In this post we comprehensively discuss the pinout details of the timer IC 555 and the working of these pinouts in actual practical circuits. We also learn the various formulas associated with the IC 555 calculations, such as frequency, timing output, duty cycle etc.
Pinout configuration of IC555
Pin-1, GROUND: It is the GROUND PIN of the IC. The negative lead of DC power source or battery is attached to this pin. At this point observe that IC555 functions constantly on solitary rail power supply and In no way on dual power supply, in contrast to operational amplifiers.
Additionally observe that this pin has to be linked straight to ground rather than via any resistor or capacitor. In case done so, the IC will likely not work well and could possibly warm up and get destroyed.
Such things happen mainly because pretty much all the semiconductor compartments inside the IC will probably be lifted by certain extent of unwanted voltage tending to harm the IC.
Pin-2, TRIGGER: It is referred to as TRIGGER PIN. Because the name implies in causes i.e. commences the timing sequence of the IC. It happens to be hooked up to the inverting input lead of trigger comparator inside the IC.
Since this pin is hooked up to inverting input lead, it approves negative voltage pulse to activate the timing sequence. Therefore it sets off when the voltage on this pin is lower than 1/3 of the source voltage (Vcc). In many operations, the IC is ought to be switched by a pulse. The amplitude and smallest pulse width necessary for setting-off is determined by the working temperature spec and supply voltage of the IC.
Typically the current necessary for initiating is approximately 0.5uA for a time period of 0.1uS. The activating voltage could be in a limit from minimum 1.67V while Vcc = 5V to optimum 5V once Vcc = 15V. The activating circuit inside the IC is extremely responsive and could very well be inadvertently initialized on account of encircling noises. To prevent this, the pin is usually attached to a pull-up resistor (10k-ohm), when this pin is employed individually.
Pin-3, OUTPUT: This becomes the OUTPUT PIN of the IC. It is designed to SINK or SOURCE an optimum current of 200mA. Sinking the current implies, whenever the output of the IC is at logic-0 status i.e. therefore it is able to suck in current into its output.
In the same way sourcing the current suggests, whenever the output of the IC is at logic-1 i.e. HIGH LOGIC therefore it is able to generate current from its output. Because of this characteristic of the IC, we are able to apply it in wide variety of standard digital products likewise.
Additionally observe that the output voltage of the IC is moderately higher than zero, whenever it happens to be in logic-0 status. In a similar fashion it will be somewhat lower than supply voltage (Vcc), in case output of the IC is within logic-1 status.
Pin-4, RESET: It happens to be the RESET PIN of the IC. Whenever it is attached to positive terminal of battery, the IC operates normally. But, in case this pinout connected to ground (either straight or using a of 100k-ohm resistor), the IC inhibits the operations altogether and its timing process quits i.e. the charging or discharging of the outside capacitor ceases, thus output of the IC is suspended in logic-0 status.
It will be intriguing to observe that the reset voltage needed in this pin is usually 0.7V at a reset current of 0.1mA. In spite of this in most operations, this pin is invariably attached to positive lead to ensure that the IC performs without issues.
Pin-5, C. VOLTAGE: It is referred to as the CONTROL VOLTAGE pin. The 2/3 of supply voltage level on the lead voltage divider is introduced off to pin-5, referred to as the control lead of the IC.
The timing sequence could be customized by putting on additional DC control voltage to this pin. This enables hands-on or digital remote controlling of the time period of the IC. The control lead is often implemented while the timer is controlled in MMV format.
However if you are probably NOT making use of this pin for any such objective, in that case this pin Needs To Be GROUNDED Via A CAPACITOR OF 0.01uF.
This stops the time period from experiencing sensitivity of stray AC or RF disturbances from the environment. Furthermore keep in mind, whenever the IC is rigged as an oscillator, in AMV format, we are able to modulate the output waveform of the IC by introducing a adjustable DC control voltage to this pin, as presented below.
Pin-6, THRESHOLD: Which is generally known as the THRESHOLD PIN. It finalises the timing cycle of the IC, whenever its voltage is equivalent to or more than 2/3Vcc, the output is at logic-0 state.
Considering this pin is associated with non-inverting pin of threshold comparator inside the IC, it receives positive moving pulse to complete the timing sequence, additionally. Observe that the standard value of threshold current is 0.1mA, in the same way as the RESET PIN. The time delay of this pulse needs to be more than or on par with 0.1uS.
Pin-7, DISCHARGE: It is usually referred to as DISCHARGE PIN. It discharges the external capacitor into through itself, however when entirely charged…! It is usually linked to the collector of an NPN transistor inside the IC.
As a result of this, the discharging current getting into this pin Simply Cannot Go beyond 50mA, if not the in-built transistor could get destroyed. It will be intriguing to observe that this pin could also be accustomed as output pin with open collector output. I have been focusing on one of these functional circuit and will post the circuit in the near future.
Pin-8, +Vcc: This is recognized as the +ve supply terminal of the IC. The battery voltage linked over this particular pin and ground pin MUST NOT GO OVER 18V. Usually the selection of operating voltage of the IC is actually 3V-18V.
Working Details
Fundamentally 555 timer is an extremely steady circuit competent at operating as a precise time-delay generator and as a free running multivibrator.
