Topic: Tech & EngineeringApplications

Last updated: January 9, 2020

?Assignment 1
ENT 216
AISHA SHAHIRA BINTI ZULKEFLI 171151442Biomedical Electronic Engineering, School of Mechatronic Engineering, Universiti Malaysia PerlisPerlis, [email protected]
Due to the usability issues in the design of the existing shoe cabinet, many designers have come up with ideas to further enhance the design of the shoe rack by using the equipment available in electronics and generate a new idea to produce a smart cabinet shoes which can be foldable and multifunction usage. This paper aims to design and develop an automated shoe cabinet by using 555 timer.

The objective of this project is to design, analysis, interpretation of data, synthesis of information to provide a valid conclusion and develop the shoe cabinet by using any electronic components and specification.

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555 timer as known as linear IC was first introduced in early 1971 by Signetics by giving the name as SE/NE 555 timer. This IC is a monolithic timer circuit that can produce accurate or extremely stable delay or swings. Time interval can be controlled by external resistors and capacitors when NE-555 operates in mono stable mode. Whereas in adjustable mode two external resistors and one capacitor controls the cycle and the frequency of the task. NE555 is an integrated circuit of time circuits (IC). It produces precise swing and delay time. It has two modes of operation namely stable mode and mono adjustable mode. In the interval of mono stable mode controlled by external resistors and capacitors. While the adjustable mode includes duty cycle controls with two external resistors and one capacitor.


SOURCE: components of each pins which consist of Ground, Trigger, Output, Reset, Supply Voltage (+Vcc), Discharge, Threshold, Control voltages. Ground – Pin 1 is connected to the ground terminal. All the voltages are measured with to this terminal. Ground refer to the lower of voltage which is low level (0 V).
Trigger – Pin 2 is the trigger, which works to start the 555 timer running. The trigger are active low trigger, which means that the timer starts when voltage at pin 2 drops to one-third of the supply voltage. When the 555 is triggered through pin 2, the output at pin 3 will be high. This pin is an inverting input to the comparator that is responsible for transition of flip-flop from set to reset. The output of the timer depends on the amplitude of the external trigger pulse which used to this pin. The Output pin going high and a timing interval starts when the input falls below 1/2 of Control voltage (which is usually 1/3 Vcc, Control becomes 2/3 Vcc by default if Control is left open). In other words, Output is high as long as the trigger is low. The Output of the timer entirely depends on the amplitude of the external trigger of voltage that used to this pin.
Output – Pin 3 is the output pin. The output is either low, which is very close to 0 V, or high, which is close to the supply voltage placed on pin 8. There are two ways in which the load can be connected to the output terminal either between Output and Ground or between Output and Supply Voltage (+Vcc). Load connected between Output and supply voltage (+Vcc) is called the normally on load while if load connected between Output and ground is called the normally off load. This output is driven to approximately 1.7 V below +Vcc, or to Ground.
Reset – Pin 4 is the reset pin, used to restart the time of 555 operation. Similar with the trigger input, reset is an active low input. Therefore, pin 4 must be connected to the supply voltage for the 555 timer to operate. If pin 4 grounded, the 555 timer operation is interrupted and does not start again until it triggered again through the pin 2. To disable or reset the timer , a negative pulse is being used to this pin as it is referred to as the reset terminal. When this pin is not to be used for reset purpose, it should be connected to Supply voltage, + Vcc to avoid any false triggering. A time interval can be reset by driving this input to Ground, but the timing does not begin again until RESET rises above 0.7 volts. Fixed a Trigger that overrides the threshold.

Control Voltage (CTRL) – Pin 5 is a control pin. This pin is simply connected to ground, typically through a small 0.01 micro Farad of capacitor. The aim of the capacitor is to level out any changes in the supply voltage which may affect the operation of the timer. The function of the terminal is to control the threshold and trigger levels. The external voltage connected to this pin can also be used to modulate the output waveform. When this pin is not in use, it should be grounded via a 0.01 micro Farad to prevent a noise problem.

Threshold – Pin 6 is called threshold. The purpose of this pin is to monitor the voltage across the capacitor being discharged by pin 7. When this voltage reaches two-thirds of the supply voltage (Vcc), the time cycle ends, and the output at pin 3 is low. This is the non-inverting input terminal of comparator 1, which compares the voltage used to the terminal with the 2/3 Vcc of the reference voltage. The amplitude of voltage used to this terminal is responsible for the flip-flop set state. The timing (Ouput high) interval ends when the voltage at threshold is greater than Control voltage (2/3 Vcc if CTRL is open).

