Analog ElectronicsTutorials

What is a 555 timer and how to use a 555 timer IC

The 555 timer IC is a low-cost, very robust and widely used integrated circuit (IC) that can be used to design a lot of circuits like delay timer, pulse generator, square wave oscillator, saw-tooth oscillator, switching circuits, flashing circuits, various frequency tone generators, frequency dividers, flip-flop, etc. any device or system that requires a delay and control can count on 555 timer for help. The IC got its name from the three serially connected 5K resistors internally made use of in the design of the circuit.
A typical 555 timer comes as an 8-pin dual-in-line package (DIL) and small outline integrated circuit (SOIC) package. See images below. There is however the dual type of the timer called 556 timer that combines two 555 timers into a 14-pin dual 555 timer IC.

Typical 555 timer and its pins
Figure 1: Typical 555 timer IC and its pins
555 Timer IC
Figure 2: 555 Timer IC
556 timer
Figure 3: 556 timer

Versions of the 555 Timer

The two main versions of the 555 timer IC are the SE and NE versions. The only difference between the two versions is that the SE version can operate comfortably within the temperature -55o C to +125o C, while the NE operates very well within temperature of 0 to 70o C. They operate within a voltage range of 4.5-15V and can handle electrical power dissipation of about 600mW.
Versions of the IC include the NE and SE versions of the 555 timer, 566 timer, 7555 and LMC555 timers.
The various electronic components used in the construction of the IC include: 25 transistors, 16 resistors, 2 diodes, 1 RS flip-flop and an output driver inverter. The transistors, resistors and diodes internally form comparators for the 555 timers. The low power versions of the IC are the complementary metal oxide semiconductor (CMOS) versions, namely, the 7555 timer and the LMC555 that use MOSFET instead of BJT.

When using the CMOS type, care should be taking not to touch the IC with bare hands because they are lightly doped, hence, chances are that static charges can easily damage them.

Modes of operation of a 555 timer

 Using the 555 timer IC to design these circuits listed above and many lies on the ability of the 555 timer IC to be operated in two main modes, namely:

  1. Monostable
  2. Astable  

Before we delve into explaining these modes of operation of a 555 timer IC, let us understand the various pins of the IC and their functions.

555 timer pin configuration
Figure 4: 555 timer pin configuration and block diagram

How a 555 timer IC works

The circuit of the 555 timer IC is a very simple-to-explain one. There are several electronic actions and switching that go on in the operation of the 555 timer IC. To give a concise understanding of how the IC works, we will focus on the actions that give out voltage at the output pin of the IC. Looking inside the IC, in the figure above, you will see three 5KΩ resistors connected in series between pin 8 and pin 1 of the IC, that is how the IC got its name (555 timer). Pin 8 of the IC is the VCC and pin 1 is the ground. So, this three 5KΩ resistors divide the VCC into three, with the inverting input pin of comparator 2 connected to 2/3VCC and the non-inverting pin of comparator 1 connected to 1/3VCC.

Here is what happens, when the voltage on pin 2 of the 555 timer IC (i.e. the trigger pin) goes below 1/3VCC, because the non-inverting input pin of this same IC is connected to 1/3VCC, this will cause comparator 1 to give a high output, this high output is connected to the set pin of the flip-flop. This will in turn cause the reset pin of the flip-flip to go high as well, thereby producing a low output at Ǭ which is connected to the driver circuit inverter that inverts the low input to a high output at pin 3 of the 555 timer IC. The output voltage produced at pin 3 of the 555 timer IC is VCC-1.5, so, if VCC is 9 volts, the output voltage will be 9-1.5Volts = 7.5Volts. for this output to go low, the reset pin of the flip-flop needs to go high, to realize this, the non-inverting input of comparator 2 needs to go higher in voltage than 2/3VCC, when this happens, comparator 2 will output a high output, since the output pin of comparator 2 is connected to the reset pin of the flip-flop, this will cause the flip-flop to flip state and send a high output to the output driver inverter that is connected to pin 3 of the 555 timer IC which is the output pin of the IC. This will cause the output pin of the 555 timer IC to go low, and at this low state, it can sink current up to 200mA of current as long as the IC is connected to power. The timing for all these switching is determined by the timer resistor(s) and capacitor network.

Functions of the pins of the pins of the 555 timer IC

Pin 1: This pin is the ground pin of the IC, i.e. the pin that is connected to the 0v terminal of the power supply or battery.

Pin 2: This is the trigger pin of the IC; it is the pin that is connected to the inverting input of comparator 1. Once the voltage here is less than 1/3VCC, the output pin of the 555 timer (pin 3) will go high, because comparator 1 will output a high voltage that will set the internal flip-flip to a high state, this high state produces toggled high output at Ǭ, this means that we now have 0v at Ǭ, now, this 0v is also connected to the output driver circuit which is an inverter circuit, the inverter will invert the 0v to a high output voltage at pin 3, i.e. the output pin of the 555 timer.

Pin 3: Output pin, this is the output pin of the 555 timer IC, it is this pin that supplies the pulse voltage or the multivibrator voltage of about VCC-1.5V. This pin can source and sink current of about 200mA, this means that this pin can serve as a high voltage source when on a high state, and as ground when on a low state. We can drive LEDs, relays, motors, buzzers with the output current at pin 3 of the 555 timer IC.

Pin 4: This is the reset pin, it is called the reset pin because, it is used to reset the state of the flip-flip from high to low. It works in active-low mode, i.e. it resets the flip-flip when it goes low. Because of this, it is always connected to VCC to avoid unwanted switching. This pin is very important, in the sense that it can be connected to another circuitry from where it can be used to control the behavior of the 555 timer. For example, you can use it to halt oscillation for some reasons and continue the oscillation after some time.

