1. https://www.electronics-tutorials.ws/waveforms/555_timer.

html
2. https://www.allaboutcircuits.com/textbook/experiments/chpt-6/555-
ramp-generator/

We have seen that Multivibrators and CMOS Oscillators can be easily

constructed from discrete components to produce relaxation
oscillators for generating basic square wave output waveforms. But
there are also dedicated IC’s such as the 555 timer especially
designed to accurately produce the required output waveform with
the addition of just a few extra timing components.
One such device that has been around since the early days of IC’s
and has itself become something of an industry “standard” is
the 555 Timer Oscillator which is more commonly called the “555
Timer”.
The basic 555 timer gets its name from the fact that there are
three internally connected 5kΩ resistors which it uses to generate
the two comparators reference voltages. The 555 timer IC is a very
cheap, popular and useful precision timing device which can act as
either a simple timer to generate single pulses or long time delays,
or as a relaxation oscillator producing a string of stabilised
waveforms of varying duty cycles from 50 to 100%.
The 555 timer chip is extremely robust and stable 8-pin device that
can be operated either as a very accurate Monostable, Bistable or
Astable Multivibrator to produce a variety of applications such as
one-shot or delay timers, pulse generation, LED and lamp flashers,
alarms and tone generation, logic clocks, frequency division, power
supplies and converters etc, in fact any circuit that requires some
form of time control as the list is endless.
The single 555 Timer chip in its basic form is a Bipolar 8-pin mini
Dual-in-line Package (DIP) device consisting of some 25 transistors,
2 diodes and about 16 resistors arranged to form two comparators,
a flip-flop and a high current output stage as shown below. As well
as the 555 Timer there is also available the NE556 Timer Oscillator
which combines TWO individual 555’s within a single 14-pin DIP
package and low power CMOS versions of the single 555 timer such
as the 7555 and LMC555 which use MOSFET transistors instead.
A simplified “block diagram” representing the internal circuitry of
the 555 timer is given below with a brief explanation of each of its
connecting pins to help provide a clearer understanding of how it
works.

555 Timer Block Diagram

 • Pin 1. – Ground, The ground pin connects the 555 timer

to the negative (0v) supply rail.
 • Pin 2. – Trigger, The negative input to comparator No 1.
A negative pulse on this pin “sets” the internal Flip-flop when
the voltage drops below 1/3Vcc causing the output to switch
from a “LOW” to a “HIGH” state.
 • Pin 3. – Output, The output pin can drive any TTL circuit
and is capable of sourcing or sinking up to 200mA of current
at an output voltage equal to approximately Vcc – 1.5V so
small speakers, LEDs or motors can be connected directly to
the output.
 • Pin 4. – Reset, This pin is used to “reset” the internal Flip-
flop controlling the state of the output, pin 3. This is an
active-low input and is generally connected to a logic “1”
level when not used to prevent any unwanted resetting of
the output.
  • Pin 5. – Control Voltage, This pin controls the timing of
the 555 by overriding the 2/3Vcc level of the voltage divider
network. By applying a voltage to this pin the width of the
output signal can be varied independently of the RC timing
network. When not used it is connected to ground via a 10nF
capacitor to eliminate any noise.
 • Pin 6. – Threshold, The positive input to comparator No
2. This pin is used to reset the Flip-flop when the voltage
applied to it exceeds 2/3Vcc causing the output to switch
from “HIGH” to “LOW” state. This pin connects directly to the
RC timing circuit.
 • Pin 7. – Discharge, The discharge pin is connected
directly to the Collector of an internal NPN transistor which is
used to “discharge” the timing capacitor to ground when the
output at pin 3 switches “LOW”.
 • Pin 8. – Supply +Vcc, This is the power supply pin and
for general purpose TTL 555 timers is between 4.5V and 15V.
The 555 Timers name comes from the fact that there are
three 5kΩ resistors connected together internally producing a
voltage divider network between the supply voltage at pin 8 and
ground at pin 1. The voltage across this series resistive network
holds the negative inverting input of comparator two at 2/3Vcc and
the positive non-inverting input to comparator one at 1/3Vcc.
The two comparators produce an output voltage dependent upon
the voltage difference at their inputs which is determined by the
charging and discharging action of the externally
connected RC network. The outputs from both comparators are
connected to the two inputs of the flip-flop which in turn produces
either a “HIGH” or “LOW” level output at Q based on the states of its
inputs. The output from the flip-flop is used to control a high current
output switching stage to drive the connected load producing either
a “HIGH” or “LOW” voltage level at the output pin.
The most common use of the 555 timer oscillator is as a simple
astable oscillator by connecting two resistors and a capacitor across
its terminals to generate a fixed pulse train with a time period
determined by the time constant of the RC network. But the 555
timer oscillator chip can also be connected in a variety of different
ways to produce Monostable or Bistable multivibrators as well as the
more common Astable Multivibrator.
The Monostable 555 Timer
The operation and output of the 555 timer monostable is exactly
the same as that for the transistorised one we look at previously in
the Monostable Multivibrators tutorial. The difference this time is
that the two transistors have been replaced by the 555 timer device.
Consider the 555 timer monostable circuit below.

