Have you ever had the chance to use the NE555 timers? I have been using it since 36 years ago. I still use it now and it’s not out of date.
Let’s not waste time with questions. Why do we like using it?
Let me explain its use in a simple way. I will also be learning it with you. The NE555 is one of the handiest IC’s to be ever invented.
The 555 (Pronounced “triple Five”) is a short form way of saying LM555 or SE555 or NE555.
It is one of the handiest IC’s to be invented and is specially designed to operate as a multivibrator. It can give time from microseconds to several hours.
It is a TTL device so it is not sensitive to static electricity. But its power consumption is more than a CMOS IC. So, we must keep this point, especially in mind when designing for battery operation.
The NE555 takes 10mA from the supply when the output is “high” and 1mA when the output is “low”. When operating as an oscillator, we can consider the power consumed to be equal to that of a LED.
A CMOS version has been introduced with part number LM7555. But it is considerably more expensive yet. When the price falls it will be a very good choice as it consumes only 120uA.
Read next: Monostable multivibrator using 555 timer
To date, there are many limitations such as output current capability & maximum voltage levels on the triggering pins. But these may be overcome in later versions.
- Can accurate time delays or oscillation
- Direct replacement for SE555/NE555
- Timing from microseconds through hours
- Operates in both astable and monostable modes
- Adjustable duty cycle
- Output can source or sink up to 200 mA
- Output and supply TTL compatible. Such as with a 5-V supply, output levels are compatible with TTL inputs.
- Temperature stability better than 0.005% per ˚C
- The output can be normally turned ON and normally turned OFF
- Available in 8-pin MSOP package
- Pulse generation
- Time delay generation
- Sequential timing
- Precision timing
- Pulse width modulation
- Pulse position modulation
- Linear ramp generator
Recommended Operating Conditions
See the details of NE555 on the operating free-air temperature range. ( unless otherwise noted)
- VCC—the supply voltage: 4.5V to 16V
- Vi—the input voltages(all): VCC
- Io—the output current: 200mA
- TA—Operating free-air temperature: 0 – 70C
Read related: AC dimmer for LED Bulbs using IC-555
The 555 timers are contained in a tiny 8 pin Dual-In-Line package look like a 741 op-amp or LM386 audio amplifier. Look at its pinout below.
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Full 555 schematic
Look at the circuit diagram. This full schematic diagram is mainly to show its complexity. Obviously it would not be worth your while making this circuit from the normal components.
But whole components to a chip cost less than 40 cents. It incorporates 28 transistors and a set of resistors housed inside the 8 pin package.
In addition, if you like to learn electronics from transistor circuits you can research more.
The “Three Fives” Discrete 555 Timer kit from Evil Mad Scientist Laboratories is a faithful and functional transistor scale replica of the classic NE555 timer integrated circuit. Learn more: evilmadscientist.com
Read also: Simple 555 IC Tester Circuit Diagram
555 Block diagram
The schematic can be simplified somewhat to a block diagram making the operation of the circuit slightly easier to understand.
In this diagram, you can see a free-running flip-flop which is triggered via pins 2 and 6 to drive the output pin 3.
The 555 timer can provide time delays ranging from several minutes for one cycle of operation to many thousands of cycles per second.
Read next: 555 Sound effect generator circuit
Any circuit which cycles more than a few times per second is called an oscillator.
Below this frequency we say it is “cycling”. The frequency of oscillation is measured in cycles per second (cps) —now called Hertz(Hz).
Remember 555 pin Functions in short
- Pin 1-Ground. We connect it to the 0V rail.
- Pin 2-Trigger: It will detect the 1/3 of the supply voltage. It makes the output turn on. While the pin 2 is LOW, it has very high impedance (about 10M) and will trigger about 1uA.
- Pin 3 OUTPUT: If it is HIGH (about the supply voltage) it delivers up to 200mA. In contrast, LOW is about 0.5V above 0V.
- Pin 4 RESET: In normal state, connect HIGH to turn on IC. Maybe internally connected HIGH via 100K Resistor. It must be taken below 0.8v to reset the chip.
