This circuit, when completed, causes a light-emitting diode (LED) to flash or pulse alternately on and off at a very slow rate, that is, at a low frequency. It could be used as a dummy car alarm indicator. You sometimes see these tiny pulsating lights in expensive cars protected by sophisticated car security systems, warning you that the car alarm is armed and ready. 

For this design, the LED flashes on for a brief period and stays off for a relatively longer period. Because additional current is consumed from the battery every time the LED comes on, this long rest period conserves battery power. The brightness of the indicator LED is also limited, again in order to preserve battery power.

Circuit Description
The 555 timer integrated circuit (IC) is configured in an oscillating mode that results in a continuous train of pulses being generated. The frequency determining components have been chosen to give a very slow pulse rate.Two resistors and a capacitor determine the flashing rate of the LED. Power is supplied from a 9-volt battery.

Parts List
IC1: LM 555 timer
R1 = 100kohm
R2 = 10kohm
R3 = 1kohm
C1 = 10mF
C2 = 0.01mF
C3 = 0.1mF
C4 = 100mF
Additional Parts and Materials
LED1: Light-emitting diode
S1: Miniature SPST toggle switch
B1: 9-volt battery
9-volt battery snap
8-pin IC socket
General purpose circuit assembly board
Hook-up wire (solid and stranded)

Pin Connections (see Figure 17-1)
Pin #1
This is always connected to ground. Because all of the circuits described
here use the positive voltage as the supply voltage, that means that the ground
connection is the same as the negative battery terminal.

Pin #2
This is first connected to pin #6 and then is connected via capacitor C1
(10mF) to ground. The value of the capacitor is one of the components that
determine what the output frequency of the oscillator will be.A high-capacitor
value results in a lower frequency, whereas a low-capacitor value raises the

Pin #3
The output frequency is taken from this pin and will go to the display
LED1 to give a visual indication of the frequency.To limit the current flowing
through LED1, a series resistor, R3 (1kohm), is used. A high-resistance value
conserves power but results in a dimmer light output.

Pin #4
This pin always goes to the positive supply voltage.

Pin #5
This pin is generally taken to ground via a capacitor, C2 (0.01mF).

Pin #6
This pin is connected to pin #2 and is connected to pin #7 via a resistor,
R1 (100kohm).

Pin #7
This pin is joined to pin #6 via the previous resistor, R1, and is connected
to the positive supply voltage via a second resistor, R2 (10kohm).

Pin #8
This pin is always connected to the positive supply voltage. A disc
ceramic capacitor, C3 (0.1mF), and an electrolytic capacitor, C4 (100mF), combination
is connected across the positive supply line and ground. These components
are used to stabilize the operation of the circuit and are found with all
the projects described herein.

Component Identification
Figure 17-1 shows the electrical schematic for the fixed low-frequency
LED flasher. Referring to the above list of connections, carefully identify the
various connections on the figure. Start with pin #1 and work up to pin #8.

This step is important, so take care. Next, we move on to identifying the actual
Start with the resistors first. There are three resistors used here. The
values are
R1 = 100 kohm/color code = brown, black, yellow
R2 = 10 kohm/color code = brown, black, orange
R3 = 1 kohm/color code = brown, black, red

How do you know which end of the color bands to start reading from?
There is an additional color band, typically gold, to indicate that the resistors
have a 5 percent tolerance. This means that a nominal value of 1kohm can
either be 1 kohm plus 5 percent (that is, 50ohm), giving a total of 1.050kohm,
or 1 kohm minus 5 percent for a total of 0.950kohm.The color bands should be
read from left to right. Thus for R3 (1kohm), the bands will be brown, black,
red, and finally gold. (Generally speaking, ignore this color when identifying
the resistor values.)

In a similar manner, verify the correct values for R1 and R2. To doublecheck
that you have the correct resistors, use your multimeter set to the resistance
range, making sure that you have zeroed the meter first (if required)
and avoided placing your fingers across the resistor (this will act as a shunt,
giving an erroneous reading). Resistors can be placed either way into the
circuit board.

Next, move on to identifying the capacitors. C1 has a value of 10mF and
is termed an electrolytic capacitor. It is polarity sensitive, which means that it
has a positive and negative terminal and must be inserted into the circuit
exactly as shown. Look carefully along the body of the capacitor and notice
that a distinguishing line of minus signs is marked on one edge of the capacitor.
This line of minus signs matches up with one of the leads, indicating that it
is the negative terminal.This particular type of capacitor is barrel-shaped and
has two leads coming from the same end of the component. The value of the
capacitor is actually marked on the body—10mF.

The next capacitor, C2, is quite different in shape and is called a ceramic
disc capacitor. The value is printed on the body in a coded form. For the
0.01-mF value, the coded number is 103¢. The way this is derived is as follows:
103¢ is a shortened way of defining the value in picofarads (pF). The last
number on the right—3¢—tells us how many zeros should be added after the
first two numbers. Thus 103¢ = 10 and three zeros, or 10,000pF. Picofarads are
related to microfarads (mF) by the relation: 1mF = 1,000,000pF. Therefore, we
can see that 10,000 pF is the same as 10,000 Π 1,000,000mF, or 0.01mF. This
capacitor is not polarity sensitive and can be inserted either way.

