eet 157 - new jersey institute of technologyecelabs.njit.edu/fed101/resources/555_timer.pdf ·...
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Electrical Engineering Technology
EET 157
Electronics Circuit Analysis # 05
Revised by Dr. Athula Kulatunga
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Overview
♦Multivibrators
♦ 555 Timer
♦ 555 Timer Monostatble Multivibrators
♦555 Timer Astable Multivibrators
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Waveforms
sine square rectangle
A dc source can be used to generate all the above shapes
triangle exponentialsawtooth
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Multivibrators
Pulse generatorsTypes Characteristics
Astable or free-running no stable stateMonostable or one shot one stable stateBistable two stable states
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable or Free-running
Astable Multivibrator
Vout
unstableVH
VL unstable
TL
T
TH
W
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Duty Cycle
Defined for rectangular waveforms
%100×=TW
D
Square wave has 50% duty cycle.
Why?
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Duty Cycle Example 1
A rectangular signal is high for 4 ms and low for 6 ms. Find the signal’s period, pulse width, duty cycle, and frequency.
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Duty Cycle Example 2
A rectangular waveform has a frequency of 1k Hz and a duty cycle of 60%. Find the signal’s repetition rate, period, and pulse width.
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Monostable Operation
unstable
VL stable
VH
W
T1
Monostable Multivibrator
VoutVin
T1 T1
The monostable multivibrator produces a well-defined pulse out for an input signal that may not be well defined (a narrow pulse or a spike)
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Bistable OperationThe bistable multivibrator has two stable states and utilizes one input pulse to take the output high and another input pulse to take the output low.
VH
VL stable
stable
Bistable Multivibrator
VoutVin_1
T1
Vin_2 T1 T2
T2
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer IC - Schematic
SET
RESET
Q
Q
RS Flip-FlopUTP
LTP
To +Esupply
VCC
8
1COMMON
2TRIGGER
THRESHOLD6
5CONTROL
4RESET
3OUTPUT
DISCHARGE
7
R
R
R
U1
U2
Q1
Inverter
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer IC - Features• Q FF output−high or low output• Q Complement (opposite) of Q−low or high
output• SET FF input−high signal sets high and low• RESET FF input−high signal in resets low and high• U1 Noninverting comparator driving the FF set input• U2 Inverting comparator driving the FF reset input• UTP Upper trip point for U1 set by 2R/3R of Esupply
• LTP Lower trip point for U2 set by R/3R of Esupply
• Q1 BJT used to discharge external capacitor when Q high• Pin 7 Open or tied to common through BJT switched on
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer - Symbol & Chip
1
2
3
4
5
6
7
8
555
LM
555
1
2
4
3
7
5
6
8
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer – Monostable Connection
12
3
4
5
6
78
555
+5 V
vout
R1 k
C1 µ
0.01 µ
0 V
3.33 V
T1
W
+5 V
less than 1.67 V
T1
W
T1
+5 V
0 V
input
RCcharge
capacitordischargethrough BJT
Output stays at 0 Vuntil input dipsbelow 1.67 V.
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer – Monostable Operation
SET
RESET
Q
Q
RS Flip-FlopUTP
LTP
To +E supply
VC C
8
1COMMON
2TRIGGER
THRESHOLD6
5CONTROL
4RESET
3OUTPUT
DISCHARGE7
R
R
R
U1
U2
Q1
Inverter
+5Vdc
1k
1uL
H ON
0V
L
5VL
Remains SET
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer – Monostable Operation
SET
RESET
Q
Q
RS Flip-FlopUTP
LTP
To +E supply
VC C
8
1COMMON
2TRIGGER
THRESHOLD6
5CONTROL
4RESET
3OUTPUT
DISCHARGE7
R
R
R
U1
U2
Q1
Inverter
+5Vdc
1k
1uH
L OFFStart
Charging
H
H
RESET
Drop below
1.67V
3.3V
SET
H
L
ONStart
Discharging
0V
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Monostable Pulse Width Calculation
ms1F1k1 =µ×Ω==τ RC( ) τ/
C eV5V0V5)( ttv −−+=ms1/
C eV5V5)( ttv −−=ms1/eV5V5V33.3 t−−=
ms10.1=t ms10.1=W
The pulse width W is the time required to charge the capacitor; therefore,
vout
T1
W
+5 V
0 V
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer – Astable Connection
12
3
4
56
7
8
555
+5 V
vout
R21 k
C1 µ
0.01 µ
R11 k
(R1+R2) Cchargetowardsupply
(R2) Cdischarge
through pin 7and BJT short to
common
W
T
TLTH
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Purdue University EET 157 - 05 Electronics Circuit Analysis
555 Timer – Astable Operation
SET
RESET
Q
Q
RS Flip-FlopUTP
LTP
To +E supply
VC C
8
1COMMON
2TRIGGER
THRESHOLD6
5CONTROL
4RESET
3OUTPUT
DISCHARGE7
R
R
R
U1
U2
Q1
Inverter
+5Vdc
R1: 1k
1u L
ON
R2: 1k
H
Discharge
towards 0V
<1.67V
H
RESET
L
H
OFF
Charge
Towards 5V
from 1.67V
<3.3V
L
H
SET
H
L
ON
Discharge Towards
0V through R2
Length Depend on (R1+R2)C
Depend on (R2)C
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Width CalculationThe pulse width W depends upon the (R1 + R2) C
time constant τH and the capacitor charging from 1.67 V to 3.33 V while the output is in the high (or 5 V) state. The capacitor must follow the universal dc transient expression
τ/ssinitssC e)()( tVVVtv −−+=
( ) ms2F1k2C2R1R =µ×Ω=+=
( ) ms2/C eV5V67.1V5)( ttv −−+=
ms2/C eV33.3V5)( ttv −−=
τ
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Width Calculation
The capacitor charges to 3.33 V (or 2/3 of VCC) when it transitions back to its stable state. Set the capacitor voltage expression equal to 3.33 V and solve the expression inversely for the time t required to charge the capacitor to 3.33 V.
ms2/eV33.3V5V33.3 t−−= ms386.1=t
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Width Calculation
The pulse width W is the time required to charge the capacitor, so
ms386.1=W
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Space Calculation
uThe time TL of the pulse depends upon the (R2) C time constant τL and discharging the capacitor from 3.33 V to 1.67 V while the output is in the low (0 V) state. The capacitor must follow the universal dc transient expression
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Space Calculation
The capacitor starts with a value of 3.33 V (initial value) and attempts to fall to 0 V
( ) ms1F1k1C2RL =µ×Ω==τ
( ) ms1/C eV0V33.3V0)( ttv −−+=
ms1/C eV33.3)( ttv −=
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Space Calculation
u The capacitor discharges to 1.67 V (or 1/3 of VCC), when it transitions back to its stable state. Set the capacitor voltage expression equal to 1.67 V and solve the expression inversely for the time t required to discharge the capacitor to 1.67 V.
ms1/eV33.3V67.1 t−=
ms693.0=t
ms693.0=t
ms693.0L =T
ms693.0L =T
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Astable Pulse Space Calculation
T = TH + TLT = 1.386ms +0.693 msT = 2.079 ms
f = 1/T f = 481 Hz
W
T
TL
TH
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Purdue University EET 157 - 05 Electronics Circuit Analysis
Overview
♦Multivibrators
♦ 555 Timer
♦ 555 Timer Monostatble Multivibrators
♦555 Timer Astable Multivibrators