Monostable 555 Timer

When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of

the Monostable configured 555 Timer oscillator, the internal
comparator, (comparator No1) detects this input and “sets” the
state of the flip-flop, changing the output from a “LOW” state to a
“HIGH” state. This action in turn turns “OFF” the discharge transistor
connected to pin 7, thereby removing the short circuit across the
external timing capacitor, C1.
This action allows the timing capacitor to start to charge up through
resistor, R1 until the voltage across the capacitor reaches the
threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage
divider network. At this point the comparators output goes “HIGH”
and “resets” the flip-flop back to its original state which in turn turns
“ON” the transistor and discharges the capacitor to ground through
pin 7. This causes the output to change its state back to the original
stable “LOW” value awaiting another trigger pulse to start the
timing process over again. Then as before, the Monostable
Multivibrator has only “ONE” stable state.
The Monostable 555 Timer circuit triggers on a negative-going
pulse applied to pin 2 and this trigger pulse must be much shorter
than the output pulse width allowing time for the timing capacitor to
charge and then discharge fully. Once triggered, the 555
Monostable will remain in this “HIGH” unstable output state until the
time period set up by the R1 x C1 network has elapsed. The amount
of time that the output voltage remains “HIGH” or at a logic “1”
level, is given by the following time constant equation.

Where, t is in seconds, R is in Ω and C in Farads.

555 Timer Example No1

A Monostable 555 Timer is required to produce a time delay
within a circuit. If a 10uF timing capacitor is used, calculate the
value of the resistor required to produce a minimum output time
delay of 500ms.
500ms is the same as saying 0.5s so by rearranging the formula
above, we get the calculated value for the resistor, R as:

The calculated value for the timing resistor required to produce the
required time constant of 500ms is therefore, 45.5KΩ. However, the
resistor value of 45.5KΩ does not exist as a standard value resistor,
so we would need to select the nearest preferred value resistor
of 47kΩ which is available in all the standard ranges of tolerance
from the E12 (10%) to the E96 (1%), giving us a new recalculated
time delay of 517ms.
If this time difference of 17ms (500 – 517ms) is unacceptable
instead of one single timing resistor, two different value resistor
could be connected together in series to adjust the pulse width to
the exact desired value, or a different timing capacitor value
chosen.
We now know that the time delay or output pulse width of a
monostable 555 timer is determined by the time constant of the
connected RC network. If long time delays are required in the 10’s of
seconds, it is not always advisable to use high value timing
capacitors as they can be physically large, expensive and have large
value tolerances, e.g, ±20%.
One alternative solution is to use a small value timing capacitor and
a much larger value resistor up to about 20MΩ’s to produce the
require time delay. Also by using one smaller value timing capacitor
and different resistor values connected to it through a multi-position
rotary switch, we can produce a Monostable 555 timer oscillator
circuit that can produce different pulse widths at each switch
rotation such as the switchable Monostable 555 timer circuit shown
below.

A Switchable Monostable Circuit

We can manually calculate the values of R and C for the individual
components required as we did in the example above. However, the
choice of components needed to obtain the desired time delay
requires us to calculate with either kilohm’s (KΩ), Megaohm’s (MΩ),
microfarad’s (μF) or picafarad’s (pF) and it is very easy to end up
with a time delay that is out by a factor of ten or even a hundred.
We can make our life a little easier by using a type of chart called a
“Nomograph” that will help us to find the monostable multivibrators
expected frequency output for different combinations or values of
both the R and C. For example,

Monostable Nomograph

So by selecting suitable values of C and R in the ranges of 0.001uF

to 100uF and 1kΩ to 10MΩ respectively, we can read the expected
output frequency directly from the nomograph graph thereby
eliminating any error in the calculations. In practice the value of the
timing resistor for a monostable 555 timer should not be less than
1kΩ or greater than 20MΩ.