Capacitor C3 (0.1mF) is a disc ceramic type with a coded value of 104¢.
This code corresponds to 100,000pF, which is the same as 100,000pF Π
1,000,000mF, or 0.1mF.
Capacitor C4 (100mF) is also an electrolytic type, and it is essential to
correctly identify the polarity of the leads.The negative terminal is always connected
to ground for the circuit projects described herein.

Integrated Circuit
This is an eight-pin device with the marking 555¢ on the top. A circle
etched onto the package identifies pin #1.The pins are arranged four per side.
The numbering scheme for the IC is as follows.With the IC positioned so that
the legend reads the correct way up and the circle is positioned at the lower
left-hand corner, pin #1 is nearest the circle and, going from left to right, the pin
numbers are #2, #3, and #4. Moving upward to the top row of pins, the numbering
now runs from right to left—#5, #6, #7, and #8.What you should thus
have is as follows: pin #1 at the lower left-hand corner, pin #4 at the lower righthand
corner, pin #5 at the upper right-hand corner, and pin #8 at the upper lefthand

Mechanical Components
There are three mechanical components plus the printed circuit assembly
board to complete the list of parts.The 9-volt battery can be purchased anywhere
and should be familiar to you as a power source for such everyday items
as radios and smoke detectors. There is a positive and negative polarity. Use
your voltmeter on the dc voltage range to verify the positive terminal (this is
also marked on the battery).The battery snap is fitted onto the battery terminals
and has two wires (red for positive and black for negative) leading out
from it.The attachment to the battery is made via these wires. Do not connect
the battery to the snap while assembling the components. Leave the battery
connection until the last step. For testing purposes, however, insert the clip
onto the battery and verify that the snap leads do, in fact, correspond with the
battery polarities.Take care that the bare ends of the wires do not touch each
other; otherwise the battery will be rapidly drained of current.The snap terminals
should fit snugly onto the battery; if not, squeeze carefully and slightly
with pliers to ensure a tight fit.A loose fit will cause an intermittent connection
and lead to no end of problems.

The switch, S1, is a miniature single pole, single throw (SPST) component
and has either two or three terminals (depending on the actual type obtained);
the difference is not important. For a two-terminal type, use the two terminals;
for a three-terminal type, use the center terminal and either one of the other
two terminals.The switch can be tested easily.Wire the switch in series with the
battery clip and the voltmeter, and you should have the following connections:
The black clip wire goes to the black lead of the voltmeter (an alligator clip
makes a useful means of temporarily attaching two wires together); the red clip wire goes to one of the switch terminals; and the remaining switch terminal
goes to the red voltmeter lead.

Connect the battery. Depending on which way the switch toggle is set,
the meter will read either 0 or 9V. Flip the switch toggle—the meter will
change in reading. For your own preference, when you come to mount the
switch later, you can choose whether you want the down position to represent
on or off.

Assembly Board
NOTE: The layout shown is a suggestion only. There is
plenty of space on the assembly board, and for beginners it is best to allow
ample space between components. Because of differences in component sizes,
feel free to vary the actual layout to suit yourself. Just make sure that the electrical
connections are still as shown. Go through this diagram carefully, making
sure that you follow each connection point. Match this diagram with the earlier
electrical schematic in Figure 17-1.

The custom SINGMIN PCB is a universal hobby project board that
greatly improves the chances of having your circuit work the first time it is
switched on. Figure 17-2 shows the layout used. Follow this pattern carefully
first, before attempting to solder. Place the components in the positions as
shown and carefully check that all connections are exactly as indicated. It is
vitally important that you have already practiced soldering and can make good
solder joints that are shiny, have no excess solder, and do not impart excess
heat to the board.

