555 Timer operation and its application ( Astable and monostable multivibrator )

1. 555 TIMER
1Dept of ECE , NHCE

2. V1
V2
V0 = (V2 – V1) AOL
If V2 > V1 then Vo= + Vsat (VCC – 1) = 11 V
If V2 < V1 then V0 = - Vsat (-VEE + 1) = - 11 V
If -VEE = 0 then
If V2 > V1 then Vo= + Vsat (VCC – 1) = 11 V
If V2 < V1 then V0 = 0
COMPARATOR BASICS
2Dept of ECE , NHCE

3. 555 Timer Pin diagram
• It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts
supply voltage.
Pin Configuration The function of each pin of the IC is
given below –
•Pin–1: Ground
•Pin–2: Trigger
•Pin–3: Output
•Pin–4: Reset
•Pin–5: Control Voltage
•Pin–6: Threshold
•Pin–7: Discharge
•Pin–8: Vcc
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4. 555 Timer Functional diagram
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5. The basic building blocks of 555 timer are:-
a) A Voltage divider
b) Two comparators
c) RS Flip Flop,
d) Discharge transistor
• The voltage divider consists of three identical 5k resistors which create two reference
voltages at 1/3 and 2/3 of the supplied voltage (VCC) , which can range from 5 to 15V.
• The first comparator negative input terminal is connected to the 2/3 reference voltage
at the voltage divider and the external “control” pin, while the positive input terminal to
the external “Threshold” pin.
• On the other hand, the second comparator negative input terminal is connected to the
“Trigger” pin, while the positive input terminal to the 1/3 reference voltage at the
voltage divider.
• So using the three pins, Trigger, Threshold and Control, we can control the output of
the two comparators which are then fed to the R and S inputs of the flip-flop.
5Dept of ECE , NHCE

6. • The RS flip flop takes input from comparators and produces output according to truth table
Below
• The Q-bar output of the flip-flip goes to the inverted output stage
• The Q-bar output of the flip-flip is also connected to a transistor that connects the
“Discharge” pin to ground.
6Dept of ECE , NHCE

7. Modes of Operation
555 IC Timer applications can be classified into two main categories:
1. Monostable Multivibrators : Always in stable state zero when negative trigger pulse
is applied it produces high pulse for fixed amount of time and again returns to low
state .
2. Astable Multivibrators : It doesn’t have a stable state and continuously switches
between High and Low
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8. Monostable Multivibrators
Here On time is 1.1 RC
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For understanding purpose

9. • The trigger input is held High by connecting to VCC That means that the lower comparator
will output 0 to the S input of the flip-flop.
•On the other hand, the Threshold pin is has zero volts and that makes the Upper
comparator output 0 as well. The Threshold pin is actually Low because the Q-bar output of
the flip-flop is High, which keeps the discharge transistor on , so the voltage coming from
the source(VCC) is going to ground through that transistor.
• In order to change the 555 we apply negative going trigger pulse thus the lower
comparator will output 1 to the S input of the flip-flip.
• This will cause the Q-bar output to go Low and the 555 Timer output High. At the same
time, we can notice that the discharge transistor is turned off, so now the capacitor C1 will
start charging through the resistor R1.
• When trigger voltage becomes Vcc , S input to flip-flop becomes zero with S=0 and R = 0
the out retains its state ( Q = 1 , Qbar = 0 ) until threshold voltage is 2VCC/3
• When threshold voltage( capacitor voltage) tries to increase more than 2VCC/3 the upper
comparator will output 1 to the R input of the flip-flip.
•This will bring the circuit into the initial state. The Q=0 and Qbar=1, which will activate the
discharge transistor discharging capacitor and IC output is Low again.
• Here On time of output is 1.1 RC
9Dept of ECE , NHCE

10. Dept of ECE , NHCE 10
Step Capacitor S R Q Qbar Transistor V0 Result
Initial
step
(Trigger
input at
VCC )
Zero 0 0 0 1 0n 0 Capacitor discharged
Negative
going
trigger
pulse
Zero 1 0 1 0 off 1 Capacitor starts charging
Trigger
input at
VCC
Continues
to charge
0 0 1 0 Off 1 Capacitor continues
Charging.
Trigger
input at
VCC
2VCC/3 0 1 0 1 On 0 Capacitor starts
discharging
Trigger
input at
VCC
Less than
2VCC/3
0 0 0 1 on 0 Capacitor discharges
completely and it will
reach to initial step

11. Dept of ECE , NHCE 11
Astable Multivibrator
• It doesn’t have a stable state and continuously switches between High and Low state
• The connections for Astable multivibrator is as shown in diagram
• Initially when capacitor voltage is zero S=1 and R=0 , therfore Q=1 and Qbar = 0 there by
transistor turns off and capacitor starts to charge through resistors RA and RB.

12. Dept of ECE , NHCE 12
• When Capacitor voltage increases by VCC/3 . Lower comparator outputs zero and S becomes 0 and with
S=0 and R =0 it flip flop retain its state ( Q=1 and Qbar= 0) and capacitor continues to charge
• Once it reaches 2VCC/3, the upper comparator will output 1 to the R input of the flip-flop then Q
becomes zero and Qbar becomes one .
•This will active the discharging transistor and now the capacitor will start discharging through the resistor
RB . As capacitor voltage reduces from 2VCC/3 ,R becomes 0.
•At this moment the output of the 555 Timer is Low.
• When capacitor voltage discharges to VCC/3 lower comparator becomes one I,e S=1 and Vo=1 .
Again above explained operation continues with capacitor charging through RA and RB .
• This Cycle repeats unless power is off.
Step Capacitor S R Q Qbar Transistor V0 Result
1 Zero 1 0 1 0 off 1 Capacitor starts charging
2 VCC/3 0 0 1 0 off 1 Capacitor continues charging
3 2VCC/3 0 1 0 1 On 0 Capacitor starts discharging
4 Less than
2VCC/ 3
0 0 0 1 On 0 Capacitor continues discharging
5 VCC/3 1 0 1 0 off 1 Capacitor starts charging(from
step 2 continues )

13. Dept of ECE , NHCE 13

14. Dept of ECE , NHCE 14
For understanding purpose
To find THigh
To find TLOW