(bright music)
- Welcome, this is our first 555 tutorial
in our miniseries.
We're gonna do four tutorials total.
And in this tutorial we are just going to
describe what a 555 timer is, what's going on inside,
and just give you a better understanding of how it works,
and then we'll also get into in the next three tutorials
about how you can use it in the most common configurations.
I just wanna do a shout out to Jayesh.
This is his first tutorial that he has done for us.
He's done some FAQs.
And he has done an amazing job.
So if you wanna see the written tutorial,
which always has more depth and information,
definitely go check that out.
As it is, let's jump into this.
So a 555 timer, which you've probably heard of,
if you've been in the electronics industry at all,
is simply a timer that can create a pulse
of varying pulse widths
or it can be a one shot where you push a button,
and it'll make your output go high
for a certain controllable amount of time.
Or you can make it so you push a button
and it stays one state,
push another button, it stays another state.
And you can use this in all sorts of different applications.
It's basically just really helpful
if you're doing anything timing.
If it's for LEDs, any sort of signal or clock generation,
stuff like that, and you don't wanna do a microcontroller
where you're programming it and this and that
and all that sort of stuff.
A 555 timer is a very simple, robust way
to get that good pulse modulated signal
or control the timing in some other way.
But it's kind of interesting on the inside.
And we're gonna go over it.
I was gonna draw it out,
but I realize that this circuit's crazy enough
that my drawing skills are just gonna be terrible.
So I'm going to use the internal schematic
that we have on our website.
I'm just gonna use it and I'm gonna point around
and we're gonna walk through it
so you can see exactly how it works and what's going on.
And it's a little bit overwhelming at first.
And I think the majority of the problem,
that I have at least, is that there's a lot of things
where it goes in high here and then it's low here
and it's inverted here and it goes out here.
And so it's like, wait, this is high,
which makes that low, which makes that high,
which makes that low.
Okay, so that makes...
Oh goodness.
So it's a little bit easy to get confused on that stuff.
Don't worry about it.
It's just gonna take a little bit of time.
And in the next tutorials we'll try and make sure
that we go through it very clearly
so that it's easy to follow
how each thing affects each other.
So with that, let's jump to the screen recording
where I'm going to just go through
and you're gonna see exactly how this all fits together.
Okay, so on this circuit diagram,
we decided to basically organize this logically
instead of in the normal
one, two, three, four, five, six, seven, eight organization.
We started with six and two over here on the left
and then moved to the right,
basically the way the logic flows and the circuit flows.
And so it makes it a little bit weird from a pin standpoint,
but also just really helps understand
exactly what is going on here.
So with that, let's actually go
and briefly go over those inputs and outputs
and then we'll come back and go into them
in a little bit more detail.
So here we have the threshold and the trigger,
and those are your two main inputs.
Basically those are the two inputs
that you would use to control it.
Now there's other things,
but those are the main ones that you would worry about.
And then you have, of course, your power, VCC, and ground,
and then control.
And this is just a way to override something,
which we'll get into later.
Don't worry about it too much.
You have your reset, which is also another thing
to override your SR flip-flop here,
your discharge and then your output here.
So of these, again, you mostly use these inputs.
Of course you use power and ground,
discharge and output,
which we will go over this,
because that's pretty interesting
and a critical part of the monostable configuration.
But with that, let's work through this,
and we'll just go through each one of these chunks
and talk about exactly how they all fit together.
So this first little portion right here
is what gives the 555 timer its name.
It's a three five-kiloohm resistors
that creates a voltage divider.
So five, five, five.
5,000, 5,000 5,000.
Anyway, this voltage divider circuit
is providing reference voltages
for these comparators right here.
So as you look at it,
typically VCC on a 555 timer can be
anywhere from five volts, or like 4.5,
up to somewhere around 15 volts.
I don't remember exactly, but it's a pretty decent amount.
If you look here, assume that you have 10 volts input,
you'll have 6.6 volts here, or 2/3 of VCC,
and 3.3 volts here, or 1/3 of VCC right here.
And so frequently you'll hear,
when talking about 555 timers,
about the 2/3 VCC and the 1/3 VCC,
and this is where it's getting that from.
Because, of course, VCC is going to change,
even though I said 6.6 and 3.3.
If you had seven volts as an input,
then that's not gonna work, or seven volts as power,
then obviously those voltages are going to be off.
So to be more accurate we say 2/3 and 1/3.
So, with that, we get this 2/3 VCC
going into the inverting input of this top comparator,
and on the non-inverting input we have our threshold input.
So, depending on the voltage on our threshold
versus 2/3 VCC,
we'll control what our output is here.
On this bottom comparator,
we have as our reference,
1/3 VCC going into our non-inverting input
and our trigger going into our inverting input.
So, if you don't remember exactly how comparators work,
we just did some tutorials on op-amps,
and we went over comparator circuits.
So you can go check that out if you want more detail.
Just as a quick reminder, what happens is
if you have a higher voltage on your non-inverting input
than on your inverting input, your output will be high.
If you have a lower voltage on your non-inverting,
or another way to say it,
if you have a higher voltage on your inverting input,
then your voltage will be low.
So that's the concept there.
So these comparators right here
go over here and drive this SR flip-flop.
So the SR flip-flop can be really confusing,
because on the internet
sometimes people will create truth tables
and instead of just having S and R,
which stands for set and reset,
it'll have SBAR and RBAR, basically the inverse,
and so the output will be different,
and it gets really confusing sometimes.
