.

Part 1: The Photodiode

Step 1:

Set the oscilloscope controls as follows: V MODE switch to CH1, CH1 VOLTS/DIV switch to .2 V, CH1 AC-GND-DC switch to DC, and the TIME/DIV switch to 2 mSEC. (Use your own experience to select the remaining settings.)

Step 2:

Connect the short lead (cathode) of the photodiode to ground and the long lead (anode) to CH1 of the scope. You can use the BNC clip leads for this, but better way (which leaves your hands free) is to plug it into the breadboard and wire it to the socket strip on the interface board. (With the interface board connected as described above, pin 1 will be CH1 and pin 14 will be ground.)

Step 3:

Use the GND position of the CH1 AC-GND-DC switch to establish the zero reference. Return the switch to the DC position.

Step 4:

Note the voltage produced by the photodiode. How does it change when you cover the photodiode with your hand.

Step 5:

Set the CH1 AC-GND-DC switch to the AC position and the VOLTS/DIV switch to 5 mV. Pull out the CH1 magnifier. Adjust the timebase to produce a stable trace. What are the amplitude and frequency of the signal?

Question 3:

Explain the waveform you observed in the previous step.

Part 2: Light Emitting Diode

Step 1:

Set up the power supply: turn both voltage controls to zero and set the meter selector to the 6V supply. Don't connect the supply yet.

Step 2:

Using a 220 ohm (red-red-brown) resistor and your red LED, wire the following circuit.

First wire the resistor and LED on the breadboard.

There are two ways to connect the power supply to the circuit (use one or the other, not both):

The first way: Plug your BNC-banana adapter into the 6V supply terminals.

There is a bump on one side of the adapter to denote which prong is connected to ground. Be sure to plug this prong into the black terminal of the power supply.

Then use the clip leads to connect to the LED and resistor.

The other way: Use the BNC adapter as above, but use a BNC patch cord to connect the power supply to J1-3 on the interface board. Use two pieces of wire to connect ground (pin 14) to the LED and the J1-3 signal (pin 3) to the resistor.

Step 3:

Turn on the power supply. Slowly increase the voltage until you see the LED just begin to glow. Measure the voltage across the LED. If the LED doesn't light by the time the meter on the power supply reads 3 V, check your circuit to make sure the diode is wired in the correct orientation. Unlike a resistor or light bulb, the LED is polarized. The anode must be positive for it to glow. Reverse the LED and verify that this is the case.

Step 4:

Set the supply voltage (as read by the front panel meter) to 3, 4, and 5 volts. At each step note the brightness of the LED and the voltage across it.

Step 5:

Disconnect the BNC cable from the power supply and connect it to the Main Output of the function generator. Leave the other end connected to J1-3 of the Interface board.

Step 6:

Set the function generator to product a 100 Hz square wave. Make sure the -20dB Attenuator is Out and set the amplitude to minimum.

Step 7:

Increase the function generator amplitude until the LED begins to glow. Is the glow steady?

Step 8:

Slowly reduce the frequency of the function generator. At what frequency does the appearance of steady glow stop and noticeable flicker begin?

Question 4:

How does the number you measured in the previous step relate to the frame rate of television and motion pictures?

Part 3: Optical Communication, Take 1

Step 1:

Disconnect the BNC patch cord from CH1 of the scope and connect the BNC clip leads. Connect the photodiode to the clip leads.

Step 2:

With the LED still connected to the function generator as in the previous part, set the frequency to 100 Hz.

Step 3:

Hold the photodiode (pointing down) above the LED (pointing up). Adjust their relative positions to maximize the signal displayed on the scope. (It may help to shield the components from ambient light with your hand.) Describe the waveform. Is it what you would expect?

Step 4:

Set the function generator to produce a triangle wave. Sketch the waveform you see on the scope. Is it what you expected? Can you explain it?

Step 5:

Reset the function generator to produce a square wave. Slowly separate the photodiode from the LED, adjusting the alignment to maintain the best signal. What is the maximum distance over which you can transmit a recognizable signal. Hint: switch the scope to the AC position and increase the gain.

Question 5:

We now have several of the components we will need for the optical communication system we will build in Lab 9. What components are missing? What problems remain to be solved?