Bistable 555 Timer

As well as the one shot 555 Monostable configuration above, we
can also produce a Bistable (two stable states) device with the
operation and output of the 555 Bistable being similar to the
transistorised one we look at previously in the Bistable
Multivibrators tutorial.
The 555 Bistable is one of the simplest circuits we can build using
the 555 timer oscillator chip. This bistable configuration does not
use any RC timing network to produce an output waveform so no
equations are required to calculate the time period of the circuit.
Consider the Bistable 555 Timer circuit below.

Bistable Flip-flop Circuit

The switching of the output waveform is achieved by controlling the

trigger and reset inputs of the 555 timer which are held “HIGH” by
the two pull-up resistors, R1 and R2. By taking the trigger input (pin
2) “LOW”, switch in set position, changes the output state into the
“HIGH” state and by taking the reset input (pin 4) “LOW”, switch in
reset position, changes the output into the “LOW” state.
This 555 timer circuit will remain in either state indefinitely and is
therefore bistable. Then the Bistable 555 timer is stable in both
states, “HIGH” and “LOW”. The threshold input (pin 6) is connected
to ground to ensure that it cannot reset the bistable circuit as it
would in a normal timing application.

555 Timer Output

We could not finish this 555 Timer tutorial without discussing
something about the switching and drive capabilities of the 555
timer or indeed the dual 556 Timer IC.
The output (pin 3) of the standard 555 timer or the 556 timer, has
the ability to either “Sink” or “Source” a load current of up to a
maximum of 200mA, which is sufficient to directly drive output
transducers such as relays, filament lamps, LED’s motors, or
speakers etc, with the aid of series resistors or diode protection.
This ability of the 555 timer to both “Sink” (absorb) and “Source”
(supply) current means that the output device can be connected
between the output terminal of the 555 timer and the supply to sink
the load current or between the output terminal and ground to
source the load current. For example.

Sinking and Sourcing the 555 Timer Output

In the first circuit above, the LED is connected between the positive
supply rail ( +Vcc ) and the output pin 3. This means that the
current will “Sink” (absorb) or flow into the 555 timer output
terminal and the LED will be “ON” when the output is “LOW”.
The second circuit above shows that the LED is connected between
the output pin 3 and ground ( 0v ). This means that the current will
“Source” (supply) or flow out of the 555 timers output terminal and
the LED will be “ON” when the output is “HIGH”.
The ability of the 555 timer to both sink and source its output load
current means that both LED’s can be connected to the output
terminal at the same time but only one will be switched “ON”
depending whether the output state is “HIGH” or “LOW”. The circuit
to the left shows an example of this. the two LED’s will be
alternatively switched “ON” and “OFF” depending upon the output.
Resistor, R is used to limit the LED current to below 20mA.
We said earlier that the maximum output current to either sink or
source the load current via pin 3 is about 200mA at the maximum
supply voltage, and this value is more than enough to drive or
switch other logic IC’s, LED’s or small lamps, etc. But what if we
wanted to switch or control higher power devices such as motors,
electromagnets, relays or loudspeakers. Then we would need to use
a Transistor to amplify the 555 timers output in order to provide a
sufficiently high enough power to drive the load.

555 Timer Transistor Driver

The transistor in the two examples above, can be replaced with a
Power MOSFET device or Darlington transistor if the load current is
high. When using an inductive load such as a motor, relay or
electromagnet, it is advisable to connect a freewheeling (or
flywheel) diode directly across the load terminals to absorb any back
emf voltages generated by the inductive device when it changes
state.
Thus far we have look at using the 555 Timer to generate
monostable and bistable output pulses. In the next tutorial
about Waveform Generation we will look at connecting the 555 in an
astable multivibrator configuration. When used in the astable mode
both the frequency and duty cycle of the output waveform can be
accurately controlled to produce a very versatile waveform
generator.

