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How to Generate PWM Using IC 555

The IC 555 is an extremely useful and versatile device which can be applied for configuring many useful circuits in the field of electronics. One very useful feature of this IC is its ability to generate PWM pulses which can be dimensioned or processed as per the needs of the application or the circuit.

PWM stands for pulse width modulation, the process involves the control of the width of the pulses that’s generated from a particular source as per the circuit specs.
Basically PWM is used for dimensioning or trimming the output voltage or power of a particular load as per individual or application requirements. It is a digital way of controlling power and is more effective than analogue or linear methods.
There are many examples which illustrates the effective use of PWM in controlling the given parameters.
It’s used for controlling the speed of DC motors, in inverters for controlling the RMS of the output AC or for producing modified sine wave outputs.
It can be also seen in SMPS power supplies for controlling the output voltage to precise levels.
It’s also applied in LED driver circuits for enabling LED dimming functioning.
It’s extensively used in buck/boost topologies for deriving stepped down or stepped-up voltages without using bulky transformers.
So basically it may be used for tailoring an output parameter as per our own preferences.
With so many interesting application options, does it mean the method may be too complicated or expensive to configure??
The answer is definitely, no. In fact it can be very simply implemented using a single IC the LM555.

How to Use IC 555 for Generating PWM Outputs

Let’s learn the following simple circuit configuration:
The diagram shows the IC 555 wired up in an easy monostable multivibrator mode. We know that in this mode the IC is able to generate a positive pulse at pin#3 in response to every single negative trigger at its pin#2. The pulse at pin#3 sustains for some predetermined time period depending upon the values of Ra and C.

We can also see the pin #2 and pin#5 assigned as clock and modulation inputs respectively.

The output is taken from the usual pin#3 of the chip.

In the above straightforward configuration the IC 555 is all set for generating the required PWM pulses, it just requires a square wave pulse or a clock input at its pin#2, which determines the output frequency, and a variable voltage input at pin#5 whose amplitude or the voltage level decides the pulse width dimensions at the output.

The pulses a pin#2 generates a correspondingly alternating triangle waves at pin#6/7 of the IC, whose width is determined by the RA and C timing components.

This triangle wave is compared with the instantaneous measure of voltage applied at pin#5 for dimensining the PWMs pulses at pin#3 output.

In simple words we just need to supply a train of pulses at pin #2 and a varying voltage at pin #5 for achieving the required PWM pulses at pin#3 of the IC.

The amplitude of the voltage at pin#5 will be directly responsible in making the output PWM pulses stronger or weaker, or simply thicker or thinner. The modulation voltage can be a very low current signal, yet it would give the intended results.

For example suppose we apply a 50 Hz square wave at pin#2 and a constant 12V at pin#5, the result at the output will show PWMs with an RMS of 12V and frequency of 50Hz.

For reducing the RMS we just need to lower the voltage at pin#5. If we vary it the resultant will be a varying PWM with varying RMS values.

If this varying RMS is applied to a mosfet driver stage at the output, any load that is supported by the mosfet will also respond with correspondingly varying high and low results. If a motor is connected to the mosfet, it will respond with  varying speeds, a lamp with varying light intensities while an inverter with modified sine wave equivalents.

The Output Waveform


The above discussion can be witnessed and verified from the given waveform illustration below:

The topmost waveform represents the modulation voltage at pin#5, the bulge in the waveform represents the rising voltage and vice versa.

The second waveform represents the uniform clock pulse applied at pin#2. It’s just for enabling the IC to switch at a certain frequency, without which the IC wouldn’t be able to work as a PWM generator device.

The third waveform depicts the actual PWM generation at pin#3, we can see that the width of the pulses is directly proportional to the top modulation signal. The pulse widths corresponding to the “bulge” can be seen as much wider and closely spaced which proportionately becomes thinner and sparse with the fall in the modulation voltage level.

The above concept can be very easily and effectively applied in power control applications as discussed earlier in the above article.

For more info, you may read this article: http://www.ti.com/lit/ds/symlink/ne555.pdf 


48 Comments

  1. Hi Sir Swatatam.

    Hi! How are you? I'm very glad I found this article from the net. Thank you very much for this. BTW, I'm interested on simple SMPS as charger for lead acid batteries and I found this PWM control of yours, I'm wondering if how this will work on EN13007 (even just 120HZ boost from 60 HZ, I would be very glad if it's more :)). I'm not good in electronics, I just know conventional types and I'll be very glad to learn much from you.

    Thanks very much.

    • Thanks PSK,

      The above concept is specifically suited for creating modified sine waveforms for applying in modified sive wave inverters. SMpSs employ much advanced topologies and ICs since AC mains is involved, so may be the above concept will not hold good for applying in smps circuits….

      anyway you are welcome to share your discussions here 🙂

  2. Thanks for the reply sir and sorry for the wrong spelling of your name. I'll be sharing my questions here.

    Oh I see. I want to learn about how PWM works on correcting the output on the final stage of the supply(Just like the conventional ATX power supply w/c the performance depends on the demand load, but not that very complex design).Sorry I get the simple logic but I'm still confuse. I've seen your designed here of your CHEAP SMPS here:

    http://homemadecircuitsandschematics.blogspot.com/2012/05/cheapest-smps-circuit-using-mje13005.html#uds-search-results

    I'll be using Two Transistor Forward design.

    I'm very glad your here to accommodate me sir. Thanks a lot again.

  3. Thank you very much Sir Swagatam.

    I now somehow understand the operation. Can I modify this circuit for higher amperage rating? I'm planning to make a charger w/c has smaller transformers so that it would be handy.

    Thanks again and regards !