Discharge – Pin 7 is called as the discharge. This pin is used to discharge the external capacitors that work together with the resistors to control the time interval. In most circuits, pin 7 is connected to the supply voltage through the resistors and to the ground through the capacitors. This pin is internally connected to the transistor collector and most of the capacitors are connected between the terminal and the ground. It is called the release terminal because when transistors being saturated, the capacitor will discharge through the transistor. When the transistor is cut off, the capacitor is charged at the rate determined by the external resistors and the capacitors.

Supply Voltage (+Vcc) – Pin 8 is connected to the positive supply voltage. Supply voltage of + 5 V to + 18 V is applied to this terminal with respect to ground (pin 1).
Like other commonly used op-amp, this IC is also very reliable, easy to use and cheaper in cost. It has a multiple of applications including monostable and astable multivibrators, dc converters, waveform generators, and control devices, voltage regulators etc. Timer basically operates in one of the two mods either monostable (one-shot) multivibrator or as an astable (free-running) multivibrator. The SE 555 is designed for operating temperature range from 55°C to 125°C while the NE 555operates over a temperature range of 0°C to 70°C. The 555 timers are operating from a wide range of power supplies ranging from + 5 V to + 18 V of supply voltage. The 555 timers also sink or sourcing 200 mA of the load current and the external components should be chosen correctly so that the time intervals can be made within minutes. Proper selection of only a few external components allows a time interval of several minutes together with frequencies exceed over several hundred kilo hertz. The timer has a high current output; the output can drive TTL (transistor- transistor logic). It has a temperature stability of 50 parts per million (ppm) per degree Celsius changes temperature, or equivalent to 0.005 %/ °C. The duty cycle of the timer can be adjusted with the maximum power dissipation per package is 600 mW and the trigger and reset of the inputs is logic compatible.

The 555 timer consists of two comparators, a flip flop, a discharge transistor, and a resistive voltage divider. It is a two-state device whose output can be at either a high voltage or low voltage as shown in figure 1.

Figure 1: Internal diagram of 555 integrated circuit timer.

SOURCE: state of the output can be changed with right input signal. The resistive voltage divider is used to set the voltage comparator level. All three resistors are the same value; therefore, the upper of the comparator has a reference voltage of 2/3 Vcc and the lower comparator has a reference voltage of 1/3 Vcc. The comparator output controls the flip-flop state. When the trigger voltage is below 1/3 Vcc, the flip-flop set and the output convert to its high level. When the external capacitor voltage exceeds 2/3 Vcc, the upper of comparator resets the flip-flop, which in turn switches the output back to its low level. When the device output is low, the discharge transistor is activated and provide a passage for the rapid discharge of the external timing capacitor. This basic operation allows the timer to be configured with the external components as an oscillator, a one-shot or a time delay element.

The 555 timer IC has basically three operating modes which in Astable mode, Bistable mode and Monostable mode.
Astable mode is known as free- running mode, which means there is no stable level at output. Therefore, the output will swing between high and low. In this mode, the IC will produce a nonstop pulse with the exact of frequency that depends on the value of the two resistors, R1, R2 and capacitor, C1. The threshold input is connected to the trigger input. The external components are the resistors and capacitors from the timing circuit that sets the frequency of the oscillation. The 0.0l?F capacitor connected to the Control input is for decoupling and has no effect on this operation.

Figure 2 : 555 Timer in Astable Mode
SOURCE:, when the power is turn on, the capacitor, C1 is uncharged and therefore the trigger voltage is at 0 v. This causes the output of the lower comparator high and the output of the comparator low, forcing the output of the flip-flop to be low and maintain the transistor off. When capacitor voltage reaches 1/3 Vcc, the lower comparator switches to its low output state, and when the capacitor voltage reaches 2/3 Vcc, the upper comparator switches to a high output state. It reset the flip-flop, causing the base of Q1 to go high, and turns on the resistor. The capacitor now starts to discharge which causes the upper comparator to be low. When capacitor discharges to the +Vcc, the lower comparator switches high, setting the flip-flop, which makes the base of Q1 low and turns off the transistor. The whole process are repeats and another charging cycle will begin.
Bistable Mode known as Schmitt trigger, IC can operate as flip-flop, if the Discharge pin is not connected and no capacitor will be used. In this mode, the IC produces two stable states: high and low. The output signals of both states are forbidden by a trigger and reset input pins, not by the charging and discharging of the capacitors.