Previous
Waveform Generators

Next
555 Oscillator Tutorial
Read more Tutorials inWaveform Generators
 1. Electrical Waveforms
 2. Monostable Multivibrator
 3. Bistable Multivibrator
 4. Astable Multivibrator
 5. Waveform Generators
 6. 555 Timer Tutorial
 7. 555 Oscillator Tutorial
 8. 555 Circuits Part 1
 9. 555 Circuits Part 2

555 Ramp Generator

PDF Version

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PARTS AND MATERIALS

 Two 6 volt batteries

 One capacitor, 470 µF electrolytic, 35 WVDC (Radio Shack
catalog # 272-1030 or equivalent)
 One capacitor, 0.1 µF, non-polarized (Radio Shack catalog
# 272-135)
 One 555 timer IC (Radio Shack catalog # 276-1723)
 Two PNP transistors—models 2N2907 or 2N3906
recommended (Radio Shack catalog # 276-1604 is a
 package of fifteen PNP transistors ideal for this and other
experiments)
 Two light-emitting diodes (Radio Shack catalog # 276-026
or equivalent)
 One 100 kΩ resistor
 One 47 kΩ resistor
 Two 510 Ω resistors
 Audio detector with headphones

The voltage rating on the 470 µF capacitor is not critical, so long as

it generously exceeds the maximum power supply voltage. In this
particular circuit, that maximum voltage is 12 volts. Be sure you
connect this capacitor in the circuit properly, respecting polarity!
CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 13: “Capacitors”
Lessons In Electric Circuits, Volume 4, chapter 10: “Multivibrators”

LEARNING OBJECTIVES

 To illustrate how to use the 555 timer as an astable

multivibrator
 To show a practical use for a current mirror circuit
 To help in understanding the relationship between
capacitor current and capacitor voltage rate-of-change

SCHEMATIC DIAGRAM

ILLUSTRATION
INSTRUCTIONS
Again, we are using a 555 timer IC as an astable multivibrator, or
oscillator. This time, however, we will compare its operation in two
different capacitor-charging modes: traditional RC and constant-
current.
Connecting test point #1 (TP1) to test point #3 (TP3) using a
jumper wire. This allows the capacitor to charge through a
47 kΩ resistor. When the capacitor has reached 2/3 supply
voltage, the 555 timer switches to “discharge” mode and
discharges the capacitor to a level of 1/3 supply voltage
almost immediately. The charging cycle begins again at this
point.
Measure voltage directly across the capacitor with a voltmeter (a
digital voltmeter is preferred) and note the rate of capacitor
charging over time. It should rise quickly at first, then taper off as it
builds up to 2/3 supply voltage, just as you would expect from an RC
charging circuit.
Remove the jumper wire from TP3, and re-connect it to TP2. This
allows the capacitor to be charged through the controlled-current
leg of a current mirror circuit formed by the two PNP transistors.
Measure voltage directly across the capacitor again, noting the
difference in charging rate over time as compared to the last circuit
configuration.
By connecting TP1 to TP2, the capacitor receives a nearly
constant charging current. Constant capacitor charging
current yields a voltage curve that is linear, as described by
the equation i = C(de/dt). If the capacitors current is
constant, so will be its rate-of-change of voltage over time.
The result is a “ramp” waveform rather than a “sawtooth”
waveform:
The capacitor’s charging current may be directly measured by
substituting an ammeter in place of the jumper wire. The ammeter
will need to be set to measure a current in the range of hundreds of
microamps (tenths of a milliamp). Connected between TP1 and TP3,
you should see a current that starts at a relatively high value at the
beginning of the charging cycle, and tapers off toward the end.
Connected between TP1 and TP2, however, the current will be much
more stable.
It is an interesting experiment at this point to change the
temperature of either current mirror transistor by touching it with
your finger. As the transistor warms, it will conduct more collector
current for the same base-emitter voltage. If
the controlling transistor (the one connected to the 100 kΩ resistor)
is touched, the current decreases.
If the controlled transistor is touched, the current increases. For the
most stable current mirror operation, the two transistors should be
cemented together so that their temperatures never differ by any
substantial amount.
This circuit works just as well at high frequencies as it does at low
frequencies. Replace the 470 µF capacitor with a 0.1 µF capacitor,
and use an audio detector to sense the voltage waveform at the
555’s output terminal. The detector should produce an audio tone
that is easy to hear. The capacitor’s voltage will now be changing
much too fast to view with a voltmeter in the DC mode, but we can
still measure capacitor current with an ammeter.
With the ammeter connected between TP1 and TP3 (RC mode),
measure both DC microamps and AC microamps. Record these
current figures on paper. Now, connect the ammeter between TP1
and TP2 (constant-current mode).
Measure both DC microamps and AC microamps, noting any
differences in current readings between this circuit configuration
and the last one. Measuring AC current in addition to DC current is
an easy way to determine which circuit configuration gives the most
stable charging current.
If the current mirror circuit were perfect—the capacitor charging
current absolutely constant—there would be zero AC current
measured by the meter.