Student name:_______________________
Hands-On Technical Training (HOTT) Workshop:
Basic Soldering and PWM Motor Speed Control
Table of Contents
Hands-On Technical Training (HOTT) Workshop: Basic Soldering and PWM Motor
Speed Control...................................................................................................................... 1
Table of Contents ............................................................................................................ 1
Workshop objectives ....................................................................................................... 1
Prerequisites .................................................................................................................... 1
Soldering Theory ................................................................................................................ 2
Soldering Techniques.......................................................................................................... 2
Pulse Width Modulation (PWM) Applications and Theory ............................................... 4
The PWM Motor Speed Control Project ............................................................................ 5
Overview ......................................................................................................................... 5
Detailed Circuit Explanation........................................................................................... 5
Fabrication Instructions .................................................................................................. 6
Test Procedure ................................................................................................................ 9
PWM Motor Speed Control PC Board Layout as Viewed from Component Side ....... 12
Finished PWM Motor Speed Control Board ................................................................ 12
PWM Motor Speed Control Board Project Schematic Diagram .................................. 13
Workshop objectives
1. Learn the theory of Pulse Width Modulation (PWM) and some typical
applications.
2. Learn how to safely and effectively make good solder joints on printed circuit
boards with through-hole components.
3. Learn how to recognize a bad solder joint.
4. Build and test the finished project: A simple PWM DC motor speed controller.
Prerequisites
There are no prerequisites for this workshop, although background in electronic circuits
will be very helpful.
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Soldering Theory
Electronic soldering uses an alloy with a fairly low melting point (350° F typical). Since
this is well below the melting point of copper or steel, the electronic components and
circuit boards do not themselves melt; rather the solder fills in the spaces between
components and bonds them together.
The hand soldering you will be doing today uses wire solder that contains flux in the
core. This flux improves the quality of the solder joints by deoxidizing the surface of the
materials being soldered, thereby improving solder wetting, flow, and adhesion. The flux
is made of a material called Rosin. Note that some solders contain acid flux in the core.
This is intended for plumbing and should NEVER be used for soldering electronic
components.
Soldering Techniques
SAFETY WARNINGS:
- Avoid injury when cutting off excess lead lengths. Hold the
leads so they cannot fly toward you or anyone else.
- Wear safety glasses when soldering or desoldering.
1. Wear safety glasses while soldering to prevent injuries due to airborne solder or
pieces of component leads which have been trimmed.
2. Keep the soldering iron tip clean. Wipe it often on a wet sponge or cloth, and then
apply solder to the tip to give the entire tip a wet look. This process is called
tinning, and it will protect the tip and enable you to make good connections.
When solder tends to “ball” or does not stick to the tip, the tip needs to be cleaned
and retinned.
3. Use only enough solder to make a good connection. Remember that this is a
BAD saying when it comes to soldering: “The bigger the blob, the better the job.”
4. Lift the soldering iron straight up from the circuit board when the solder joint is
complete.
5. Due to the small foil area around the circuit board holes and the small areas
between foils, use the utmost care to prevent solder bridges between adjacent foil
areas (see Figure 1).
6. If a solder bridge should develop, turn the circuit board foil-side-down and heat
the solder between connections. The excess solder will run onto the tip of the
soldering iron, and this will remove the solder bridge.
7. Place the soldering iron tip so that it contacts both the protoboard copper foil and
the component lead and apply solder to them rather than directly to the soldering
iron. Even so, it may be necessary to touch solder to the iron to initiate the flow
of molten solder.
8. Hold the soldering iron steadily in place as much as possible. Do not try to use it
as a knife, spreading the solder like peanut butter.
9. Try not to hold the soldering iron in place for more than a 4-second count, but
realize that the components in this project are quite forgiving and will most likely
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endure heat application for 10 seconds or longer. This will NOT be true for every
project. In general, the 4-second rule is a good guideline.
