1.0 INTRODUCTION

As an introduction, mini project is a part of syllabus in ELE 526 where need cooperation

among members in a group to think for how to solve about the tittles given. Our group has been

given tittles of “SCHMITT TRIGGER SWITCH”. In reality, Schmitt trigger is used when

interfacing an analogue signal to digital signal. One of its functions is to detect level of sensed.

When the input signal reached to a given level, it will switch from its low state to high state.

Transistor is a component used as a transistor switch and able to control high power devices such

as lamp, motors and digital electronic device. Two types of transistor are NPN and PNP which

both of them are different on how the current flow. For an ideal transistor switch, between

collector and emitter current resulting too zero means off state and on state while the resistance

reach zero and have maximum current flow.

In this project, it is required for the schmitt trigger circuit detect an input signal between

ranges of 4V to 8V.After an input signal have been detected, that value also can be used to

control light emitting diode (LED) through a switch. Switch in this circuit have been replace by a

NPN transistor. Then, this transistor was ON the light emitting diode (LEDs) as the value

detected decrease to 4V and OFF the LEDs when increase to 8V.

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2.0 CIRCUIT THEORY

FIGURE 2.1

The circuit consist different type of component use to make sure the LED will light up when

the input voltage is 4 V. Led will turn off when the input voltage reach 8V. The component use

in this circuit is:

1. power source2. resistor (15.4k,10k,6.8k)3. op-amp UA741 4. transistor5. relay6. light emitting diode (LED)

Inverting op-amp is use in this circuit because the input voltage (Vin) is connected at

negative terminal op-amp. The op-amp is connected by using Schmitt trigger. Schmitt trigger is

use to compare a regular or irregular waveform with a reference signal and convert the waveform

to a square or pulse wave. While reference voltage (Vref) is connected to the positive terminal of

the op-amp.LED is use as an output. It is connected to the relay which in open circuit.

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When the current is 4V the positive output value will produce. Transistor use in this circuit

is NPN type. The positive output value at op-amp will pass throw resistor and relay that

functioning as switch will force the circuit in close condition. Because the magnetic coil in relay

will pull the internal trigger. So, the LED is connected with the circuit and turn on. The power

supply B1 is connected to the op-amp to make sure the op-amp will functioning.

At 8V input voltage the value of output voltage will become negative. This negative value

cannot pass through transistor perfectly and make the relay in open condition. It is because the

magnetic coil in relay is not functioning to pull the internal trigger. As the result the LED will

turn off.

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2.1.1 FLOWCHART

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START

Vo< 2.7v

Vout – I R4 ≤ 0.7v

Transistor offTransistor on

Vout – I R4 ≥ 0.7v

Vo> 2.7v

Vin< 6 Vin> 6

input

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END

Open loop

Relay off

Closed loop

Relay on

Open LED circuitClosed LED circuit

LED on LED off

3.0 THEORETICAL CALCULATION

FIGURE 3.1

Golden rules of ideal OP-AMP

Where

Therefore,

For

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For

Calculation by using Schmitt trigger

=2.4v

=4.85v

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With

FIGURE 3.2

(

So,

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IF

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Calculation for transistor

FIGURE 3.3

Where

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Therefore,

When ,

If,

Where,

Sub =-0.397mA

-2.7 + (-0.397m) (6.8k) +0.7 = 0

Therefore,

0.7≤ 5.36v

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4.0 COMPUTER SIMULATION

FIGURE 4.1

Refer to figure above .the LED is turn on when the value is set to 4v. It is because the

transistor receive the positive output voltage from op-amp that working as Schmitt trigger that

make.

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FIGURE 4.2

Refer to figure above, the LED is turn off when the value is set to 8v. It is because the

transistor receives the negative output voltage from op-amp.

FIGURE 4.3

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Refer to figure above. For ac voltage the high value frequency is use to make sure the

LED to turn ON and avoid the output value become saturated.

5.0 RESULTS

FIGURE 5.1 Light emitting diode (LED) was on when input voltage is 4V.

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FIGURE 5.2 LED still on when input voltage is 5.9V

FIGURE 5.3 LED was off when input voltage is set to 5.9V

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FIGURE 5.4 LED continuously off when reached input voltage of 8V

6.0 DISCUSSION

From the results before Figure 5.1 shows when an input signal is decrease to 4V, all light

emitting diode (LEDs) was on. The output voltage measured on R4 gives a value of 4V where

current was driven to transistor. Same goes to Figure 5.2 when an input signal increase, all LEDs

still light up but the output voltage produced was 2.77V. This situation occurred because when

the transistor on base received current from op amp, it definitely simulate to collector and the

relay will attracted to coil. This is due to magnetization in coil. However, Figure 5.3 and 5.4

shows LEDs not light up respectively. Schmitt trigger have detect input voltage started from

6.1V and above until 8V which resulting the decrease of output voltage values.

Through this project, there are problems have to faced. The main problems are when

frequency of AC source was setting to low value such as 0.01HZ, the signal produce on

oscilloscope cannot be seen and the output value reached to -10V. To overcome this problem

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frequency was change to 1Hz and output value showed the positive values. Again, AC source

have been replaced by DC source to ensure the output value is same as calculated.

7.0 CONCLUSION

This mini project was completely done, a group of four students are able to achieved the

task that ask to design a smith triggered circuit that will be drive another component in order to

make its works as inverter switch. Actually, the task are given to prove the syllabus that have

study in this course are able to apply in the real world. It is not just only the theory. In order

dealing with op-amp, its not be working by its self. The circuit need to pair with another

electrical component to make it function and useable. In the end of these mini project, students

will get knowledge about how to configure an op-amp and what kind of parameters are need or

will be varies. The task has done with the output of the op-amp drive a transistor to turn on and

off the LEDs by varying the input of op-amp.

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RECOMMENDATION

1. Increase the number level of input in order to see the accuracy.

2. The output type not just on or off maybe control the brightness, timing, regulating and

etc.

3. Varying in input will control more than one parameters.

4. Avoid using Vref to minimize the circuitry components.

7.1 METHODOLOGY (PROJECT PROGRESS)

Week

Activity

8 9 10 11 12

Get titles project from lecturer

Find and design circuit

Troubleshoot circuit

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Prove the calculation

Make first draft

Make a slide for presentation

Presentation

REFERENCES:

http://www.calculatoredge.com/electronics/BJT.htm

http://www.electronics-tutorials.ws/transistor/tran_2.html

http://www.circuitstoday.com/schmitt-trigger-using-op-amp

http://www.random-science-tools.com/electronics/schmitt-trigger-calculator.htm

http://www.radioelectronics.com/info/circuits/opamp_schmitt_trigger/op_amp_schmitt_triggerhp

http://www.petervis.com/GCSE_Design_and_Technology_Electronic_Products/

transistor_base_resistor_calculator/transistor_base_resistor_calculator.html

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