ELEC 243 Lab

Experiment 2.3

Optoelectronic Transducers

Components

Last week we used the photodiode to measure the ambient illumination level and other apparantly DC values. In fact the light in the lab has a significant AC component whose frequency is faster than our eyes can perceive. By using the oscilloscope to observe the photodiode output signal, we can separate the AC and DC components of the light level.

With certain materials, the process by which the photodiode converts photons to electrons can be run in reverse, converting electricity to light. The photodiode we are using is made of silicon, which is not one of those materials. However, in our parts kit are a number of diodes which are made out of such a material (e.g. gallium phosphide) which can convert electrons to photons. These diodes are called, somewhat unimaginatively, light emitting diodes or LEDs.

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 a 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:

 Repeat the previous two steps using as a source (a) the under-shelf florescent lamp and (b) the incandescent lamp. Sketch the shape of the AC component of the waveform for each source. What is its amplitude and frequency?

Remark:

 The following procedure may make it easier to study the AC component of the signal: 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.

Question 4:

 Explain the waveforms you observed in the previous Step.

Part 2: Measuring Photocurrent

The physics behind the operation of the photodiode says that the current produced by the diode should be directly proportional to the irradiance of the incident light (ideally one electron of current produced for each photon of light absorbed). We might hope that the voltage would also be proportional to the irradiance, but as our measurements last week suggested, and our measurements next week will clearly show, this is not the case.

Since the oscilloscope (and the Lab PC) can only display voltage, we need to convert the current produced by the photodiode (called photocurrent) to a voltage in order to make quantitative measurements. We will find more sophisticated ways of doing this in Labs 5 and 6, but for now the only technique we have is to send the current through a resistor and allow Ohm's law to give us a proportional voltage.


Step 1:

 Keeping the setup from the previous Part, connect a 1~sp kΩ resistor (brown-black-red) in parallel with the photodiode.

Step 2:

 Adjusting the VOLTS/DIV switch as necessary, repeat the measurements of Part 1 using the same three light sources (overhead florescent lamps, under-shelf florescent lamp, incandescant lamp).

Part 3: 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.
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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.



Note
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 IN 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 5:

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

Part 4: Optical Communication, Take 1

In Part 1 we measured environmental light sources over which we have little control. In Part 2 we were able to create an optical signal containing whatever information we chose (or at least whatever we could get the function generator to produce). By comining the components of both Parts we can create an optical communication link.


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.