I don't know why there's no standard way of doing that.
But for the way we're doing it, just so you know,
when we're talking about it,
we are assuming that it's just S and R,
Q and QBAR, or Q inverted.
And so with this, again, this is really high level,
but with a flip-flop, SR flip-flop,
if your reset is zero and your set is one
then your output Q will be one, and QBAR will be zero.
Now the other way around,
if your reset is one and your set is zero,
then your Q will be zero and your QBAR will be one.
And if both your set and reset are zero
then whatever the previous state was, it stays the same.
And so that's why SR,
or flip flops in general are considered sequential
digital logic instead of just combination digital logic,
in that it matters what the previous state was.
So if S and R are both zero,
then whatever the output was it stays the same,
and you should never have both inputs being one,
'cause you basically don't know what your output can be,
and it'll be kinda crazy.
So you want to avoid having both of those be one,
which we do when we're dealing with 555 timers.
But just, if you ever deal with a flip-flop yourself later.
So, again, if S is one and R is zero, Q is one.
If R is one and S is zero, Q is zero.
And of course QBAR is the opposite.
So this is important because it goes out to this output,
and you think why is the output QBAR?
This output not only provides the power, it also inverts it.
So even though we are pulling internally from QBAR,
the actual output here at pin three is the same as Q.
And that's really important to remember.
And, like I said, it's part of the confusion of,
okay you got your value here,
that's compared to here, that's going into here,
that's going into here, that goes actually inverse here,
and then goes here, oh my goodness, what's going on?
And again, it can be complicated,
but you break it down, work it,
take copious notes as you're trying to figure it out.
At least that's what worked for me to make it click.
So with that one final thing that I want to talk about
is this discharge.
So the strange thing is this is kind of an input
and kind of an output.
This pin is designed specifically to discharge
an external capacitor.
That is literally its purpose in life
is to be connected to a capacitor,
and when QBAR goes high, it turns on
and closes this switch, basically, this BJT,
and it takes wherever your capacitor is
and just discharges it to ground.
And that's its whole purpose in life.
And we will go over that
in the other modes of why that's important.
But it is important
because that way you can get some cool timing effects
and stuff like that.
So this literally just is here to make it
so you can discharge a capacitor.
Again, reset is to control your SR flip-flop
and control is here to override this top comparator.
So if you want to do something specific
with your threshold and control
and don't want to be dependent on this 2/3 VCC here,
you can manually change that voltage.
And that's about it for the internal aspect of a 555 timer.
So that's the internal workings of the 555 timer.
And I just went over the basic
here are the parts and here's the relation.
I didn't really show any
of the ways that they've worked together,
because the best way to do that is with an example.
Our next tutorial is going to be
on a monostable configuration.
And I'll talk about that in a moment.
And in that tutorial we'll go over
and we'll go very in depth on how these are all related
and how you change this to get that.
And we'll try and keep everything as clear as possible.
So we will do that later.
But for this, I just wanted to show you
what's going on inside, so it's not just a black box.
And frankly, if you don't care,
all of this information is out there.
You could look up, oh, I want a monostable circuit,
and you just grab it, grab the diagram off the internet,
and they say, hey, to get this timing
you just need to multiply this resistance
and this capacitance and that'll give you the timing
and you don't need to know what's going on in here.
But it helps so much to actually
understand how it works, and it frankly,
it's really quite cool.
So with that I want to mention
the three next tutorials that we have
and their configurations,
because I think this is a good thing to be repetitive on.
And that's basically there are three
very common configuration for a 555 timer.
And if you hear the names of them,
it should make a lotta sense,
but it's good to just hear it over and over again.
And that is the monostable configuration,
the bistable configuration, and the astable configuration.
And basically the monostable configuration
is where there's only one stable state for the 555 timer,
that the output wants to settle to a certain state.
And typically that's low.
And so when that happens, you push a button, push a switch,
it'll change states, and then after a certain amount of time
it'll go back to its stable state.
So it has one, or mono, one stable state.
Now a bistable configuration
is actually where you have usually two buttons,
and you press one button it'll switch to one state forever.
You press the other button
it'll switch to the other state forever.
And that then has two stable states
that you can just push the button, it makes that switch.
It won't flip-flop back on you
unintentionally or intentionally.
You just push the button
and that's the stable configuration,
pushing the other button,
that's the other stable configuration.
And then the third one is the astable configuration.
And, as you might guess,
that means there is no stable configuration,
and that is basically where you're using a 555 timer
as a PWM, or basically a square wave generator,
and it'll just oscillate back and forth,
and you can control the speed of the oscillation
and the width of the pulses and all that sorta stuff.
But that is the astable configuration,
because you don't have any buttons or switches.
It's just constantly oscillating
between one and zero on the output.
And again, I'll go over those.
One, I'll go over each one of those in depth
in the next three tutorials.
And two, I will probably go over
all three of the names again,
just so it clicks and it sinks in a little bit harder,
because for some reason it took a while
for me to remember which one was which,
and I'm hoping that you don't have the same problem I did.
Okay, that's all we have for this,
for this overview of the 555 timers.
I hope you found that helpful.
And I hope you found the concept
and the inner workings of the 555 timer fascinating.
I hope you'll join us with the next tutorial
with the monostable configuration.
If you did enjoy this video, give it a like,
subscribe to our channel, all that good stuff.
As it is, we will catch you in the next one.
Have a great day.
(toaster clicking)