Pin 5: This is the voltage control pin, because this pin is connected to 2/3VCC point, latching this pin to a voltage higher than 2/3VCC will override this 2/3VCC reference for comparator 2 inverting input pin, with this we can vary the width of the output signal without the consent of the RC components.

Pin 6: This is the threshold pin, it is connected to the non-inverting input pin of comparator 2, when the voltage on this pin is greater than 2/3VCC, the comparator 2 goes high, this sends a high state voltage to the reset pin of the flip-flip, this will cause the flip-flip to reset, thereby causing the output driver inverter circuit to send a low voltage state to the output pin. Though the 555 timer gives a low output voltage which is zero voltage in this situation, but it can sink current in this mode instead of sourcing current. The voltage on pin 6 is determined by the timer capacitorresistor network.

Pin 7: This is the discharge pin of the 555-timer. This is the pin through which the capacitor in the RC timer network is discharged. This pin is connected to the collector of an NPN transistor. When the flip-flip is set, state at Ǭ is toggled, for example, when the flip-flip set pin goes high, Ǭ becomes 0v, this 0v enters the output driver which is an inverter to produce high voltage at the output pin. We should not forget that this Ǭ is also connected to the base of the NPN transistor (see figure 4 above), so, because Ǭ is 0v, it will not bias the transistor and collector current will not flow, hence, the capacitor connected to pin 7 will not be discharged. But, when the reset pin of the flip-flop goes high, the state of Ǭ toggles to a high state voltage, this causes the output driver inverter circuit to give a low output at pin 3, at the same time, the base of the NPN transistor will be biased and that will cause the charged capacitor connected to pin 7 to discharge and a new cycle of the process begins.

Pin 8: This is the voltage supply pin of the 555 timer IC, where the positive terminal of the power supply of battery is connected.

Having explained the various pins of the 555 timer IC and its internal circuitry, let’s talk about the various multi-vibrators we can design with it. First, we will design a monostable multi-vibrator.

Monostable multivibrator

The monostable multi-vibrator produces a one-shot signal when triggered, i.e. a signal that goes high and stays high for some time and then go low and remain low until the circuit is triggered again.

Below is the circuit diagram of a monostable multi-vibrator

Monostable multivibrator with 555 timer IC
Monostable multivibrator with 555 timer IC

With this circuit, we can design a timer device that stays on for a certain amount of time when triggered, and goes off after some time has elapsed. This circuit has its applications in so many automated and control systems. Below is a clear explanation of how the system works, to understand the circuit explanation better, please make reference to the block diagram of the 555 timer in figure 4 above.

How 555 timer monostable multivibrator works

In the circuit above, once power is connected to the circuit without pressing the trigger button, pin 4 that is connected to VCC will cause the flip-flop to reset causing Ǭ to have a high logic output, this output at Ǭ is inverted by the output driver inverter, which causes 0V to appear as output on pin 3, at the same time, since there is some non-zero voltage on Ǭ, this voltage and its current biases the base of the NPN transistor, when this happens, the capacitor that is connected to pin 7 of the IC will be discharged through the collector of the NPN transistor to ground. In this way, the circuit does not give out any output voltage on pin 3 of the 555 timer IC. However, when the trigger button is pressed, pin 2 which is the inverting input of the comparator 1 is pulled to ground causing the input voltage to the comparator on that end to be lower than 1/3VCC, this will produce a logic high output on the comparator which is connected to the set input pin of the flip-flop. Once the set pin goes high, the output on Ǭ toggles to a logic low output, this cause current to stop flowing to the base of the NPN transistor and the output driver inverter inverts this 0v to logic high on pin 3 that has a voltage value corresponding to VCC-1.5V. while this this happens, the capacitor C1 will start charging through resistor R1, and it will charge up to VCC with respect to its time constant. Because pin 7 and pin 6 are tied together, when the voltage of the capacitor is above 2/3VCC, it will cause the non-inverting input voltage of comparator 2 to be higher than inverting input voltage, this will cause comparator 2 to output a logic high that resets the flip-flip, causing the voltage on Ǭ to toggle once again to logic high, this in turn will cause the output driver inverter to invert this voltage and output 0V on pin 3 and at the same time causing the base of the NPN transistor to be biased which also causes the charged capacitor to discharge through the NPN transistor. In this way, we can obtain a device that can stay on for some time and go off.
How long the output stays ON is determined by the time constant of the capacitor, the formula for calculating this time is given below. because large capacitors discharge not so quickly, to get a quick discharge for optimized response, a capacitor of smaller value should be used.

How long the 555 output stays on = 1.1RC

i.e. On time = 1.1RC

using this formula, lets calculate the on time of the monostable multivibrator above.

R = 90.9K = 90,900Ω

C = 10uf = 0.00001F

Hence, we have that on time = 1.1 X 90,900 X 0.00001 = 1sec

Therefore, the circuit will be ON for 1 second and go OFF afterwards.

So far in this tutorial, we have been able to explain in details how a 555 timer IC works and how to use it to design a monostable multivibrator. In the next tutorial which will come with a video, we will learn how to use the 555 timer IC to design an astable multivibrator and also a simulation video on Proteus how it works.
Before then, you can keep learning by checking other wonderful simulation videos we have made for you in the links below, as well as wonderful Arduino projects and tutorials, feel free to give us feedback on the comment section. Thank you.

Arduino projects
Electronic tutorials

Leave a Reply

Your email address will not be published. Required fields are marked *