Oscillators, Pulse Generators, Clocks...
- Capacitors and the 555 Timer IC
.
Introduction
As electronic designs
get bigger, it becomes difficult to build the complete circuit. So
we will use prebuilt circuits that come in packages like the one shown
above. This prebuilt circuit is called an IC. IC stands for
Integrated Circuit. An IC has many transistors inside it that are
connected together to form a circuit. Metal pins are connected to
the circuit and the circuit is stuck into a piece of plastic or ceramic
so that the metal pins are sticking out of the side. These pins allow
you to connect other devices to the circuit inside. We can buy simple
ICs that have several inverter circuits like the one we built in the LED
and Transistor tutorial or we can buy complex ICs like a Pentium Processor.
The Pulse - More than just an on/off
switch
So far the circuits
we have built have been stable, meaning that the output voltage stays the
same. If you change the input voltage, the output voltage changes
and once it changes it will stay at the same voltage level. The 555
integrated circuit (IC) is designed so that when the input changes, the
output goes from 0 volts to Vcc (where Vcc is the voltage of the power
supply). Then the output stays at Vcc for a certain length of time
and then it goes back to 0 volts. This is a pulse. A graph
of the output voltage is shown below.
.
The Oscillator (A Clock) - More than just a Pulse
The pulse is nice but
it only happens one time. If you want something that does something
interesting forever rather than just once, you need an oscillator.
An oscillator puts out an endless series of pulses. The output constantly
goes from 0 volts to Vcc and back to 0 volts again. Almost all digital
circuits have some type of oscillator. This stream of output pulses
is often called a clock. You can count the number of pulses to tell
how much time has gone by. We will see how the 555 timer can be used
to generate this clock. A graph of a clock signal is shown below.
.
The Capacitor
If you already understand capacitors
you can skip this part.
 
The picture above on the left shows two typical
capacitors. Capacitors usually have two legs. One leg is the
positive leg and the other is the negative leg. The positive leg
is the one that is longer. The picture on the right is the symbol
used for capacitors in circuit drawings (schematics). When you put
one in a circuit, you must make sure the positive leg and the negative
leg go in the right place. Capacitors do not always have a positive
leg and a negative leg. The smallest capacitors in this kit do not.
It does not matter which way you put them in a circuit.
A capacitor is similar to a rechargable battery
in the way it works. The difference is that a capacitor can only
hold a small fraction of the energy that a battery can. (Except for
really big capacitors like the ones found in old TVs. These can hold
a lot of charge. Even if a TV has been disconnected from the wall
for a long time, these capacitors can still make lots of sparks and hurt
people.) As with a rechargable battery, it takes a while for the
capacitor to charge. So if we have a 12 volt supply and start charging
the capacitor, it will start with 0 volts and go from 0 volts to 12 volts.
Below is a graph of the voltage in the capacitor while it is charging.
.
.
The same idea is true when the capacitor is discharging.
If the capacitor has been charged to 12 volts and then we connect both
legs to ground, the capacitor will start discharging but it will take some
time for the voltage to go to 0 volts. Below is a graph of what the
voltage is in the capacitor while it is discharging.
.
We can control the speed of the capacitor's
charging and discharging using resistors.
Capacitors are given values based on how much
electricity they can store. Larger capacitors can store more energy
and take more time to charge and discharge. The values are given
in Farads but a Farad is a really large unit of measure for common capacitors.
Common capacitors use measurements of pf and uf. Pf means picofarad
and uf means microfarad. A picofarad is 0.000000000001 Farads.
So a 33pf capacitor has a value of 33 picofarads or 0.000000000033 Farads.
A microfarad is 0.000001 Farads. So a 10uf capacitor is 0.00001 Farads
and a 220uF capacitor is 0.000220 Farads. If you do any calculations
with formulas using the value of the capacitor you have to use the Farad
value rather than the picofarad or microfarad value.
Capacitors are also rated by the maximum voltage
they can take. This value is always written on the larger can shaped
capacitors. For example, the 220uF capacitor in this kit has a maximum
voltage rating of 25 volts. If you apply more than 25 volts to them
they will die.
The 555 Timer
Creating a Pulse
The 555 is made
out of simple transistors that are about the same as on / off switches.
They do not have any sense of time. When you apply a voltage they
turn on and when you take away the voltage they turn off. So by itself,
the 555 can not create a pulse. The way the pulse is created is by
using some components in a circuit attached to the 555 (see the circuit
on the next page). This circuit is made of a capacitor and a resistor.
We can flip a switch and start charging the capacitor. The resistor
is used to control how fast the capacitor charges. The bigger the
resistance, the longer it takes to charge the capacitor. The voltage
in the capacitor can then be used as an input to another switch.
Since the voltage starts at 0, nothing happens to the second switch.
But eventually the capacitor will charge up to some point where the second
switch comes on.
The way the 555
timer works is that when you flip the first switch, the Output pin goes
to Vcc (the positive power supply voltage) and starts charging the capacitor.
When the capacitor voltage gets to 2/3 Vcc (that is Vcc * 2/3) the second
switch turns on which makes the output go to 0 volts.
The pinout for
the 555 timer is shown below
Deep Details
Pin 2 (Trigger)
is the 'on' switch for the pulse. The line over the word Trigger
tells us that the voltage levels are the opposite of what you would normally
expect. To turn the switch on you apply 0 volts to pin 2. The
technical term for this opposite behavior is 'Active Low'. It is
common to see this 'Active Low' behavior for IC inputs because of the inverting
nature of transistor circuits like we saw in the LED and Transistor Tutorial.
