SYAFIQAH ZAWANAH ZAHRAA’ BT MOHAMED ZAHARI
Biomedical Electronic Engineering, School of Mechatronic Engineering, Universiti Malaysia Perlis
555 timer is one of the most useful Integrated Circuit( IC)s in the electronic circuit and it is used in many projects because it is easy to build which only needed only a few part of external components.555 Timer consists of 2 comparators, 1 Flip-flop , 1discharge resistor , 1 resistive voltage divider . It includes 23 transistors, 2 diodes and 16 resistors at a silicon chip installed in an 8- pin mini dual-in-line package (DIP).
Figure 1 Pin in 555 Timer
The 555 timer has 3 principle operating modes:
a) Astable mode which can operate as an electronic oscillator by connect two external resistors and one capacitor in it specific arrangement. There is no stable output that causing the waveform become high and low continuously. It uses include LED, logic clocks, alarm and so on.
Figure 2 astable mode of 555 Timer
b)Monostable mode which this mode function as a “single-shot” pulse generator where it arranged one push to on switch, 2 resistors and a capacitor of it specific arrangement. It uses include timer, bounce free switches, frequency divider and so on.
Figure 3 Monostable mode of 555 Timer
A. Feature of the 555 timer IC
• Operates in large range of power from + 5 v to + 18V voltage supply
• Sinking or sourcing 200 mA of load current.
• The timer duty cycle can be adjust
• As it has high current output , the output can drive a transistor –transistor logic (TTL)
• In order to made timing intervals into several minutes along with the frequencies exceeding several hundred kilohertz the external components will need to be choose properly.
• The temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature is similar to 0.005%/ °C.
• Its maximum power dissipation per package is 600mW.
• Lastly, it reset and triggers input has logic compability.
II. FLOWCHARTS/ BLOCK DIAGRAM
A. 555 timer IC or NE 555
The diagram 3.1 shows the NE 555 componet and it symbol
Pin numbers Function
Pin 1 GROUND TERMNAL
All the voltages in the circuit are measured with respect to the grounf
Pin 2 TRIGGER TERMINAL
The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state.
Pin 3 OUTPUT TERMINAL
The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output.
Pin 4 RESET TERMINAL
When a negative pulse is applied to pin 4 this 555 Timer will be reset and disabled .If this pin is not going to be used as a reset purpose it will be connected to +Vcc to avoid any possibility of false triggering .
Pin 5 CONTROL VOLTAGE
Controlled the threshold and trigger levels of this timer in order to modulate the output waveform ,the external voltage will be applied to this pin. Therefore, when the comparator is switched, the amount of voltage applied to this terminal can be decide. If this pin is not going to be used , it should be bypassed to ground through 0.01 micro Farad in order to avoid any noise problem.
Pin 6 THERSHOLD TERMINAL
The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
Pin 7 DISCHARGE TERMINAL
It connected to the collector of the transistor. As the transistor saturates, capacitor will discharges through the transistor. Whenever, the transistor is cut-off , then the capacitor will charge at a rate which will be determined by the external resistor and capacitor
Pin 8 Supply Terminal
This is the pin where supply voltage is applied to the terminal with respect to the ground. The range of the supply voltage is +5v to +18v.
Table 1 shows the pin numbers’ functions
B. Transistor BC547
The diagram 3.2 shows Transistor
C. Capacitor (50 µF, 10 nF)
The diagram 3.3 shows Capacitor
Capacitor used to store energy.
D. Battery (5V to 9V)
The diagram 3.4 shows Battery
The range voltage of battery is 5V to 9V. Thus, the suitable battery use is 9V
The diagram 3.5 shows LED
F. Resistor ( 220 ? , 100k ?,10k ?)
The diagram 3.6 shows Resistor
IV. CIRCUIT DESIGN
This circuit system will cause the LED inside the cabinet to lights up and turn off in specific time. This is cause by the application of monostable mode of IC 555 Timer. The circuit initial condition at it output is zero which means the door is closed and it is due to the flip-flop in HIGH and both comparators inside the timer is in LOW. When the door is open , the switch will toggle down automatically which then will change IC 555 Timer output state from LOW to HIGH. The push button also will be ground led the input state become zero and the comparator will output .This will cause the Flip-Flop being LOW while the output become HIGH and the LED will lights up. Whereas, when the door is closed , the LED will also turn off automatically. This is cause by the discharged transistor is turned off and the capacitor started to charge to the R1 until the voltage through the C1 reach 2/3 of the supplied voltage .This then will cause the threshold higher and become one. All of this will bring the circuit to its initial state which then Q bar output become HIGH and will activate the discharge transistor and make the output IC LOW again .
NO I/O DESCRIPTION
5 I Controls the threshold and trigger levels.
7 I Toggles the output from high to low when volt reaches 2/3 of supply volt.
1 O Ground reference voltage
3 O Output driven waveform
4 I Negative pulse applied to this pin to disable or reset the timer
6 I Responsible for the set state of the flip-flop
2 I Responsible for transition of the flip-flop set to reset
8 I Supply voltage with respect to GND
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