Solder
Bridge
Figure 1: Circuit Board With and Without Solder Bridges
10. Refer to Figures 2 & 3 for examples of some good and bad soldering techniques.
Figure 2: How to Make a Good Solder Connection
Figure 3: Bad Solder Connections to Avoid
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Pulse Width Modulation (PWM) Applications and Theory
Pulse width modulation is a technique commonly used for various applications, including
but not limited to:
 DC motor speed control
 Variable frequency AC motor speed control
 LED brightness control
 High power audio amplifiers (Class D).
Advantages:
 Very power efficient
 Easily controlled by digital devices such as microcontrollers.
In this workshop, we’ll focus on PWM for motor speed and LED brightness control.
The speed of a simple DC motor is roughly proportional to the applied voltage. Instead
of sending pure DC to a motor, PWM uses pulses that have precisely controlled ON and
OFF times. If the pulses are ON longer, then the average DC voltage sent to the motor
will be higher and the motor will run faster. Conversely, narrowing the pulses’ on time
will slow the motor down. A typical PWM signal might look something like this:
Figure 4: PWM Signal
This technical explanation of PWM is provided from www.wikipedia.org:
Pulse width modulation uses a square wave whose duty cycle is modulated
resulting in the variation of the average value of the waveform. If we consider a
square waveform f(t) with a low value ymin, a high value ymax and a duty cycle D
(see figure), the average value of the waveform is given by:
As f(t) is a square wave, its value is ymax for
. The above expression then becomes:
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and ymin for
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This latter expression can be fairly simplified in many cases where ymin = 0 as
. From this, it is obvious that the average value of the signal ( )
is directly dependent on the duty cycle D.
Keep in mind that although the ON and OFF times may vary, the frequency of a PWM
signal will typically remain constant and will typically be in the tens of kilohertz range.
This reduces the likelihood of creating noise within the range of human hearing. The
circuit you will be building runs at approximately 25 kHz. The important variable here is
not frequency, but duty cycle. By definition, “duty cycle” is the portion of each cycle
that the signal is ON compared to how long it is OFF, expressed as a percentage. Again,
this duty cycle is what controls motor speed and LED brightness.
The PWM Motor Speed Control Project
Overview
You will learn and practice through-hole soldering techniques by building a PWM motor
speed control circuit board. A photograph of an assembled board is shown in Figure 5.
Figure 5: Finished PWM Project Board
Detailed Circuit Explanation
Refer to the schematic diagram near the end of this handout. The heart of this module is
an LM555 IC chip configured as a variable duty cycle square wave oscillator. When
pushbutton S1 is pressed, 9 Volt power is applied to the other components on the circuit
board. IC1 begins oscillating immediately at a frequency and duty cycle determined by
capacitor C1 and the setting of speed control potentiometer R2. D1 reduces the changes
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in frequency that would otherwise be caused by changing the R2 setting. Optional
capacitor C2 improves circuit reliability and gives you additional soldering practice. D2
removes counter-EMF voltage spikes caused by the rapid switching of voltage to motor
MOT1, thereby protecting IC1 from damage. LED1 will appear to dim and brighten as
R2 is adjusted, although it is actually turning on and off very rapidly with a varying duty
cycle. R1 limits the current through LED1, protecting it and IC1.
Fabrication Instructions
1. Your workshop facilitator will provide you with a printed circuit board, 9 Volt
battery, and the necessary soldering supplies.
2. Procure the needed parts from the E109 Supply Room, using the following Bill of
Material and photographs as your guide.
PWM Motor Speed Control Bill of Material
Quantity
1
1
1
1
1
1
2
1
1
1
1
1
MOT1:
Description
Motor, 9 VDC
Diode, 1N914B
Diode, 1N4007GP
Integrated Circuit, LM555CN
IC Socket, 8-pin
Switches, NO_PB
Capacitor, 10nF (0.01 uF)
5kilohm Potentiometer
Resistor, 330Ω (orange, orange, brown)
LED, red
Battery Clip, 9 Volt
Battery, 9 Volt
RefDes
MOT1
D1
D2
IC1
X1
S1
C1, C2
R2
R1
LED1
n/a
V1
D1:
D2:
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X1 and IC1:
Pushbutton S1:
C1 and C2:
R2:
R1:
LED1:
Battery Clip:
3. Switch on your soldering iron to begin warming it up.
4. If necessary, review the Soldering Techniques section discussed earlier in this
handout.
