Download 1 MONOSTABLES A monostable circuit provides a single output

MONOSTABLES
A monostable circuit provides a single output pulse (one-shot) of fixed
duration when a short pulse is applied at its trigger input.
T
MONOSTABLE
Trigger
pulse
Q
Output
pulse
Fig. 1. Function of a monostable
Under normal conditions, the monostable will be in a stable state with its
output Q at logic level 0. When it receives a short trigger pulse, it enters
an unstable state and output Q changes to logic level 1. It remains in this
state for a time (T) then changes back to its stable state. The duration of T
depends upon values for resistors and capacitors in the timing circuit.
While in its unstable state, the monostable ignores any pulses applied at
its input. It can only be re-triggered after it has returned to its stable state.
Some monostable circuits are triggered by the rising edge of the trigger
pulse while others are triggered by the falling edge. You will come across
examples of both types in your Practical Exercise.
PRACTICAL APPLICATIONS
TIMERS
Monostables are often used as timers e.g. to switch on a light for a fixed
period of time after being triggered.
PULSE STRETCHERS
Logic probes are often fitted with pulse stretching circuits so that an indicator
stays on for a few seconds when a very short pulse appears in a logic circuits.
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THE 555 TIMER IC
The 555 IC was the first general-purpose timer (ASIC - Application Specific Integrated
Circuit)capable of both monostable and astable operation. The device is available in an
8-pin dual in line (DIL) package with pin outs shown in fig.2.
Ground
Trigger
Output
Reset
1
2
3
4
8
7
6
5
Fig. 2
+Vcc
Discharge
Threshold
Control voltage
The pin layout of the 555
THE 555 AS A MONOSTASBLE
The 555 timer can be configured as a monostable using only 3 external components.
Vcc (3 to 15 V)
R
8
Output
555
7
Trig
Input
4
6
3
2
5
C
1
T
10nF
0V
Fig. 3 The 555 as a monostable
A 10nF capacitor is connected to pin number 5 to reduce noise at the input of the
comparator.
2
The trigger is active low, producing an output pulse of duration:
T = 1.1 R x C
It is recommended that R is in the range of 10 kΩ to 10 MΩ
100pF to 1000uF for power economy and predictability.
and C in the range of
Practical use:
To produce a delay of 5 seconds the values of R and C must be derived. The simplest
method is construct the resistor from a fixed 1kΩ and a 100kΩ variable. The fixed value
limits the lower resistance.
Therefore R is in the range 1 to 101 kΩ.
If we assume that R is a mid value approximately 50kΩ then we are able to find a value
for C
T = 1.1 x R x C
C = T/ ( 1.1 x R)
C = 5 / ( 1.1 x 50x103)
C = 9.1x10-5 F= 91µF
The closest value would be a 100µF capacitor, the 100kΩ variable resistor could be
adjusted to produce a 10 second delay.
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ACTIVITY
In this Activity you will be setting up a 555 IC Timer to form a monostable
circuit.
EQUIPMENT AND COMPONENTS REQUIRED
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•
•
Resistors (1k, 47k, 100k)
Capacitors (10nF, 10µF, 100µF, 220µF, 470µF)
555 TIMER IC PACKAGE
INFORMATION
The following diagram shows how a 555 IC Timer can be set up as a
monostable.
+5V
8
R
4
6
Output
555
7
Trig
Input
3
2
5
C
1
T
10nF
0V
The monostable is triggered by applying a falling edge at pin 2.
The duration (T) of the output pulse provided at pin 3 is given by:
T
=
1.1 x R x C
T is in seconds if R is in ohms and C is in Farads.
Investigate changing the values of R and C, produce a table of output pulse time for
different values.
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