Objective
To analyze IC 555 Timer and design a system that will be automatically turn off or turn on the light by presence and absence of the light.
Introduction
What Is Ic 555 Timer? IC 556 Timer is a integrated circuit consists of : 2 comparators 1 Flip-Flop 1 discharge transistor 1 Resistive voltage divider Features Of The Ic 555 Timer: 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.
Block Diagram IC 555 Timer comes in 8 Pin as the figures below:
PIN 1: Ground Terminal. All the voltages in the circuit are measured respect to the ground.
PIN 2 : Trigger Terminal. It is an inverting input of a comparator and also is responsible to the transition of flip –flop in the IC from set to reset. It also used to feed the trigger input as the IC 555 Timer is set up as monostable multivibrator. The output of this IC 555 Timer is depending on the amplitude of the external trigger pulse that is applied to this pin. DC level greater than Vcc/3 with a negative pulse will be applied to this terminal. As the trigger passes through Vcc/3 in the negative edge , the lower comparator output will become high and the complementary of Q becomes zero. Therefore, the IC 555 Timer will have high output voltage in quasi stable state.
PIN 3: Output Terminal. This pin control the output of this Timer. Load can be connected in two ways to the output terminal. First is by connected it between output pin ( pin 3) and ground pin ( pin 1) . Second , by connect it between pin 3 and supply pin ( pin 8). Connection between output and ground supply pin is known as normally on load whereas the connection between output and ground pin is known as normally off load.
PIN 4: Reset Terminal. When a negative pulse is applied to pin 4 this IC Timer will be reset and disabled. If this pin is not going to be used as a rest purpose it will be connected to + Vcc to avoid any possibility of false triggering.
PIN 5: Control Voltage. This pin will controlled the threshold and trigger levels of this timer. 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 used , it should be bypassed to ground through 0.01 micro Farad in order to avoid any noise problem.
PIN 6: Threshold Terminal. This pin is used as non- inverting input terminal of comparator 1 which then will compare the voltage that is applied to the terminal with a reference voltage 2/3Vcc . The amplitude of the voltage that is applied to this terminal will be responsible to the set of flip-flop in the timer.As the voltage applied to this terminal greater than 2/3Vcc , the upper comparator will switch to +Vsat and the output will reset.
PIN 7 :Discharge Terminal. This pin is connected to the collector of the transistor. As the transistor saturates, capacitor will discharges through the transistor. If 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.
Charge And Discharge IC 555
Timer can be set up to operate as a VCO by using the same external connections as for astable operation , with the exception that a variable control voltage is applied to the CONT input ( pin5). The control voltage ( VCONT) changes the threshold values of 1/3 VCC and 2/3 VCC for the internal comparators. With the control voltage , the upper value is VCONT and the lower value is ½ VCONT. When the control voltage is varied , the output frequency also varies. An increase in VCONT increase the charging and discharging time of the external caacitor and causes the frequency to decrease. A decrease in VCONT decreases the charging and discharging time of the capacitor and causes the frequency to increase. Circuit Design for light system in the cabinet
Components Used Ic 555 Timer
Light Dependent Resistor Diode ( 1N4007 or 1N4001) Capacitor, 47uF Capacitor, 0.1uF Resistor -10k?,470 k? ,100? Breadboard Connecting Wires
How The Circuit Works ?
This circuit is supply with 9V battery. In dark condition or when the cabinet closed, the Light Dependent Resistor ( LDR) will be nearly 1M ? same as given in datasheet . Output voltage of the potential divider will appear across the capacitor and will remains in charged condition which is I voltage higher than 2/3 Vcc so the output is LOW. In bright condition or in the presence of light when the cabinet is open , the light will trigger Light Dependent Resistor ( LDR) which then will lower the resistance of Light Dependent Resistance and will cause discharge of the capacitor which in RC combination on 30 secs.
As the voltage lower than 2/3 Vcc oscillation at the certain frequency will start by the output and it is HIGH. So, Charging of the capacitor will occur and reach a threshold which then will continue by the discharging of the capacitor . Next , once again the capacitor charges and reaches a threshold continued by discharge of the capacitor. This continues till the LDR resistance goes high which will happen in the absence of light which when the door is closed.