    • Hello sir swagatam

      I'll just be using silicon Laminated steel E Cores. Will it be good?? I'm targeting to have 45v 15a and 2x15v 5a each. It's ok for me to have minimal 2ndary filters because I'll just be using it for charging batteries.

      I'm very thankful you take time replying to my concern.

    • Hello PSK,

      If you are intending to derive more power (voltage+current) from a low power source then I am afraid that won't be possible, because the overall power will be always equal to:

      Output < Input….efficiency can never be more than 100%

      You can raise the voltage by sacrificing current, that would be possible.

      Regards.

    • Hi Sir Swagatam.

      Oh I see. Unfortunately I can't find ferrite cores here in our place. Maybe I'll stick to linear type of supply. I really want to imitate pc supply but lesser components and less complexity.

      BTW, I'm very thankful sir for your attention and for having me taught about smps. Thanks a lot.

      Regards.

    • Hi PSK,

      To make smaller transformer power supply you will have to go for ferrite cored transformers to be operated at very high frequencies.

      With iron cored transformers it would be just the opposite…..bulky transformers, no circuit required, to be plugged directly to the AC mains for the results.

      Thanks and regards.

    • Hi sir Swagatam

      Can Iron Core transformers handle high frequencies? I'm currently winding my own. but unfortunately, I don't have a smps design to look on so I stick with linear type.

      Regards

  4. Sir,
    can i get a circuit to shift pwm pulse by say 10 ms.how to delay the pwm signal from 555 timer (to make it start after some preset delay) .pls can you help me out with this.
    thank you sir

    • you can probably do it by adding 6 NOT gates from IC4049 with the output of a standard 555 IC astable.

      join all the NOT gates in series, join the first gates input with pin3 of IC555 via a resistor, also connect a capacitor across this input of the gate and ground.

      The value of the above RC will decide the ms shift of the pulses.

      the relevant pulses may be acquired at pin3 of 555 and the output of the 6th NOT gate.

      alternatively you may refer to the datasheet of IC LM567, i think this IC can be also modified for generating such pulses.

  5. Hello Sir,

    Please can you give a PWM lead acid battery charger circuit, which take 220v power from electric company.
    In other words i need a non solar PWM charger.

    Thanks.

    Rashid

  6. Hi Swagatam,

    This is a great post, with some inspiring information. I have a question if I may. I have a SMPS converting 230vAC down to 24vDC @ 40A. I need to control the output voltage from 0-24v. I'm looking into using PWM and so would the 555 be the right solution? i'm not an electrical engineer but I need to have a clue about whats possible before I try and commission the product.

    Many thanks.

    David

  7. Hello Mr Swagatam,

    Many thanks for your article which I came across because I am looking to construct a very basic VCO which has an analog input variable voltage that should be converted to an alternating sine wave corresponding to the input voltage.

    I was thinking of using the 555 to generate a square wave corresponding to my varing voltage and convert it to a sine wave, what do you think?

    Many thanks in advance.

    • Hello Ahmad,

      Yes it's possible, first the varying may be applied at pin5 of the IC 555 (pin2 connected with a square wave source) for generating a correspondingly varying PWMs, and then these PWMs may be fed to another 555 IC integrator circuit for converting them into correspondingly varying sine pulses…..wider the PWMs higher the amplitude of the sinewaves and vice vera.

  8. Hello Mr Swagatam,

    Many thanks for your prompt reply.

    I didn't quite understand what you meant by connecting pin-2 to a square wave source, is it the feedback connected back from pin-3 through a resistor? This is how I have designed my circuit:

    Pins 2 and 6 are connected together and joining a capacitor to the ground, pin-2 is connected through a resistor to pin-3 to have the same charge and discharge path through same resistor and by this way I can obtain 50% duty cycle.

    My input to pin-5 is actually an analog signal with a voltage range that I choose (between 2 –> 8 Volts), if I do understand correctly then the lower the control voltage on pin-5 the higher the output frequency and vice versa, true?

    In addition, some experimental studies referred to the usage of electrolytic capacitors on pin-3 to generate an average sine wave, what do you think?

  9. Hello,

    Many thanks again.

    Please do accept my apologies but electronics is not really my strength point and I struggle with some concepts.

    I understood what you said with creating the PWM, however I don't see where can I input my own varying signal onto the IC-555? In addition, the second link gave me the page of a Tachometer, I didn't manage my way through to be honest 🙁

    • It's just an idea that I have suggested, I am myself not sure if the idea would work.

      The input will need to be applied at pin5 of IC2 in the referred first diagram.

      In the tachometer, see only upto the transistor base, ignore the left side ignition system.

      The PWMs from the first diagram could be applied to the base of this transistor for getting an equivalent sine wave across C8

  10. hello
    swagatam i need a help from u pls
    i want pwm ciruit diagram with specificitions 0-24v dc using 555ic and mosfet output upto 30amps using controling preset r variable can u help me pleace

    • Dear Faris, Pin#2 needs to be fed with a square wave pulse and pin5 with some constant DC voltage, that's all….the resultant will be a dimensioned constant PWM equivalent in RMS value to the DC input at pin#5…the PWM will vary as the pin#5 DC is varied, if it's in the form of a sinewave then the PWM will follow this and produce a equivalently varying PWM train at pin#3

  11. Hello sir,

    I have a few queries could you please clarify that,

    Is this possible to control the load of 24v using PWM if possible can you please send the circuit diagram.

    my requirement is "on" and "off" the load depends on the time period like 2 minutes "on" and 1 minutes "off", and 3 minutes "on" and 1 minutes "off", the time duration for "on" and "off" is should variable.

    Thanks in advance.

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