Schmitt Trigger

Schmitt Trigger

Introduction

In electronics, a Schmitt trigger is a comparator circuit that incorporates positive feedback.

In the non-inverting configuration, when the input is higher than a certain chosen threshold, the

output is high; when the input is below a different (lower) chosen threshold, the output is low;

when the input is between the two, the output retains its value. The trigger is so named because

the output retains its value until the input changes sufficiently to trigger a change. This dual

threshold action is called hysteresis, and implies that the Schmitt trigger has some memory. In

fact, the Schmitt trigger is a bistable multivibrator.

Schmitt trigger devices are typically used in open loop configurations for noise immunity and

closed loop positive feedback configurations to implement multivibrators.

A comparator circuit, whether using an operational amplifier or some other electronics

technology will work well under some conditions but it is not always ideal. If there is a slow

waveform, or one with some noise ion it, then there is the possibility that the output will switch

back and forth several times during the switch over phase as only small levels of noise on the

input will cause the output to change.

This may not be a problem in some circumstances, but if the output from the operational

amplifier comparator is being fed into fast logic circuitry, then it can often give rise to problems

as the circuit will see several low high or high low transitions and will respond to each one. This

can easily cause many problems.

Under these circumstances circuits that combat this problem are require. One known as the

Schmitt trigger has been in use for many years, having been originally invented by an American

scientist named Otto Schmitt. The Schmitt trigger switches at different voltages depending upon

whether it is moving from low to high or high to low, employing what is termed hysteresis.

WS Project Report 1

Schmitt Trigger

Invention

The Schmitt trigger was invented by US scientist Otto H. Schmitt in 1934 while he was still a

graduate student,[1] later described in his doctoral dissertation (1937) as a "thermionic trigger". It

was a direct result of Schmitt's study of the neural impulse propagation in squid nerves.

Symbol

The symbol for Schmitt triggers in circuit diagrams is a triangle with an inverting or non-

inverting hysteresis symbol. The symbol depicts the corresponding ideal hysteresis curve.

Standard Schmitt trigger

   

Inverting Schmitt trigger

Implementation

A Schmitt trigger can be implemented with a simple tunnel diode, a diode with an "N"-shaped

current–voltage characteristic in the first quadrant. An oscillating input will cause the diode to

move from one rising leg of the "N" to the other and back again as the input crosses the rising

and falling switching thresholds. However, the performance of this Schmitt trigger can be

improved with transistor-based devices that make explicit use of positive feedback to implement

the switching.

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Schmitt Trigger

Schmitt trigger circuit

The problem can be solved very easily by adding some positive feedback to the operational

amplifier or comparator circuit. This is provided by the addition of R3 in the circuit below and

the circuit is known as a Schmitt trigger.

Operational Amplifier Schmitt Trigger Circuit

The effect of the new resistor (R3) is to give the circuit different switching thresholds dependent

upon the output state of the comparator or operational amplifier. When the output of the

comparator is high, this voltage is fed back to the non-inverting input of the operational amplifier

of comparator. As a result the switching threshold becomes higher. When the output is switched

in the opposite sense, the switching threshold is lowered. This gives the circuit what is termed

hysteresis.

The fact that the positive feedback applied within the circuit ensures that there is effectively a

higher gain and hence the switching is faster. This is particularly useful when the input

waveform may be slow. However a speed up capacitor can be applied within the Schmitt trigger

circuit to increase the switching speed still further. By placing a capacitor across the positive

feedback resistor R3, the gain can be increased during the changeover, making the switching

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Schmitt Trigger

even faster. This capacitor, known as a speed up capacitor may be anywhere between 10 and 100

pF dependent upon the circuit.

It is quite easy to calculate the resistors needed in the Schmitt trigger circuit. The centre voltage

about which the circuit should switch is determined by the potential divider chain consisting of

R1 and R2. This should be chosen first. Then the feedback resistor R3 can be calculated. This

will provide a level of hysteresis that is equal to the output swing of the circuit reduced by the

potential divide formed as a result of R3 and the parallel combination of R1 and R2.

The effect of using a Schmitt trigger (B) instead of a comparator (A)

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Schmitt Trigger

Practical Schmitt Trigger Circuit

The output characteristic has exactly the same shape of the previous basic configuration, and the

threshold values are the same as well. On the other hand, in the previous case, the output voltage

was depending on the power supply, while now it is defined by the Zener diodes (which could

also be replaced with a single double-anode Zener diode).

A Practical Schmitt Trigger Circuit

In this configuration, the output levels can be modified by

appropriate choice of Zener diode, and these levels are resistant

to power supply fluctuations (i.e., they increase the PSRR of the

comparator). The resistor R3 is there to limit the current through

the diodes, and the resistor R4 minimizes the input voltage offset

caused by the comparator's input leakage currents.

The value of the threshold T is given by,

Also, the maximum value of the output M is the power supply rail.

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Schmitt Trigger

Applications

Schmitt triggers are typically used in open loop configurations for noise immunity and closed

loop positive feedback configurations to implement multivibrators.

Noise immunity

One application of a Schmitt trigger is to increase the noise immunity in a circuit with only a

single input threshold. With only one input threshold, a noisy input signal near that threshold

could cause the output to switch rapidly back and forth from noise alone. A noisy Schmitt

Trigger input signal near one threshold can cause only one switch in output value, after which it

would have to move beyond the other threshold in order to cause another switch.

For example, in Fairchild Semiconductor's QSE15x family of infrared photosensors, an

amplified infrared photodiode generates an electric signal that switches frequently between its

absolute lowest value and its absolute highest value. This signal is then low-pass filtered to form

a smooth signal that rises and falls corresponding to the relative amount of time the switching

signal is on and off. That filtered output passes to the input of a Schmitt trigger. The net effect is

that the output of the Schmitt trigger only passes from low to high after a received infrared signal

excites the photodiode for longer than some known delay, and once the Schmitt trigger is high, it

only moves low after the infrared signal ceases to excite the photodiode for longer than a similar

known delay. Whereas the photodiode is prone to spurious switching due to noise from the

environment, the delay added by the filter and Schmitt trigger ensures that the output only

switches when there is certainly an input stimulating the device.

WS Project Report 6

Select which Circuit is to be

Collect its Circuit

Collect The Components of Desired

Get The Copper Clad

Draw The Layout of Circuit

Schmitt Trigger

Manufacturing Process:

Primary Stage:

Initially, Collect the circuit’s name which is to be manufactured. Then, Collect the Circuit

diagram & its required component of proper value.

Second Stage:

After the competition of primary stage, collect the copper clad, Draw the layout

of circuit diagram, and Drill the points where the components are to mount & also drive & drill

the testing points. After this go for the etching process. In the process, all the unwanted copper

will be removed. Then, with the help of Digital Multimeter, check the continuity of all the tracks.

If any track is discontinued with the help of metal wire & soldering gun, it can made continued.

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Drill The Points Where Components Are to

Etch The Copper

Check The

If Any Discontinues, Solder

Test The

Schmitt Trigger

Final Stage:

After the PCB is get ready, tested components are mounted on the PCB. Cut the

unwanted material of the component. Test the output of the component. Test the output of the

circuit. At last, give the finishing touch to the circuit i.e. filling touch to the soldered points &

again verify the Output.

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Solder The Components

Cut The Unwanted

Filing The Tips of Cut Components

Verify The Output

Circuit is Ready!

Schmitt Trigger

Thank You!

WS Project Report 9

The End