- Pin 5 CONTROL VOLTAGE: Normally we often see this pin connected to the ground via 0.01uF to 0.1uF capacitor. The capacitor to this pin removes external noise.
Also, the voltage applied to this pin will vary the timing of the RC network (quite considerably). We can adjust frequency via an external pot.
- Pin 6 THRESHOLD: It detects 2/3 of the supply voltage to turn IC off, make output LOW only if pin 2 is HIGH. This pin has very high impedance (about 10M) and will trigger about 1uA.
- Pin 7 DISCHARGE: It will go LOW when pin 6 detects 2/3 the supply voltage. But pin 2 must be HIGH.
If pin 2 is “HIGH”, pin 6 can be HIGH or LOW. And pin 7 remains LOW.
Then, the pin 7 will go OPEN (HIGH) and stays HIGH when pin 2 detects 1/3 supply voltage.
- Pin 8 +VCC. Connects to the positive rail supply.
How to use the 555
There are a lot of ways to use the 555 IC. We can use them in hundreds of different circuits to create many clever things. But they can all be put into three groups. In different types of oscillators:
- Astable Multivibrator – constantly oscillates
– For frequencies above 1 cycle per second, it is called an oscillator (multivibrator or square wave oscillator).
– For frequencies below 1 cycle per second, it is called a TIMER or DELAY.
Recommended: Simple 555 Timer circuit
- Monostable – changes state only once per trigger
pulse – also called a ONE-SHOT
- Voltage Controlled Oscillator – called a VCO.
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How does the 555 timer work
You may not be able to see a clear picture of how the 555 timer runs. Seen in the circuit diagram is a standard 555 circuit. We often use Astable Multivibrator mode. So we should learn it before using.
When we draw a circuit diagram, always draw the 555 as a building block, as shown below with the pins in the following locations. This will help you to instantly recognize the function of each pin:
Look at the block diagram again. I hope you will understand it better. Credit: Colin Mitchel.
In short, the 555 timer chip works by detecting threshold voltage levels.
- Pin 2 detects a voltage below 1/3 of the supply voltage to turn the IC on.
- And, pin 6 detects a voltage above 2/3 of the supply voltage to turn the IC off.
When we connect the IC to the supply, the capacitor C1 begins to charge through R1 and R2.
Then the capacitor voltage rises to 2/3 of the supply voltage, pin 6 detects this level and turns the IC off. While pin 7 connects to the ground through the circuit inside the IC.
So, the capacitor begins to discharge via R2 until its voltage is 1/3 of the supply and pin 2 detects this and turns the IC on again.
Also, the R1 prevents pin 7 damage when it goes down to 0v then pin 6 detects 2/3 rail voltage.
Its resistance is small compared to R2 and does not come into the timing of the oscillator.
In contrast, it does not connect pin 7 to the ground. So the capacitor C1 can charge up again. During this charging period, Pin 2 has no effect and it is disconnected.
Basic 555 oscillator circuit calculator
The 555 output is suitable for driving loads such as relays and LEDs without the need for a buffer transistor.
In the astable (or free-running) mode as shown in the circuit diagram, the 555 triggers itself and timing capacitor and charges through R1 and R2 and discharges through R2 only.
How do you calculate a 555 timer?
Look at the basic oscillator circuit, 1Hz at 50% duty cycle.
Read also: 555 PWM LED dimmer circuit diagram
Set duty cycle
We can adjust both resistors, R1 and R2 to set the duty cycle precisely. It can be calculated from:
D = R2/ (R1 + 2R2)
R1: 3.9K and R2: 68K
Putting R values into the formula.
D = 68,000 / (3900+136,000)
D = 50% (approx.)
In this formula, you can see the top resistor R1 has such a low value with respect to R2 that it has very little effect on the duty cycle.
Find frequency output
The frequency of oscillation of the 555 timers follows this formula:
F = 1.44/ (R1 + 2R2)C1
R1 = 3.9K, R2 = 68K, C1 = 10 uF
If we take the values for the slow clock rate and insert them into the formula:
F= 1.44/ (3900 + 136,000)10×10-6
= 1HZ (approx.)
It is easier if you use NE555 Astable Circuit Calculator
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