LAB3 – Potentiometers, Photocells, Balanced Circuits and LEDs Objective: In this lab, you will become familiar with variable resistors (potentiometers, or pots).
Using simple sensors, you will convert nonelectrical parameters (light intensity) to electrical
resistance. You will use the breadboard on the CY3214 evaluation board and your digital
multimeter for this lab.
Items needed:
CY3214 eval board
Wire package
Digital Multimeter with probes Radio-Shack #22-810
9-volt battery Radio-Shack #23-875
5 Cadmium-Sulfide Photocells Radio-Shack #276-1657
10kΩ Potentiometer Radio-Shack #271-282
470Ω resistor
2 5mm High-Brightness White LED #276-017
Flashlight or bright light
Potentiometers
A potentiometer is a resistor with a variable resistance. A potentiometer has three leads, with a rotator
dial. The total resistance is constant, but by changing the dial (or slider) knob, the values of the
resistances between the middle lead and the two outer leads changes.
Symbol for a potentiometer
There are two variable resistors on the CY3214
board; they are labeled VR1 and VR2. You can
access the slider position using the headers next to
the breadboard.
Using the multimeter, measure the resistance across
VR1 with the knob turned fully clockwise; repeat
with the knob turned fully counterclockwise.
VR1clockwise
=
___________________
VR1counter-clockwise = ___________________
Access the schematic for the CY3214. Find the two pots on the schematic. Is one end of each of the
pots wired (or tied) to a specific value? If so, what value__________________
Draw the circuit for the potentiometer below.
The potentiometers on the CY3214 are wired in a way that a user only has access to one half of the
potentiometer. We want to use a potentiometer in a way that allows us access to both sides of the pot.
To access both sides of a potentiometer, we will not use the pots on the board, but rather the 10kΩ
potentiometer. Notice the potentiometer has three leads – place each of the leads into separate rows of
the breadboard. Using the multimeter, measure the resistance across the two outside leads of the
potentiometer. Move one of the probes to the middle lead. Measure the resistance with the knob turned
fully clockwise; repeat with the knob turned fully counterclockwise.
= ___________________
Vpottotal
Vpotclockwise
=
___________________
Vpotcounter-clockwise = ___________________
Photocells or Photoresistors
Sensors allow electrical engineers to measure nonelectrical physical quantities and convert the values to
electrical values. A photoresistor, or photocell, is a resistor whose value is dependent on the intensity of
light. A typical light-to-dark resistance ratio for a photocell is 1:1000. This means that its resistance
could range from 500 ohms in the light to 5000 kilohms in the dark. Photocells are used in various types
of control and timing circuits.
You will measure the light-dark resistance ratio of the photocells.
The precision of the photocells can
be very different. It is important that
the resistance the photocells in our
circuit be as evenly matched as
possible. Note that even though the
photoresistors look very similar in
appearance, the resistance
characteristics can be quite different.
Initially, place the probes from the multimeter (set the value to 2KΩ) on the leads of the one of the
photoresistors. To determine the resistance of the photocell in bright light conditions, place the
photocell directly on the lightbulb or as close as possible. To measure the resistance in darkness, cover
the photoresistor with a dark cloth (or under a desk). Repeat the procedure for five photocells. Make a
table of results. Use labels to keep track of the values for each photoresistor.
Rphotocell
open
Rphotocell
covered
PhotoResistor1
PhotoResistor2
PhotoResistor3
PhotoResistor4
PhotoResistor5
Try to find two pairs of photocells that are that have similar values for both light and dark conditions.
Ask classmates in order to obtain two sets that are similar. Each person should have two pairs of
matched photocells. (I paired together a photocell that measured 448Ω light, 1.48M dark and a
photocell that measured 498Ωlight and 1.53M dark; the other pair measured 1.48K light, 100K dark and
1.36K light and 65M dark.) You will be placing one photocell from each pair to create a balanced
circuit. Using two photocells in series creates a more balanced circuit since there is so much variation in
individual photocells.
Photodiodes and phototransistors are other types of optical devices that work as variable resistors.
Balanced Voltage Divider Circuits
Notice the circuit on the next page. R1 is a resistor similar to the one used in lab 2. R2 is a
potentiometer – a variable resistor. R3-R6 are photocells – variable resistors that are sensitive to
the amount of light received. The arrows indicate light entering the diode. The arrows on the
symbols for optical electronic devices either point toward the circle, if they use light, or away
from the circle, if they produce light (LEDs are an example). Create the circuit on your
breadboard. R3 and R4 should be one photoresistor from each set of pairs determined earlier.
Working in pairs of students, measure the values of V1 and V2. Using two multimeters
simultaneously will make your measuring much easier. The circuit may not look like a complete
circuit – often the supply voltage (+9V) is written in this manner. It is assumed that the other
end of the battery is attached to ground. Before you connect the battery place the dial on the
potentiometer so the resistor is evenly divided between the two sides.
V1 light _______________ V2light __________________
V1 light _______________ V2dark __________________
V1dark _______________
V2light__________________
V1dark _______________
V2dark __________________
Using the law of voltages that two voltages in parallel must be equal, what is happening to the
voltage across the potentiometer as the voltages across the photoresistors changes?
Does the voltage across the 470Ω resistor ever change?
How is this circuit a voltage divider circuit?
V1
V2
Light Emitting Diodes (LEDs)
LEDs are another type of optical electronic device. LEDs (light emitting diodes) were developed
to replace the fragile, short-life incandescent light bulbs used to indicate on/off conditions on
panels. The life expectancy of the LED is very long, over 100,000 hours of operation. A
LIGHT-EMITTING DIODE is a diode which, when forward biased, produces visible light.
Common colors for LED lights may be red, green, or amber, depending upon the material used
to make the diode. Other colors such as yellow, blue and white are also available. LEDs have
been in the news recently as new composite materials are being developed to reduce the cost of
high-brightness white lights.
The LED operating voltage is small, about 1-1.5 volts forward bias voltage, depending on type,
and generally about 10 milliamps. White LEDs have a larger forward bias voltage,
approximately 3 volts. We will be using white LEDs in this lab for their brightness.
An LED has two leads labeled Anode and Cathode. The Anode is the positive end and is usually
longer than the cathode. We want to apply voltage to the positive end and ground to the cathode,
but have to be careful to not overdrive the LED. A current-limiting resistor (the 470Ω resistor)
should always used to prevent blowing the LED. What happens to the LEDs as the resistance of
R2 changes? Create the circuit below.
The CY3214 board has four LEDs labeled LED1-LED4. Find the LEDs in the schematic. Draw
the circuit for LED1 below. Does it require a high voltage or a low voltage to turn it on?
When we use the LEDs on the board, the output of the microprocessor should be LOW/HIGH?
What is the value of the supply voltage (VCC) on the CY3214 board?