Wan Nur Wafaa Qaisara bt Wan Hilmi
Biomedical Electronic Engineering, School of Mechatronic Engineering, Universiti Malaysia Perlis
Abstract— 555 timer IC is a device that can be used to provide time delay whether it can be like oscillator or flip-flop. This device can provide the exact timing as it can be a timer that can generate single pulses or longer time delays, or as an oscillator that produce a stable signal waveform.
Keywords—555 timer IC, oscillator, flip flop
Task that had been given to me is that I need to construct a circuit that can function as a timer which when I open the cabinet, I can see clearly whats inside and for the easy use of daily life and after a certain period of time, the light will turn off. The method is more or less the same as when we open the fridge. The light will light up when we open the door of the cabinet and will turn off when the door is closed. The main component I need to use is 555 Timer IC because technically this device can adjust the time delay and have a very stable state. This component have two comparator which that it will compare the input which is determined by the charging and discharging action of the externally connected RC network and thus will give the after affect towards flip flop to produce the reliable output, which is when the light will on or off. 1
There are few components that I need to use when designing the circuit. Firstly, monostable 555 Timer IC will be used so that the output will produce a single pulse of current within a certain period of time. 3 The timing of the light will continue to light up depending on values of resistor and capacitor. Time can be get from the equation t=1.1RC. 2 Next, as mention previously, resistors and capacitors will be used. Normally closed switch because it works dependently with LED. LED will become the output for the circuit. Lastly, power supply or VCC is needed in order the circuit to function.
B.Flow of the circuit
When the door is open, the light will turn on. And after a certain period of time the light will turn off. The light also will be turn off when the door is closed. Thus, when the power supply is given, LED will light up until a certain period off time. 555 timer IC will compare the input given in order to produce the output. Initially, 555 Timer IC is in stable mood. Example when the output at PIN 3 is low. When voltage is applied at PIN 2, lower comparator becomes HIGH and flip flop is Set so we will get HIGH OUTPUT at PIN 3. Transistor Q1 becomes OFF, and Timing capacitor C1 disconnected from the Ground and start charging though the Resistor R1.This state is called the quasi stable state and time will remains for a required amount. When capacitor starts charging and reaches to a certain value of voltage at Threshold, PIN 6 becomes greater than the voltage of Upper comparator. Thus, Upper comparator becomes HIGH and Flip flop is being Resets. The OUTPUT of the chip at PIN 3 becomes LOW. Transistor Q2 becomes ON, and capacitor starts discharging to the Ground, through the Discharge PIN 7. 555 IC fall back again to stable state (LOW) after the required time determined by the RC network. The time duration can be calculated using the formula as stated above. 2
III.Preparation to design the circuit
A.Abbreviations and Acronyms
?NE555 Monostable, 555 Timer IC
Use SI units as primary units :
?Second , s for time
?Ampere, A for current
?Volts, V for voltage
?Ohm, ? for resistor
?Farard, F for capacitor
Equations that will be used in this circuit to calculate the delaying time is
t = 1.1RC
?Error in calculation when calculating the time due to wrong inserting of value.
?Less taking the value of decimal places performing a less precise value.
?The value of resistor and capacitor is not the same as theoretical as they have tolerance.
?Do not confuse the connection of series and parallel when designing the circuit.
?Do not mistaken the components when designing the circuit in multisim application.
Based on the mechanism, I can conclude that timer with the characteristic of monostable is the best device that can be used because it response accordingly with the input and perform output that this task wanted.
1 555 Timer Tutorial – The Monostable Multivibrator. (2018, April 29). Retrieved from https://www.electronics-tutorials.ws/waveforms/555_timer.html Accessed December 03, 2018