EE215, Homework 2



Operational Amplifiers: Infrared Remote Control Tester

Due: At recitation week of May 14-18

Objectives

At the end of this lab you will be able to:

Construct and test an infrared (IR) sensor circuit - see the invisible! (well, see if the invisible is present, anyway)

Amplify a small signal with an op amp

Construct a variable gain amplifier from an op amp

(Extra credit) Build a bar graph output display

Hint: datasheets can be found at and other IC manufacturer sites

Materials and Supplies

In addition to the usual parts, you will need to obtain an infrared remote control from a TV, VCR, cable box, stereo system, or other. If you do not have one at home, try borrowing one from your group. You will also need the following new types of parts from your kit:

Infrared Phototransistor

LM741 Op Amp

a. LM3915 Integrated Circuit (Bar Graph display driver)

Bar Graph Array (Display)

b. .01μF capacitor

The infrared phototransistor is the clear bullet shaped plastic thing that looks like a clear LED. The flat on the side is next to the short lead, which is the collector. This particular transistor allows infrared light to substitute for the base current. The more light, the more collector current flows.

The LM741 Op Amp is the small 8-legged black plastic bug-like thing stuck in the black foam. The foam is conductive and protects the integrated circuits from static electricity. It's a good place to keep them until you put them in a circuit. The top of my LM741 is marked HA17741, and appears to be from Hitachi.

Since the bar graph and driver are part of the extra credit work, you can figure out which is which on your own! The capacitor is a brown disk-like shape with two parallel leads. The smaller one will be the 0.01 μF, marked 103M in my kit.

Procedure 1

Construct Circuit 1 on your breadboard. The flat on the rim of the phototransistor body, which is on the side of the short lead, is the collector - the lead without the arrow, marked pin 2. Connect your voltmeter (multi-meter) across the 4.7kΩ resistor R3.

a. (5 points) Measure the voltage across R3 under the following conditions:

(1) Remote control off (no button pressed)

(2) Remote control turned on at various distances (1”, 2”, 3” etc) from the phototransistor. (Comment: You have to push a button to turn on the IR emitter in the remote. I recommend using the volume button. The remote actually transmits a series of pulses, not a constant signal. You will observe the average voltage with your meter. Make sure you point the remote directly at the phototransistor.)

•

b. (10 points) Plot the voltage across R3 as a function of distance between the phototransistor and the remote control. What type of relationship do you observe on the plot (exponential, linear, etc.)?

b.

c. (5 points) Determine the distance between the remote and the phototransistor at which the voltage seen on your meter ceases to change significantly when you turn on the remote control.

Note: The phototransistor is sensitive to visible light as well as IR. Also, IR is heat and your body produces IR. This experiment is best done in a darkened room with out much movement on your part. Consider building a tubular shield around the phototransistor.

[pic]Circuit 1: The Phototransistor

Procedure 2

Add the LM741 op amp to your remote control tester circuit as shown below in Circuit 2. You can find a pin-out (which pin is which) for the 741 in Chapter 6 of your text or at the web site listed above. A small dot indented in the top of the circuit marks pin 1. The op amp package is called a dual in-line package, and has pins at the same spacing as your breadboard. The op amp should straddle the trough in the center of your breadboard, with each pin fitting in to a hole in a different row.

a. (5 points) Measure the voltage at the input (Phototransistor Pin 2) and at the output (op amp Pin 6) with respect to the reference node (negative battery terminal) under the following conditions:

• (1) Remote control is turned off

• (2) Remote control is turned on at various distances (1”, 2”, 3” etc) away from the phototransistor

b. (5 points) Calculate the gain provided by the op amp circuit from the measurements in part a. (The gain of the overall circuit, not the op amp itself.)

b.

c. (5 points) Plot the input voltage to the op amp circuit as a function of distance between the phototransistor and the remote control.

c.

d. (5 points) Plot the output voltage of the op amp as a function of distance between the phototransistor and the remote control for the same distances that you plotted the input.

d.

e. (10 points) What is the function of the op amp circuit in Circuit 2? What is the theoretical gain, calculated using nominal resistor and op amp values? How does the theoretical value compare with the measured value you calculated in part b?.

[pic]Circuit 2

f. (10 points) For Circuit 2, at what distance between remote control and phototransistor does the LED stop lighting? Is this distance more or less than the distance in Step 1 that ceased to produce a measurable voltage across the resistor R3? If they are different, why? If they are not different, why not?

e.

Procedure 3

Redesign circuit 2 so that the op amp circuit gain is variable and can be controlled externally (using a knob or tweaker) to adjust the range of your remote control tester.

a. (10 points) Put a drawing of the new circuit in your lab report. Build the circuit, and (20 points) show it to the TA in recitation (bring a remote control with you).

b. (5 points) How much can you vary the gain using your method?

c. (5 points) Plot the change in performance for at least two different settings of the LM741 and its resistors using your new configuration. Your plots should contain output voltage vs. distance between phototransistor and remote control for each of at least two settings of your new circuit.

Extra Credit (10 points max)

f. Add the display driver integrated circuit LM3915 and the bar graph array to your circuit from Procedure 3 to construct a circuit like Circuit 3, but with your Procedure 3 modification. Find the datasheet for the LM3915 (probably not in your text!). What is the LM3915 doing for your circuit?

Hold the infrared remote control 1” and 2” away from the phototransistor. How does your output change as compared to the circuit from Procedure 3 (what additional information is provided in Circuit 3 as compared to Procedure 3)?

g. Is the output from your remote control constant? If not, what does it do? Why?

If you did the extra credit circuit, bring it in place of the Procedure 3 circuit to the recitation.

(Note: the bar graph array is symmetrical from top to bottom, so you will have to figure out which way the LED’s are pointing. You can do this by placing a battery and resistor in series with one of the LED’s in the array and noting the orientation of the array when the LED turns on.).

[pic]Circuit 3

1.

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