Monostable Mode, in this mode, the IC only produces a single pulse when the timer gets a signal from the trigger input. The period of the pulse depends on the values of the resistor and capacitor. This configuration consists of a stable and an unstable state. If the stable output is set at high , the output of the timer is high. If the application of an interrupt, the timer output turns low. Since the low state is unstable, it will be high automatically after the interruption passes. The width of the output pulse is determined by the time constant dependent on the capacitor, C and a resistor, R. The output pulse ends when the voltage on the capacitor equals to 2/3 of the supply voltage.

Block diagram of 555 timer

Figure 3: Block diagram of 555 timer
SOURCE: with block diagram consisting of 2 comparators, a flip-flop, voltage divider, a discharge transistor and an output stage.
The voltage divider consists of three same values of resistors (5k ohm) which create two reference voltages at 1/3 and 2/3 of the supplied voltage, which can range from 5 to 15V. Next, there are the two comparators. A comparator is a circuit element that compares two analog input voltages at the positive (non-inverting) and negative (inverting) input terminal. If the input voltage at the positive terminal is higher than the input voltage at the negative terminal, the comparator of the output is high. In contrast, if the voltage at the negative input terminal is higher than the voltage at the positive terminal, the comparator of the output low. The first comparator negative input terminal is connected to the 2/3 reference voltage on the voltage divider and the external “control” pin, while the positive input terminal to the external “Threshold” pin. Meanwhile, 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.

Therefore, using the three pins, Trigger, Threshold and Control, the output of the two comparators can be controlled that are then fed to the R and S inputs of the flip-flop. Flip-flop will output high when R is low and S is high, and in contrast, it will output low when R is high and S is low. The flip-flop can be reset through the external pin called “Reset” which can override the both inputs, thus reset the entire timer at any time.

The Q-bar output of the flip-flip goes to the output stage which can either source or sink a current of 200mA to the load. The flip-flip output is also connected to a transistor that connects the “Discharge” pin to the ground.

Field of the Invention
For a safety reasons, it is advisable if user could see the inside of the cabinet. In that shoe cabinet, we need to have LED inside as we can see more inside of the cabinet. We have to propose a circuit design by using 555 IC timer to rigger the LED and close it automatically within a period of time.

Components and requirements

Components Specification Name
Resistors 10k ohm R1
Resistors 470 ohmR2
Resistors 2.2k ohm R3
Resistors 100k ohm R4
555 Timer IC IC1
Capacitor 1uF C1
Capacitor 100nF C2
Transistor BC 547 Q1
Battery 3V-9V V1
Switch S1
Block Diagram
A) When the shoes cabinet is opened

Figure 4 : block diagram when the shoes cabinet opened
B) When the shoes cabinet is closed

Figure 5 : Block diagram when the shoes cabinet closed
Data Sheet

Figure 6 : Data Sheet for IC 555 timer

Figure 7: Data Sheet for LED

Figure 8: Data Sheet for transistor
Circuit Diagram

Working Explanation
The automatic shoes cabinet uses 9 volts of battery as the source of power. Opening or closing the shoes cabinet can make this door open sensor switch to work. User will push the button switch at the outside of the shoes cabinet. When the switch turn on, the circuit will be installed into the cabinet as the user open the door. The circuit gets a powerOnce the cabinet is open, the push button will be connected automatically, and then change the 555 timer output state to high and ground the push button cause the input state become zero and comparator will produce output high because the Q’ flip-flop low and output become high so that LED will turned on.. The LED that inside the cabinet will turn on and turn off after the cabinet close, within a specific time. After some time, the LED will turned off automatically when the cabinet close because 555 timers IC in Monostable Mode. LED will remain turn ON for 1.1*R1*C1 second. When the output high, the LED immediately turns on the light. It stays only a minute to save the battery. Then the LED will turn off.

Philips Semiconductors. (2005, Sept. 14). 74HC4040
12-Stage Binary Ripple Counter Product Data Sheet.

Koninklijke Philips Electronics. p.1, 4, 6.

Electronics area, “Refrigerator door alarm circuit with two 555 timers” online
Available: May 19, 2016
Circuits today, “Fridge Door Open Alarm” online
Available: February 2, 2016
Maitam, Martial. “History of Garage Door Openers”online
Available: Dec. 3, 2009


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