# 555 timer-how it works

## Main Question or Discussion Point

http://pokit.org/get/a93c079249cf42fc35c8be34ce9db362.jpg [Broken]

So I built this. With some minor adjustments. From my previous experience with astable multivibrators, I know that capacitor size means a lot.

So I put 100uF instead of 1uF and I put a variable resistor, linear 1k, instead of R2 so I can control the rate of flashing.

It just happened that I had linear potentiometers 1k laying around so I figured. And what do you know it works!!

I am quite new to ICs, I only just "understood" how op amp works.

Google didn't give me much results.

Can anybody help me a bit with understanding of what I built? Anybody can build this but I want an understanding.

You don't have to go IC level a lot, just a bit insight into this oscillator circuit.

Thank you

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AlephZero
Homework Helper
Which part of the data sheet (e.g. pages 7 - 10 of http://www.national.com/ds/LM/LM555.pdf [Broken] ) don't you understand?

If the application notes don't have enough detail, which part of the block diagram on page 2 and/or the circuit diagram on page 1 don't you understand?

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Which part of the data sheet (e.g. pages 7 - 10 of http://www.national.com/ds/LM/LM555.pdf [Broken] ) don't you understand?

If the application notes don't have enough detail, which part of the block diagram on page 2 and/or the circuit diagram on page 1 don't you understand?
Well it looks like a jungle to me to be honest.

Our professor didn't get to 555 timers yet, but I was assigned to design a circuit(outside faculty) which included 555 timer. And I succeeded and it works.

Our professor explained us the op amp through this scheme:

http://pokit.org/get/acac60a6c38d664636b21d066df67fa5.jpg [Broken]

And I understood that quiet well.

I saw some schematics, with comparators and flip flops in them, but I do not know how to bring that all together.

If the schematics you provided me with is not too hard to explain, try to in some most basic segments. Any explanation would be much appreciated as I made a circuit which works, but in a nutshell I do not know how 555 timer works.

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jim hardy
Gold Member
2019 Award
Dearly Missed
555 is a really cool device.

secret is in relation of OUT to TRIG and THRESH.

Driving TRIG below 1/3 VCC makes output high.
Driving THRESH above 2/3 VCC makes output low.
From this all else follows.

I suggest the SIGNETICS datasheet and the old Signetics applications manuals. They are easier to read than the Nationals (and that's unusual)
they are easily found via google.

there are hobby sites dedicated to 555.

i like the 555 for schmitt-triggers, just tie trig&thresh together use as input.... it'll interface to most anything.

555 is a really cool device.

secret is in relation of OUT to TRIG and THRESH.

Driving TRIG below 1/3 VCC makes output high.
Driving THRESH above 2/3 VCC makes output low.
From this all else follows.

I suggest the SIGNETICS datasheet and the old Signetics applications manuals. They are easier to read than the Nationals (and that's unusual)
they are easily found via google.

there are hobby sites dedicated to 555.

i like the 555 for schmitt-triggers, just tie trig&thresh together use as input.... it'll interface to most anything.

I will definitely check it out. To update you, that circuit, with my uncle and all, if you can recall, I built it and it works. And I probably got myself a job as electronics engineer :D(part time of course, but my heart thumped so hard when I saw my circuit is working ! )

Something to note about the 555.

There are two versions - the bipolar version and the CMOS version.

The bipolar version can source/sink considerably more current than the CMOS version. However it also presents a large current spike to the power rails when it switches. This spike can embarrass a weak supply and also cause other parts of the circuit to malfunction.

The CMOS version is much less current greedy and should therefore be used unless the greater current capacity of the bipolar is required.

go well

Something to note about the 555.

There are two versions - the bipolar version and the CMOS version.

The bipolar version can source/sink considerably more current than the CMOS version. However it also presents a large current spike to the power rails when it switches. This spike can embarrass a weak supply and also cause other parts of the circuit to malfunction.

The CMOS version is much less current greedy and should therefore be used unless the greater current capacity of the bipolar is required.

go well
How do I tell them apart?

The IC type number often longer than just '555' eg LM555 for bipolar or LMC555 for CMOS

There is also a dual version the 556.

The IC type number often longer than just '555' eg LM555 for bipolar or LMC555 for CMOS

There is also a dual version the 556.
I am using NE555. Is it fond of heating? I am worrying that my circuit will overheat. That 555 timer will be operational at least 18h a day.

The standard plastic package has a rating of 600 milliwatts for the NE555 (this is the bipolar version)

The output can be up to 200 milliamps so the dissipation (at 50%duty cycle) will be 0.5 x 200 x the dropped voltage milliwatts.

If you approach this figure a small attached heatsink would be wise.

The standard plastic package has a rating of 600 milliwatts for the NE555 (this is the bipolar version)

The output can be up to 200 milliamps so the dissipation (at 50%duty cycle) will be 0.5 x 200 x the dropped voltage milliwatts.