Pin 6 is the off
switch for the pulse. We connect the positive side of the capacitor
to this pin and the negative side of the capacitor to ground. When
Pin 2 (Trigger) is at Vcc, the 555 holds Pin 7 at 0 volts (Note the inverted
voltage). When Pin 2 goes to 0 volts, the 555 stops holding Pin 7
at 0 volts. Then the capacitor starts charging. The capacitor
is charged through a resistor connected to Vcc. The current starts flowing
into the capacitor, and the voltage in the capacitor starts to increase.
Pin 3 is the output (where
the actual pulse comes out). The voltage on this pin starts at 0
volts. When 0 volts is applied to the trigger (Pin 2), the 555 puts
out Vcc on Pin 3 and holds it at Vcc until Pin 6 reaches 2/3 of Vcc (that
is Vcc * 2/3). Then the 555 pulls the voltage at Pin 3 to ground
and you have created a pulse. (Again notice the inverting action.)
The voltage on Pin 7 is also pulled to ground, connecting the capacitor
to ground and discharging it.
Seeing the pulse
To see the pulse we will use an LED
connected to the 555 output, Pin 3. When the output is 0 volts the
LED will be off. When the output is Vcc the LED will be on.
Building the Circuit
Place the 555 across the middle line of the
breadboard so that 4 pins are on one side and 4 pins are on the other side.
(You may need to bend the pins in a little so they will go in the holes.)
Leave the power disconnected until you finish building the circuit.
The diagram above shows how the pins on the 555 are numbered. You can find
pin 1 by looking for the half circle in the end of the chip. Sometimes
instead of a half circle, there will be a dot or shallow hole by pin 1.
Before you start building the circuit, use
jumper wires to connect the red and blue power rows to the red and blue
power rows on the other side of the board. Then you will be able
to easily reach Vcc and Ground lines from both sides of the board.
(If the wires are too short, use two wires joined together in a row of
holes for the positive power (Vcc) and two wires joined together in a different
row of holes for the ground.)
Connect Pin 1 to ground.
Connect Pin 8 to Vcc.
Connect Pin 4 to Vcc.
Connect the positive leg of the LED to a 330
ohm resistor and connect the negative end of the LED to ground. Connect
the other leg of the 330 ohm resistor to the output, Pin 3.
Connect Pin 7 to Vcc with a 10k resistor (RA
= 10K).
Connect Pin 7 to Pin 6 with a jumper wire.
Connect Pin 6 to the positive leg of the 220uF
Capacitor (C = 220uF). (You will need to bend the positive (long
leg) up and out some so that the negative leg can go in the breadboard.
Connect the negative leg of the capacitor
to ground.
Connect a wire to Pin 2 to use as the trigger.
Start with Pin 2 connected to Vcc.
Now connect the power. The LED will come
on and stay on for about 2 seconds. Remove the wire connected to
Pin 2 from Vcc. You should be able to trigger the 555 again by touching
the wire connected to pin 2 with your finger or by connecting it to ground
and removing it. (It should be about a 2 second pulse.)
Making it Oscillate
Next we will make the LED flash continually
without having to trigger it. We will hook up the 555 so that it
triggers itself. The way this works is that we add in a resistor
between the capacitor and the discharge pin, Pin 7. Now, the capacitor
will charge up (through RA and RB) and when it reaches 2/3 Vcc, Pin 3 and
Pin 7 will go to ground. But the capacitor can not discharge immediately
because of RB. It takes some time for the charge to drain through
RB. The more resistance RB has, the longer it takes to discharge.
The time it takes to discharge the capacitor will be the time the LED is
off.
To trigger the 555 again, we connect Pin 6
to the trigger (Pin 2). As the capacitor is discharging, the voltage
in the capacitor gets lower and lower. When it gets down to 1/3 Vcc
this triggers Pin 2 causing Pin 3 to go to Vcc and the LED to come on.
The 555 disconnects Pin 7 from ground, and the capacitor starts to charge
up again through RA and RB.
To build this circuit
from the previous circuit, do the following.
Disconnect the power.
Take out the jumper
wire between Pin 6 and Pin 7 and replace it with a 2.2k resistor (RB =
2.2K).
Use the jumper wire
at pin 2 to connect Pin 2 to Pin 6.
Now reconnect the power
and the LED should flash forever (as long as you pay your electricity bill).
Experiment with different
resistor values of RA and RB to see how it changes the length of time that
the LED flashes. (You are changing the amount of time that it takes
for the Capacitor to charge and discharge.)
Formulas
These are the formulas
we use for the 555 to control the length of the pulses.
t1 = charge time (how
long the LED is on) = 0.693 * (RA + RB) * C
t2 = discharge time
(how long the LED is off) = 0.693 * RB * C
T = period = t1 + t2
= 0.693 * (RA + 2*RB) * C
Frequency = 1 / T =
1.44 / ((RA + 2 * RB) * C)
t1 and t2 are the time
in seconds. C is the capacitor value in Farads. 220uF = 0.000220
F. So for our circuit we have:
t1 = 0.693 * (10000 + 2200) * 0.000220 = 1.86
seconds
t2 = 0.693 * 2200 * 0.000220 = 0.335 seconds
T = 1.86 + 0.335 = 2.195 seconds
Frequency = 0.456 (cycles per second)
All the parts in this kit are included with the
Beginners
Kit and the Microcontroller
Beginner Kit. If you already have a breadboard and a power supply,
and you just want the parts for this kit, order the 555Kit.
The 555Kit includes:
2 - 555 ICs
5 - 10K ohm Resistors
5 - 2.2K ohm Resistors
5 - 510 ohm Resistors
5 - 330 ohm Resistors
1 - 220 uF Capacitor
5 - LEDs
Jumper Wires
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This page last updated on August
6, 2004. |