Construction Details
Step 1
Start by placing the IC socket in the SINGMIN PCB as shown. The
notch in the socket should face left. Bend the leads gently over to lie flush with
the board. The socket should now be self-supporting. Start with just two
opposite corner leads first. Turn the board over and check that the socket is
positioned exactly as shown. Once you are satisfied, bend the rest of the leads
in place.
Preheat your soldering iron. Allow time for the tip to heat up and have a
moist sponge on hand for periodically cleaning the tip.When ready, tin the tip
with solder, wipe it off, and apply the heated, clean tip firmly to the first corner
pin #1 (when looking from the top, with the notch in the socket facing left, pin
#1 is at the lower left-hand edge). Apply solder to the junction of the tip and
terminal.Within a few seconds, the solder will melt.
Remove the solder, then remove the tip and keep the board steady until
the solder has cooled (typically 5 to 10 seconds). Turn the board over and
check that the socket is straight and is sitting flush with the board. If this is not
the case, then the joint can be reheated (without extra solder) and repositioned;
however, this step is not critical, and it is better to put up with minor
oddities at this stage.There is the danger of imparting excess heat to the components
if you are a beginner to soldering.
Finish off the rest of the pins, anchoring the corners first to obtain a good
fit to the board.After the socket has been completed, check carefully, if needed
with a magnifying glass, that all of the pins have adequate solder, all of the
joints are shiny, and there are no solder splashes between adjacent pins to
cause a short-circuit. Do not insert the IC at this stage.
Step 2
There are seven solder links to be inserted next. Prepare suitable lengths
of solid gauge wire by removing the insulation with wire strippers. The list
below shows all of the links to be made. Proceed carefully, soldering one link at
a time. Check the integrity of each solder joint before proceeding to the next
one. Leave excess wire protruding while making the solder joint. After the
solder has cooled, the wire can be trimmed with wire cutters. Do not cut the
wire flush with the board; leave a short length showing so as not to impart
mechanical stress to the board or components. Refer to Figure 17-2 for the
position of the links, which are all marked for clarity.
Link 1: Upper ground to lower ground
Link 2: Upper positive supply to lower positive supply
Link 3: Pin #1 to ground
Link 4: Pin #4 to positive supply
Link 5: Pin #8 to positive supply
Link 6: Pin #2 to pin #6
Link 7: Pin #3 to LED1
Construction Details for 10 Simple Projects 49
Step 3
Start with LED1 and resistor R3 (1kohm). Look closely at the LED.
There is a flat surface on one edge of the LED showing that the lead nearest
this pin should go to the negative supply voltage. Temporarily hook up the
resistor R3 to either end of the LED and connect the remaining LED terminal
and free resistor end to a 9-volt battery. Note the polarities carefully. If the
LED does not light up, then reverse the connections to the battery. Note which
end of the LED goes to the positive terminal. This will be the end that will
later go to pin #3 of IC1.We are going to locate the LED off the board later, so
two short lengths (6 inches or so) of flexible wires are used as extension leads
to the LED. Solder the flexible leads to the LED and insulate the bare wires
to prevent them from touching and shorting out. Electrical insulation tape
can be used.
Do not solder in the extended LED to the SINGMIN PCB at this stage.
Resistor R3, however, can be inserted. Bend the resistor leads at right angles to
fit the appropriate holes in the board as shown. Do not bend the wires right up
to the body of the resistor, but instead use miniature pliers to isolate the
bending stresses from the resistor body. Insert the resistor into place and
solder. Once cooled, the leads can be trimmed to length.
Step 4
Resistor R1 (100 kohm) is handled in a similar fashion; however, because
it goes to two adjacent pins (#6 and #7), there is insufficient space for R1 to be
bent in the same way as the first resistor. Instead, bend just one of the leads
back on itself so that the two leads face the same direction. Now R1 will easily
fit in as shown. Once more, solder the resistor into place, allow it to cool, then
check and trim the leads.
Step 5
Resistor R2 (10 kohm) is bent to fit the connection from pin #7 and the
positive supply voltage. All of the resistor connections are now completed.
Step 6
Electrolytic capacitor C1 (10mF) is connected from pin #2 to ground
(negative). Observe the polarity carefully. The positive end of the capacitor
goes to pin #2, and the negative end goes to ground.
Step 7
Capacitor C2 (0.01mF) goes from pin #5 to ground and can go in either
way because it is not polarity sensitive.
Step 8
Capacitor C3 (0.1mF) is added across the positive supply line and

Step 9
Capacitor C4 (100mF) is also added across the positive supply line and
ground.The negative end of C4 goes to ground.
That completes all of the electronic component connections.

Step 10
The few remaining mechanical parts are connected next. Switch S1 needs
to have two flexible wires (6 inches long) soldered to two terminals, as
explained earlier. When stripping flexible wire, twist the bare ends together
before soldering and tin sparingly with solder. One end of the extended switch
connection goes to the positive supply rail as shown.The other end of S1 is soldered
to the red (positive) wire on the battery snap. The wires on the battery
snap are fragile, and to prevent excess stress on them it’s a good idea to extend
them as well. As in all cases, insulate any bare soldered wires. The negative
(black) end of the battery snap goes to ground. Finally, solder in the LED,
making sure that the notched end goes to ground. That’s it—the project construction
is complete.

Step 11
Perform a thorough check of all of the connections.

Step 12
Take IC1—the 555 timer—and locate pin #1. Position the IC so that the
notch (if there is one) faces left, or the identifier circle is in the lower left-hand
corner, and the name of the IC reads the correct way up. Place the IC over the
socket and you’ll see that the leads might be wider than the socket. Gently
bend both sets of the IC leads inward until the IC can be inserted into the
socket. Make sure all of the pins go in straight and smoothly.

Step 13
Check that the switch, S1, is in the off position. Connect the 9-volt
battery. Momentarily flick the switch on. If all is well, the LED will start flashing;
however, if there is no sign of life, then switch off immediately, disconnect
the battery, and start rechecking the circuit connections. Wrong connections,
poor solder joints, and short-circuits are the most common causes of problems.
Component failure is rarely, if ever, to blame.

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