Physics 1B
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
Introduction
This lab relates to material in Hecht, Chapter 18. In this lab you will explore the concepts of
circuits, resistors, and capacitors, by actually building a small circuit that is yours to keep! It is a
good idea to read all the steps in each part before you start. (Pages 6-9 in this lab manual are
technical specifications for the 555 timer chip and are included for those interested. They are not
required for performing the lab.)
Pictures in this lab have been taken from www.epemag.wimborne.co.uk/solderpix.htm.
Pre-Lab Homework
These Pre-Lab Homework problems are to be done before you arrive at lab. Unlike many of the
questions in previous labs, these questions are not predictions. Indeed, questions 1-4 have only one
correct answer, and they will be graded as such. (Some of the answers are contained in this lab manual.)
1. Using the resistor color chart in Fig. 17.19 of Hecht (pg. 709), determine the color bands for each of
the four resistors you will be using (listed below). Do not worry about the tolerance band for each
resistor. (For example, a 100-Ω resistor will have color bands of brown-black-brown.)
2. Assuming that resistors R1 and R2 are in series, calculate the equivalent resistance.
3. In the lab, you will build a circuit with flashing lights (LEDs). The equivalent resistance (of R1 and
R2) and the capacitor C1 set the rate at which the LEDs flash.
(a) Calculate the time constant for the resistance (R1 and R2) and capacitor C1.
(b) If you wanted your LEDs to flash twice as fast, what resistance would you use?
(c) If you wanted your LEDs to flash twice as slow, what resistance would you use?
4. Referring to the circuit diagram on page 5, what is the maximum current through the LEDs?
(Remember Ohm’s Law: V = IR, and assume the LEDs contribute no additional resistance.)
5. Briefly describe the steps you will take when you solder.
6. Which circuit components must be oriented correctly in order for your circuit to function properly?
Experimental Setup
Materials:
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1 kΩ resistor (R1)
82 kΩ resistor (R2)
Two 150-Ω resistors (R3 and R4)
Two light emitting diodes (LEDs)
2.2 µF electrolytic capacitor (C1)
0.1 µF capacitor (C2)
0.01 µF capacitor (C3)
555 Timer chip
3-V battery with holder
Several inches of solder
Soldering iron and base
Helping hands (shown in Fig. 1)
Circuit board with attached jewelry pin
© 2001 UCSD-PERG
Fig. 1: So-called “helping hands”
are a useful tool for holding a
circuit board while soldering.
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Physics 1B
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
The Flasher Circuit
Your circuit is composed of the 555 timer, certain resistors and capacitors that form the timing
circuit (R1, R2 and C1), two LEDs, additional resistors (R3 and R4) that limit the current through
the LEDs, two filter capacitors (C2 and C3) and a battery.
You may find it useful to refer to the circuit diagrams on page 5 while reading this section on the
operation of your circuit.
The 555 timer is an integrated circuit, or chip, that repeatedly turns something on and off. In the
circuit you will build in this lab, the function of the 555 timer chip is simply to output a voltage
that alternates between +3 V and 0 V. In effect, the timer chip will turn two lights (LEDs) on
and off.
The timer controls the LEDs by monitoring the voltage across the capacitor C1. The capacitor
charges up through R1 and R2. In Pre-Lab #3, you calculated the time it takes for the capacitor to
charge up. When the voltage across the capacitor reaches 2 V, the timer “turns on”
(outputs +3 V) and LED2 lights up. A moment later, the timer connects R2 to ground and the
capacitor starts to discharge. When the voltage across the capacitor drops to 1 V, LED2 turns off
and the timer disconnects itself from ground. At this point, LED1 turns on because it is supplied
with 3 Volts from the battery. Then, the capacitor starts to charge up again and the cycle repeats.
(For those who are interested, the technical specifications for this chip are included at the end of
this lab manual.)
As shown on the circuit diagram on page 5, all connections to the top wire on the circuit are at
+3 V and all connections to the bottom wire are at 0 V. C2 is known as a filter capacitor since it
filters any noise (or stray voltages) and thus stabilizes the voltage delivered by the battery. For
example, if the circuit suddenly – and, for a short time – requires much more current, the internal
resistance of the battery will cause the voltage to drop. The capacitor C2 is charged to +3 V and
for a short period it can supply power at +3 V to the circuit. C3 acts in a similar way.
The resistors R3 and R4 reduce the maximum current through the LEDs. Pin 3 of the timer chip
alternately switches from +3 V to ground (0 V). When it is at +3 V, LED2 lights up and LED 1
is off. When it is at 0 V, LED1 light up and LED2 is off. (This will help you answer Pre-Lab
#4.)
© 2001 UCSD-PERG
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Physics 1B
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
Soldering Basics
WARNING: The melted lead solder releases fumes during soldering. These fumes are harmful to
your eyes and lungs. Always work in a well-ventilated area. Hot solder is also dangerous. Be
careful not to let it splash around or drip because it will burn you almost instantly. Wash your
hands when immediately when finished handling the lead solder.
In this lab you will be learning and applying a technique called
soldering. Solder (pronounced, SOD-er) is simply a mix of metals,
usually tin and lead. When solder is melted, it can be used like
glue to hold two objects together. In this case we wish to fasten
the legs of our resistors, capacitors, etc. to the metal contacts on
the circuit board. We use a soldering iron to melt the solder and
heat the parts we want the solder to adhere to.
