Electronics Lab 7 Spring 2015

The Intelligent Light Bulb: A Negative Feedback Experiment

1. Measure the resistance of your new device (a photocell) both when it is exposed to light and when it is in the dark. Describe what you observe.

2. A Manual Feedback Circuit: Before connecting any circuitry, wire your Triple Output DC Power Supply as described on page 36 of its manual such that you can set output B to -15 V while also being able to vary output A. In addition, connect the negative terminal of the 5-V fixed supply to the common ground (green terminal). That way, when you use three supplies at the same time, they will all have the same ground. Then connect your photocell as shown on the right side of the diagram below, using your 10k POT as the variable resistor. Add about 400 of resistance in series with the POT and the -15-V terminal to protect the photocell. Note that the photocell connects to the +5 V terminal rather than +15 V to limit its power dissipation and also because you’ll need to vary the +15-V supply in the next step. Set the POT initially for zero resistance.

Use the larger of the two types of light bulbs that you have in your collection. It is rated at about 14 V and 240 mA. Turn the variable 15-V supply to its lowest output voltage and connect the light bulb as shown on the left side of the diagram. Turn on the power supply and bring the supply voltage up to about 14 volts, at which point the light bulb should be quite bright.

Connect your DMM to measure the voltage at point A (with respect to…?). Hold the bulb a few centimeters from the photocell and verify that the voltage at point A is positive. If it isn’t, adjust the POT resistance appropriately. Should you increase or decrease the POT’s resistance? Now darken the photocell and verify that point A goes negative. Why is there a change in voltage polarity at point A?

Now bring the bulb a few centimeters from the photocell. Adjust the variable +15-V supply downward until the voltage at point A is close to

zero. Move the bulb a few centimeters further from the photocell and adjust the

voltage again to keep point A at essentially zero volts. What happens to the brightness of the bulb? Continue to move the bulb away in steps of a few centimeters, adjusting the bulb voltage

each time to keep point A at essentially zero volts. When you get the bulb as far from the photocell as its wires will permit, the bulb should be at essentially full intensity. If the bulb is still dim, or if you need to get it excessively bright before you get the photocell far from it, adjust the resistance of your POT as necessary. The exact resistance isn’t critical; you just want a good range of variation in bulb brightness over the limits of the bulb’s range of position. Then repeat the experiment of moving the bulb away from the photocell, this time adjusting the voltage across the bulb continuously in an effort to keep point A at essentially zero volts. Make sure that you do not exceed 14 V across the light bulb! Before proceeding to part 3, be sure that you understand the answers to these questions:

1. Why does the bulb have to get brighter as you move it away?

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A

–15 V

RSB ~1 k

+5 V

#57 lamp

5–15 V

photocell

0-14 V

Light bulb

10k POT + 400

Electronics Lab 7 Spring 2015

2. If you keep point A at zero volts, qualitatively, what happens to the resistance of the photocell as you move the bulb away?

3. If you keep point A at zero volts, what happens to the apparent brightness of the bulb as perceived by the photocell?

4. Where is the feedback loop in this set-up?

Leave your circuit connected and proceed to part 3.

3. An Operational Amplifier/Comparator: Disconnect the bulb from the power supply. Set the positive and negative 15-volt supplies for about ±15 V. Turn off the power supply. Use your L165V operational amplifier and wire it as shown below. Leave the inverting (–) input disconnected for now, but run a wire to it so you can readily connect it elsewhere later. Check all wiring and make sure that the wires that you have attached to the L165V are not electrically touching each other before turning on the power supply; the L165V is fairly

expensive as integrated circuits go. If you would like additional cables or alligator clips, let Justin or me know. The L165V is like a 741 except that the L165V can supply up to 3 A of current. Note that the connections to the L165V are shown from the side where the plastic is thickest.After checking the wiring, turn on the power supply. Monitor the output of the op amp with your DMM. Connect the (–) input of the op amp to the +5-V supply. What happens at the output? Does the bulb light? Why or why not? Now try connecting the (–) input to the +15-V supply. Again, what happens? Now connect the (–) input to the –15-V supply. What happens? Why? What’s the diode for? Make sure that you understand what is happening before going on to part 4.

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Light bulb

10k POT + 400

Electronics Lab 7 Spring 2015

4. Putting It All Together: Now connect point A to the (–) input of the op amp, resulting in the circuit shown below.

a. Observing the Intelligent Light Bulb: Point the bulb at the photocell and increase and decrease the separation between the bulb and the photocell. What happens? Why does the bulb get brighter as you move it away from the photocell? What is happening to the voltage at point A? What is happening to the resistance of the photocell? What is happening to the apparent brightness of the bulb as perceived by the photocell? Where is the feedback loop in this set-up?

b. DC Analysis of the Intelligent Light Bulb: Connect a DMM to the output of the op amp (before the diode) and another DMM to measure the voltage at point A. Observe the voltages as you move the light bulb. Are your results consistent with what you expect? Why / why not?

c. (Optional for 104). AC Analysis of the Intelligent Light Bulb: Now connect your oscilloscope so that one channel displays the voltage at point A and the other channel displays the output of the op amp. Explain what’s going on.

5. An Optical Flip-Flop: Get together with another group and arrange your circuits so that each group’s photocell points at the other group’s light bulb. Note the states of the two bulbs. What happens when you pass your hand between a lit bulb and the photocell that it’s shining on? Explain what’s going on. Why do you think that this circuit is called a bistable circuit or a flip-flop?

6. Write-up:

1. Photocell: Describe the behavior of the photocell when exposed to light and when it is in the dark.

2. A Manual Feedback Circuit:

Where do you connect the two leads from the DMM when measuring the voltage at point A?

When you darkened the photocell, why is there a change in voltage polarity at point A?

As you increase the separation between the bulb and the photocell, what happens to the brightness of the bulb? Why?

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Electronics Lab 7 Spring 2015

If you keep point A at zero volts, qualitatively, what happens to the resistance of the photocell as you separate the bulb from the photocell?

If you keep point A at zero volts, what happens to the apparent brightness of the bulb as perceived by the photocell?

Where is the feedback loop in this set-up?

3. An Operational Amplifier/Comparator:

With the (–) input of the op amp connected to the +5-V supply, what happens at the output of the op amp? Does the bulb light? Why or why not?

With the (–) input of the op amp connected to the +15-V supply, what happens at the output of the op amp? Does the bulb light? Why or why not?

With the (–) input of the op amp connected to the -15-V supply, what happens at the output of the op amp? Does the bulb light? Why or why not?

What is the role of the diode in this circuit?

4. Putting It All Together:

a. Observing the Intelligent Light Bulb:

Describe what happens when you point the bulb at the photocell and increase and decrease the separation between the bulb and the photocell.

Why does the bulb get brighter as you move it away from the photocell?

What is happening to the voltage at point A?

What is happening to the resistance of the photocell?

What is happening to the apparent brightness of the bulb as perceived by the photocell?

Where is the feedback loop in this set-up?

b. DC Analysis of the Intelligent Light Bulb:

Describe what happens to the voltage at point A as you move the light bulb.

Are these observations consistent with what you expect? Why / why not?

c. (Optional for 104). AC Analysis of the Intelligent Light Bulb:

Explain what you observe when you look at the voltage at point A and the output of the op amp. What’s going on?

5. An Optical Flip-Flop:

What happens when you pass your hand between a lit bulb and the photocell that it’s shining on?

Explain what’s going on.

Why do you think that this circuit is called a bistable circuit or a flip-flop?

6. Summary:

In one paragraph, explain how the intelligent light bulb circuit works.

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Electronics Lab 7 Spring 2015

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