555 IC

555 IC

Figure 1 shows the input and output signals of the 555 IC having dip-8 is very popular IC. 555 IC have many different industrial application and is is used to generate a time delay . delay generated by 555 timer is very accurate and we can generate different frequency. Popular project based on 555 IC is darkness activation system, and Sireon blinker.

• 555 is a timer, pulse generation IC.
• This IC is used to provide a time delay in any basic electronic circuits with right choice of resistors and capacitors.

as they are arranged around a standard 8 pin dual inline package (DIP).

Pin 4 – Reset (RES)
A low voltage (less than 0.7V) applied to the reset pin will cause the output (pin 3) to go low. This input should remain connected to Vcc when not used.

Pin 5 – Control voltage (CON)
You can control the threshold voltage (pin 6) through the control input (which is internally set to 2/3 the supply voltage). You can vary it from 45% to 90% of the supply voltage. This enables you to vary the length of the output pulse in monostable mode or the output frequency in astable mode. When not in use it is recommended that this input be connected to circuit ground via a 0.01uF capacitor.

Pin 6 – Threshold (TRE)
In both astable and monostable mode the voltage across the timing capacitor is monitored through the Threshold input. When the voltage at this input rises above the threshold value the output will go from high to low.

Pin 7 – Discharge (DIS)
when the voltage across the timing capacitor exceeds the threshold value. The timing capacitor is discharged through this input

Pin 8 – Supply voltage (VCC)
This is is the positive supply voltage terminal. The supply voltage range is usually between +5V and +15V. The RC timing interval will not vary much over the supply voltage range (approximately 0.1%) in either astable or monostable mode.

Monostable Circuit

Figure 2 shows the basic 555 timer monostable circuit.

Figure 2: Basic 555 monostable multivibrator circuit.
Referring to the timing diagram in figure 3, a low voltage pulse applied to the trigger input (pin 2) causes the output voltage at pin 3 to go from low to high. The values of R1 and C1 determine how long the output will remain high.

Figure 3: Timing diagram for 555 in monostable mode.
During the timing interval, the state of the trigger input has no effect on the output. However, as indicated in figure 3, if the trigger input is still low at the end of the timing interval the output will remain high. Make sure that the trigger pulse is shorter than the desired timing interval. The circuit in figure 4 shows one way to accomplish this electronically. It produces a short duration low going pulse when S1 is closed. R1 and C1 are chosen to produce a trigger pulse that is much shorter than the timing interval.

Figure 4: Edge triggering circuit.
As shown in figure 5, setting pin 4 (Reset) to low before the end of the timing interval will stop the timer.

Figure 5: Resetting the timer before the end of the timing interval.
Reset must return to high before another timing interval can be triggered.

Calculating the timing interval
Use the following formula to calculate the timing interval for a monostable circuit:

T = 1.1 * R1 * C1

Where R1 is the resistance in ohms, C1 is the capacitance in farads, and T is the time interval. For example, if you use a 1M ohm resistor with a 1 micro Farad (.000001 F) capacitor the timing interval will be 1 second:

T = 1.1 * 1000000 * 0.000001   = 1.1