If you approach this figure a small attached heatsink would be wise.
Thank you, in the dawn the team will put it to the test. (without heatsink, because im home cannot install it right now) if It fails, burns out or something we will do something about it.

Currently I am tinkering with duty cycle. My desired duty cycle is 30-70 (30% on-70% off). Have a LOT of electronic components around me :D

Well in your circuit pin 3 will be switching between about 6 volts and 0.3 volts through a 1k resistor plus D1.
This means it will be switching about 4.1 volts across the resistor at 30% on cycle.
So the current demand will be 4.1/1 = 4.1 milliamps

So the power will be 0.3 x 4 x 5.7 = 7 (say 10) milliwatts.

So no biggy.

Well in your circuit pin 3 will be switching between about 6 volts and 0.3 volts through a 1k resistor plus D1.
This means it will be switching about 4.1 volts across the resistor at 30% on cycle.
So the current demand will be 4.1/1 = 4.1 milliamps

So the power will be 0.3 x 4 x 5.7 = 7 (say 10) milliwatts.

So no biggy.
I have 10k potentiometers, is it ok if I scale this up to 10k?

I presume you mean in the timing part of the circuit, feeding C1?

That is using 10k pots for R1/R2?

This is OK and will obviously alter the RC time constant and thus the rate of flashing, although you might not get a very bright pulse out of your LED at only 4 mA. (ie is R3 to big?)

Another trick with a variable rate control is to use a constant current source instead of a timing resistor. This makes for a linear control instead of the logarithmic one provided by the RC time constant.

I presume you mean in the timing part of the circuit, feeding C1?

That is using 10k pots for R1/R2?

This is OK and will obviously alter the RC time constant and thus the rate of flashing, although you might not get a very bright pulse out of your LED at only 4 mA. (ie is R3 to big?)

Another trick with a variable rate control is to use a constant current source instead of a timing resistor. This makes for a linear control instead of the logarithmic one provided by the RC time constant.
Well I just built it and tested it and it works! I just have to get ratio of R1 R2 to be just right. And yes, that diode did make my duty cycle desirable! Thanks!

I used 10k resistor for the top one, and 10k pot for the bottom one. Need to change the R1 a bit so I can get frequency between 3-15 Hz (variable). Googling some formulas atm, they are not so complex.

You obviously like burning midnight oil (or LEDs) but it is past my beauty sleep time so I wish you goodnight.

You obviously like burning midnight oil (or LEDs) but it is past my beauty sleep time so I wish you goodnight.

I can't sleep if I have a project on my desk, and I am on the spree of understanding :D

good night :D

jim hardy
Gold Member
2019 Award
Dearly Missed
congratulations!

it is prudent to use two resistors and a pot between them, so that setting the pot to bottom does not stop the circuit.

Size them so the pot gives you enough adjustment to handle worst case tolerances on the resistors, plus however much adjustability you want.
or for one of a kind, just enough adjustability to keep 'tinkering fingers' out of trouble.

i will look back for your Uncle's circuit. Did we use that discovercircuits link for starting point??

it is rewarding to see your progress and enthusiasm! You will be, in your workplace, the one who people come to for answers and explanations...

old jim

congratulations!

it is prudent to use two resistors and a pot between them, so that setting the pot to bottom does not stop the circuit.

Size them so the pot gives you enough adjustment to handle worst case tolerances on the resistors, plus however much adjustability you want.
or for one of a kind, just enough adjustability to keep 'tinkering fingers' out of trouble.

i will look back for your Uncle's circuit. Did we use that discovercircuits link for starting point??

it is rewarding to see your progress and enthusiasm! You will be, in your workplace, the one who people come to for answers and explanations...

old jim
Well my uncle is a mechanic. And the circuit that they had at work was not working.

So I combined a 555 timer circuit with classic power mosfet.

http://pokit.org/get/1c028999912716e30c4a69581a215d74.jpg [Broken]

Now instead of those 2 resistors(first picture) I put two 10k lin pots.

I want 30% duty cycle(or something around that). So I implemented a diode as you can see in the picture above. But then again, I have to have variable frequency. So when I change the period(bottom pot) I am losing my duty cycle( I think I am raising it).

But I think it doesn't have to be perfect. From my implemented test LED I can observe that its more off than on, and that is good enough for me :D.

But all in all my circuit is working, and the desired effect which they wanted is there!

In fact two days ago, I had my first prototype circuit tested(without diode and top pot) and I was so excited when it started working :D that actuator that is.

Picture of it in attachments.

I should also mention that I will replace that 10k pot(top one) with a resistor. I will adjust that pot until they are satisfied with maximum and minimum frequency(which will be controlled through bottom pot). I will measure then the resistance at which top pot is, and the replace it with corresponding value.

Still apart from 555 timer calibration,I designed that circuit :D

Not much, but still I am only one year into electric circuits.

And thank you kind sir. Without you and Studiot and many good folks here I wouldn't be where I currently am :D

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