5. Components will be placed on the board in accordance with the PC board layout
diagram at the end of this handout. Do not yet begin installing components.
6. Note that some components are polarized, that is, they have a positive and a
negative lead. Here are tips to recognizing and properly installing those
components which are polarity sensitive:
a. Diodes D1 and D2 have a black stripe identifying the negative, or cathode,
lead. These components will need to be installed as shown on the layout
diagram, matching the location of the black stripe on the diode to the black
stripe on the diagram.
b. Locate the LED1 flat side on both the diagram and the actual LED. This
is the cathode (negative side) and must be installed as shown.
c. The battery clip leads should be attached red to positive and black to
negative.
d. IC socket X1 should be installed so that the small half-round cutout is on
the left when the board is oriented as shown in the layout diagram. If your
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socket X1 has no cutout, don’t worry about it. More importantly, IC1
must be installed correctly. Some IC chips will have a semicircle imprint
as shown below. If so, that should be pointing to the left (away from the
motor). Some ICs are marked with a circle nearest to pin 1 rather than the
semicircle shown in Figure 6. If your IC has a circle, that must go toward
the lower left corner of the board.
Figure 6: IC Chip with Left Side Marker
7. Refer to the illustrations on page 12 for component layout.
8. It is usually best to place and solder a few components at a time, focusing on the
lowest profile components first. Install D1, D2, and R1 onto the board. Bend the
leads slightly outward to hold the component in place during soldering. Solder
them in place, obeying the rules outlined in the Soldering Techniques section.
9. Clip the leads close to the PC board, being careful not to send any clippings into
any soft body parts of your body or anyone else’s.
10. Install and solder IC socket X1, but do not yet install IC1 into the socket.
Remember to place X1 on the board with the cutout facing left. Bend the corner
pins to hold the socket in place while soldering.
11. Install and solder S1, trimming the leads afterward if necessary. Note: The leads
may need to be bent straight before inserting into the board.
12. Install and solder C1, C2, and LED1, paying attention to the polarity of LED1.
Trim the leads.
13. Install R2, bend the leads slightly to hold it in place and make soldering easier,
and then solder it.
14. Install and solder motor MOT1. Hints: (1) A larger soldering iron tip may be
required for maximum heat transfer, (2) Position the iron so that the maximum tip
surface area is in contact with both the copper pad on the board and the motor
lead.
15. Install and solder the battery clip.
16. Inspect the finished board for solder bridges, cold solder joints, and incomplete
solder joints.
17. Plug IC1 into socket X1, being careful to orient it correctly as mentioned
previously.
18. Optional: Place a small piece of tape onto the motor shaft to make rotation more
clearly noticeable.
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Test Procedure
1. Plug the 9 Volt battery into the clip.
2. Press pushbutton switch S1. Verify that the motor runs.
3. Adjust speed control potentiometer R2 from limit to limit while holding S1.
Verify that the motor speed changes.
4. Optional: use an oscilloscope to view the motor drive signal across diode D2
using the following procedure:
a. Turn oscilloscope Intensity knob clockwise to about the ¾ position and
note that the Power LED illuminates as shown here:
b. Connect oscilloscope probe or mini-hook leads across D2 as shown in this
photo:
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c. Connect the probe or mini-hook cable to Channel 1 input, Input slide
switch as shown here:
d. Set the Mode to CH 1, Time/Div to 10 uS, Trigger Mode to Normal, and
Trigger Source to CH 1, as shown in these photos:
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e. Adjust the black potentiometer, R2, fully clockwise. Hold the PWM
circuit board pushbutton to make the motor and adjust the Trigger Level
until the signal on the oscilloscope appears something like this:
f. Adjust R2 counter-clockwise 3-4 clicks and readjust Trigger Level as
necessary. Note that the waveform now appears something like this:
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PWM Motor Speed Control PC Board Layout as Viewed
from Component Side
X1/IC1
Finished PWM Motor Speed Control Board
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Student name:_______________________
PWM Motor Speed Control Board Project Schematic Diagram
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