Whenever applied as an oscillator the frequency and duty cycle are effectively governed by just a couple of outside components, a resistor (R) and a capacitor (C). The circuit could be activated and reset on falling input pulses. Its notable capabilities are summarized under:
- Timing from micro seconds to many hours
- Monostable and Astable operation
- Adjustable duty cycle
- Ability to operate from a wide range of supply voltages
- Output compatible with CMOS, DTL and TTL (when used with appropriate supply voltage)
- High current output that can sink or source 200 mA
- Trigger and reset inputs are logic compatible
- Output can be operated normal ON and OFF
- High temperature stability
Let's discover the internal particulars and functioning of IC555 and find out how the different characteristics could be formulated into functional circuits.
The SE and NE variants are very similar except maximum temperature ratings. The precision type SE retains important qualities within a temperature range of -55°C to +125°C whereas the general purpose which is the NE works dependably merely within a range of 0°C to 70°C.
A few companies make use of the suffix C to denote the industrial edition for general purpose applications. Each forms possess a optimum rating of 18 volts and may deal with power dissipation as high as 600 mW.
A functional block diagram of 555 timer is presented underneath. The unit includes a pair of comparators a couple of transistors, a flip-flop and buffered output stage. The reference voltages for the a pair of comparators within the 555 are developed around a voltage divider comprising 3 equal resistors of 5K ohms each.
IC 555 block diagram
In depth internal block diagram of IC555.
Observe the three series resistors of 5k each
Go through the above shown block diagram of the IC. You will find 3 resistors of 5kΩ each attached in series. All these three resistors generate 1/3 and 2/3 voltage ranges for governing the steps of triggering the threshold comparators within the IC. Because of this design using the three resistors, the IC features a standard code number as IC555.
The threshold comparator is referenced at 2/3 Vcc along with the trigger comparator which is referenced at 1/3Vcc. Both comparators regulate the flip-flop which, consequently, regulates the status of the output i.e. sometimes ON or OFF conditions
Once the timer is in the quiescent condition, the internal transistor T1 will be switched ON to represent a short circuit throughout timing capacitor C. The status of the output pin in this situation will show as a low.
In functional circuits voltage at pin-2 is actually held over a trigger point using a resistor hooked up to Vcc. Whenever a negative moving trigger pulse is applied on pin-2, leads to the voltage at that point to drop down below 1/3Vcc and therefore the trigger comparator RESETs the flip-flop.
At this point transistor T1 is cut-off and also the output status of the IC moves HIGH to some value somewhat lower than Vcc. Capacitor (C) at this point begins to charge and the voltage around it begins rising exponentially until it finally gets to 2/3Vcc.
At this moment, the threshold comparator resets the flip-flop and the output comes back to its low state-just a bit over ground supply level. Transistor T1 is switched on, discharging capacitor C in order that it gets into the standby position for the subsequent timing period.
As soon as triggered, the circuit may not be able to react to supplemental triggering before the timed period has elapsed.
The delay period, for which the output remains high, in seconds is presented by -
1.1 x C x R, where R is in Mega ohm and C is in microfarads.
For getting substantial delays in certain functional applications, the value of timing resistor probably should not go beyond 20 Mega ohm. If you are using an electrolytic timing capacitor, decide on a unit specified with low leakage. The time delay may need to be modified by altering PT to recompense for the broad tolerance of electrolytic.
A significant characteristic to be observed here is that 555, in contrast to many RC timers, give you a timed interval which is practically regardless of supply voltage Vcc. The reason being the charge rate of C as well as the reference voltages for the threshold comparator and trigger comparator are generally directly proportionate to the supply Voltage. Supply voltage may range through 3V to 18V.
Frequency Calculations
To be able to determine the output frequency of the circuit following formula is applied. Within this you must place the values of R1, R2 and the value of timing capacitor C. Remember that R1 and R2 are in Ohms and C is in Farad.
This appears great so far as assumptive calculations are involved. However when you cope with practical circuits and wish to make use of this formula, then how to proceed? The formula consists of three unknowns…! So how to estimate the output frequency?
Timing Calculations
The total time period, the On time and Off time period of the IC are shown through the identical formula. The timing calculations provides you with time in seconds, in the event the values of R1 and R2 come in Ohms and the value of timing capacitor become in Farad.
Duty Cycle
The duty cycle of the IC is truly a unique ratio of the two resistors employed in AMV circuit. Therefore the formula for duty cycle of the IC is presented through the exact same formula. The duty cycle of the circuit is usually computed with regards to percentage. You will find three principal values of duty cycle of the IC.
- Whenever duty cycle is 50%, we have an ideal square wave on the output of the circuit.
- Any time duty cycle is higher than 50%, we get a rectangular wave, in a way that ON TIME of the circuit is higher than the OFF TIME.
- Whenever duty cycle is lower than 50%, we have a rectangular wave, in a way that OFF TIME of the circuit is more than the ON TIME.
- Never forget that the value of duty cycle MAY NOT BE equal to 100% as well as it CAN'T BE equal to 0%.
- The reason being, the importance of R1 can not be zero in the circuit of AMV.
More about IC555
The 555 timer IC is an integrated circuit (chip) utilized in many different timer, pulse generation and oscillator purposes. The 555 could be applied to deliver time delays, being an oscillator, and as a flip-flop component.
Derivatives offer as much as four timing circuits in a single bundle. Launched in 1971 by Signetics, the 555 continues to be in popular use, on account of its simplicity of use, affordable price and very good stability, and is at this point manufactured by several businesses in the initial bipolar and also in low-power CMOS varieties. Since 2003, it had been approximated that 1 billion devices are produced each year.
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