Relaxation Oscillator: Schmitt Trigger Circuits.pptx

1. Relaxation Oscillator
Schmitt Trigger Circuits
Dr. Sarmista Sengupta
Asst. Prof., Dept. of ECE
Techno Main Salt Lake

2. Operation of Schmitt Trigger
• The Schmitt trigger uses positive feedback with
a loop-gain greater than unity to produce a bi-
stable characteristic.

3. Schmitt Trigger…….
Hysteresis effect
The bi-stable characteristic of the circuit occurs around the
point vI = 0, at which the output may be in either its high or low
state. The output remains in either state as long as vI remains in
the range VTL <vI < VTH . The output switches states only if the
input increases above VTH or decreases below VTL .

4. Schmitt Trigger as astable
multivibrator
R1 = R2 =R
High sat. output VH = VP
Low sat. output VL= -VP
If vo = -VP, v+= -VP/2 & when
vo = VP then, v+ = VP/2
The RXCX network sees a positive
step-increase in voltage, so capacitor
CX begins to charge and voltage vX
starts to increase toward a final value
of VP .

5. Schmitt Trigger as astable
multivibrator…..

6. Schmitt Trigger as astable
multivibrator…..

7. Schmitt Trigger as monostable
multivibrator
A monostable multivibrator
has one stable state, in
which it can remain
indefinitely if not disturbed.
However, a trigger pulse can
force the circuit into a quasi-
stable state.
In the stable state, the output is high and voltage vX is held
low by the conducting diode D1.

8. Schmitt Trigger as monostable
multivibrator…..
VP = (Vγ 1 + VZ2).
Negative pulse:

9. Monostable operation (brief)
• The negative-step change in vO causes voltage vX to
decrease exponentially with a time constant of τx =
RXCX toward a final value of −VP .
• Diode D1 is reverse biased during this time. When vX
drops just below the value of vY (=-VP), the output
switches back to its positive saturated value of +VP.
• The capacitor voltage vX then starts to increase
exponentially toward a final value of +VP .
• When vX reaches Vγ, diode D1 again becomes forward
biased, vX is clamped at Vγ , and the output remains in
its high state.

10. Waveform
Where,

11. Relaxation Oscillator
IC 555 timer Circuits

12. 555 IC??
• General purpose IC that can be used for
precision timing, pulse generation, sequential
timing, time delay generation, pulse width
modulation, pulse position modulation, and
linear ramp generation.
• It can operate in both astable and monostable
modes, with timing pulses ranging from
microseconds to hours.
• It also has an adjustable duty cycle

13. Basic operation

14. Basic operation……
Comparator 1 is called the threshold comparator, which compares its
input
with an internal voltage reference set at (2/3)V+ by the voltage divider
R3, R4, and R5. When the input level exceeds this reference level, the
threshold comparator output goes high, producing a high output at flip-
flop terminal Q’. This turns the discharge transistor on and an external
timing capacitor (not shown in this figure) starts to discharge.
Comparator 2, called the trigger comparator, compares its input
trigger voltage
to an internal voltage reference set to (1/3)V+ by the same voltage
divider as before. When the output trigger level is reduced below this
reference level, the trigger comparator output goes high, causing the
RS flip-flop to reset. Output Q’ goes low and the discharge transistor
turns off. This comparator triggers on the leading edge of a negative-
going input pulse.

15. Monostable multivibrator using IC 555
With a high voltage V+ applied to the trigger
input, the trigger comparator output is low,
the flip-flop output Q’ is high, the discharge
transistor is turned on, and the timing
capacitor C is discharged to nearly
ground potential.
When a negative-going pulse is applied to the
trigger input, the output of the
trigger comparator goes high when the trigger
pulse drops below (1/3)V+. Output Q’ goes
low, which means that the output of the 555
goes high, and the discharge transistor turns
off.

16. Astable multivibrator using IC 555
Typical external circuit connection for the 555
operating as an astable multivibrator, also
called a timer circuit or clock.
The threshold input and trigger input
terminals are connected together.
The timing capacitor C charges through RA = RB until
v(t) reaches (2/3)V+. The threshold comparator
output then goes high, forcing the flip-flop output Q’
to go high. The discharge transistor turns on, and the
timing capacitor C discharges through RB and the
discharge transistor. The capacitor voltage decreases
until it reaches (1/3)V+, at which point the trigger
comparator switches states and sends Q’ low. The
discharge transistor turns off, and the timing
capacitor begins to recharge. When v(t) reaches the
threshold level of (2/3)V+, the cycle repeats itself.

17. Astable multivibrator using IC 555
• Charging time of C TC + Discharging time of C
TD = Time period of oscillation T