ELEC 242 Lab

Experiment 2.1

Optical Coupler

Equipment

Components

The basic structure of an LED shining on a photodetector (photodiode or phototransistor) has a variety of different implementations. The simplest (called an optical coupler or optoisolator) is just an emitter and a detector imbeded in a block of plastic. These are used for transfering signals between two circuits which cannot be directly connected electrically (due to large voltage differences, noise, etc.). We will examine one of these to get a feel for the behavior, then modify it by allowing an external motion to block or unblock the path of the light, thereby serving as an electromechanical transducer.

Caution

There are three components in your parts kit with very similar appearances, but very different functions: the photodiode, phototransistor, and the high-brightness LED. Here's how to tell them apart: The photodiode has a flat topped package, the chip in the phototransistor is mounted on a flat pedestal, and the chip in the high-brightness LED is mounted in a bowl-shaped reflector. See the close-up pictures for details.


Step 1:

 Press the phototransistor into the end of the holder having the flattened side.
It should be a snug press fit. If it is loose, tape it as described in the next two steps.

Step 2:

 Punch two small holes 0.1 inch appart in a piece of black paper tape. Push the leads of your red LED through the holes with the sticky side toward the body of the LED.


Step 3:

 Push the LED into the hole in the other end of the holder and use the tape to hold it in place.


Step 4:

 Put a piece of black tape over the slot to keep out ambient light.

Step 5:

 Plug a phone plug patch cable into P4 of the interface board. Plug the LED into the 3-pin connector on the other end, connecting the long lead (anode) to the pin nearest the white stripe and the short lead (cathode) to the center pin. If you have one of the cables with the covered connector, here's how it's wired:


Step 6:

 Plug another phone plug patch cable into P5 of the interface board. Plug the phototransistor into the 3-pin connector on the other end, connecting the long lead (emitter) to the pin with the white stripe and the short lead (collector) to the center pin.


Diversion:

 As in the previous Lab we used the +20V power supply as a signal source. We will be using the other half (0-6V) as a source of power for our circuits. Since we will need to have the power voltage (and ground) available at many points in the circuit, it will be convenient to connect them to the bus strips that run through the breadboard.

Step 7:

 Connect the green binding posts on the breadboard to the upper row of the top bus strip. Connect the red binding post to the lower row. Connect the gaps at the center of the bus strip to form two full width power buses.

You now have a power and ground bus that looks like this:



Step 8:

 Set the METER SELECTOR switch on the power supply to 6V. Adjust the 0 to 6V voltage control to produce 5 volts.

Step 9:

 Make sure the green banana plug from the interface board is plugged into the green binding post on the breadboard. Use a red banana patch cord to connect the 0 to 6V Plus terminal of the power supply to the red binding post on the breadboard. With a green cord, connect the 0 to 6V Minus terminal to the green binding post.

Step 10:

 Plug your BNC-banana adapter into the COMMON and 0 TO +20V terminals of the power supply, with the ground bump in the COMMON terminal.


Step 11:

 Plug a BNC patch cord into this adapter and connect the other end to P3 on the interface board. This will bring the output of the 20 V supply to pin 30 on the interface board socket strip.

Step 12:

 Wire the following circuit. The numbers on the connector symbols ( ) are the pin numbers on the interface connector socket strip (P10).
It might look something like this:
Note the blue border on the above picture. That means you can click on it to get a larger, more detailed picture.

Step 13:

 Set the METER SELECTOR switch to the +20V position. Vary the supply voltage (as read by the front panel meter) between 0 and 20 volts in steps of 2 V. At each step measure (with the DMM) and record the voltage across each of the two resistors.

Step 14:

 From your measurements, compute the current through both the LED and the phototransistor. Make a plot of the phototransistor current vs. the LED current. Is it a straight line as we expect?