First, you should make sure that your the legs of the circuit
element (resistors, capacitors, timer, etc.) you want to solder are
firmly in place. The wires may be bent slightly as shown in Fig. 2.
Fig. 2: The “legs” of a circuit
element (such as a resistor) protrude
slightly through the bottom of a
circuit board, ready to be soldered.
Next, place the tip of the soldering iron against both the wire and
the point on the circuit board you want to join. It normally takes a
few seconds to heat up enough to solder.
Once the wire and circuit board have been heated up, you are ready
to apply solder. Touch the tip of the strand of solder to the wire
and circuit board, but not the tip of the iron. If everything is hot
enough, the solder should flow freely and fuse the leg of the
resistor to the circuit board (Fig. 3).
Once the surface is completely coated, stop adding solder and then
remove the soldering iron (in that order). Do not move the joint
for a few seconds to allow the solder time to cool. When finished,
the leg of the circuit element will be electrically attached to the
circuit board as shown in Fig. 4.
You should also keep the soldering iron tip clean. A clean iron tip
allows for more efficient heat conduction and thus, a better joint.
Use a wet sponge to clean the tip after each joint is soldered. You
should also double check joints to make sure there is complete
coverage.
© 2001 UCSD-PERG
Fig. 3: The legs of the circuit
element are first heated with the
soldering iron (left), and then fused
to the circuit board by feeding in a
strand of solder (right).
Fig. 4: Once the solder has cooled,
the leg is firmly and electrically
attached to the circuit board.
(Notice that you do not need a large
amount of solder.)
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Physics 1B
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
Building the Circuit
1.
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13.
14.
15.
Locate and identify each of the materials listed in the Experimental Setup section.
Distinguish the resistors based on the color codes you determined in Pre-Lab #1. If you
wish, you may trade in your R1 and R2 resistors for different values to make your LEDs
flash faster or slower.
Turn on your soldering iron and set the temperature gauge to 650-700 °F. Moisten the
sponge in the base of the soldering iron holder.
Using the diagram on the next page as a guide, put the 555 timer into place on the circuit
board. Make sure the timer chip is oriented properly (and not upside-down). There is a
small indent on the timer chip that matches up with the “U” on the printed diagram.
Place the circuit board in the helping hands, using the alligator clips to hold the circuit
board firmly on both sides.
Take several inches of solder and hold it in your left hand (or if you are a lefty, hold the
solder in your right hand). Pick up the soldering iron with your other hand and carefully
solder each of the timer chip’s eight pins onto the circuit board. It is better to use to little
solder than too much!
At this point you should wipe the tip on the sponge, then place the soldering iron back into
its holder and check to see if you completely soldered each pin (see Fig. 4 on the previous
page). If you are not sure, ask your TA.
Now, put R1 in place the circuit board. Make sure that you pull most of the wire from each
end of the resistor (its “legs”) through the circuit board so that the resistor is close to the
circuit board. Now, turn over the circuit board and bend the wires at a 45° angle so that the
resistor will remain in position. Solder both legs of the resistor to the circuit board.
Repeat the same steps for the second resistor, R2. Once you have soldered R1 and R2 to the
circuit board, check with your TA to make sure that your soldering looks complete and
correct. At this point, trim the excess wire to about 1/8 inch (or 0.3 cm).
Place the remaining resistors (R3 and R4) in position and solder the two resistors in place.
As before, trim excess wires where appropriate.
Locate capacitor C1: it is long, cylindrical, and blue. Notice this capacitor has one short leg
(the + side) and one long leg (– side). Refer to the circuit diagram to determine how to
place this capacitor on the circuit board. If this capacitor is oriented incorrectly, your
circuit will not flash! Once it is in place correctly, solder it to the circuit board.
Next, find C2 and C3 and place them into the circuit board. Be careful since these
capacitors look similar. Ask your TA if you are having trouble distinguishing them.
Solder these capacitors into place, trim excess wire, etc.
Locate the LEDs. Notice that they, too, have one short leg and one long leg. Look at the
printed circuit diagram to determine which way to properly orient each LED. If the LEDs
are oriented incorrectly or backwards your circuit will not work! Solder the two LEDs
onto the circuit board.
Place the battery holder into the circuit board, carefully checking to make sure that the (+)
and (–) ends of the battery holder are in the proper location. Solder the battery holder into
place.
Place the battery into the battery holder (Be careful about its +/– orientation!). Close the
jewelry pin and verify that your LEDs are flashing. Congratulations!
Once you have verified that your circuit works, you clip any extra wire off the back of the
circuit board.
© 2001 UCSD-PERG
Page 4
Physics 1B
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
Fig. 5: Circuit element diagram printed on actual circuit board to show locations
of each circuit element (resistors, capacitors, LEDs, timer, etc.).
Fig. 6: Block circuit diagram including 555 timer chip, resistors, capacitors, battery, and LEDs.
Note that pin 3 (on the timer chip) simply alternates between +3 V and 0 V. Furthermore, the top of the circuit branch with
LED1 is always at +3 V, while the bottom of the branch with LED2 is always at 0 V (GND). Pin 3 is in between both of these
branches (at the bottom of LED1 and at the top of the LED2 branch).
© 2001 UCSD-PERG
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Physics 1B
© 2001 UCSD-PERG
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
Page 6
Physics 1B
© 2001 UCSD-PERG
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
Page 7
Physics 1B
© 2001 UCSD-PERG
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
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Physics 1B
© 2001 UCSD-PERG
• FLASHERS – RC TIME CONSTANT •
Rev 3-AH
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