(gentle music)
- Welcome to this second tutorial
in our mini-series about the 555 timers.
In this tutorial, we are going to be going over
the monostable configuration.
So in the last one, we just went over very briefly
what's going on inside and kind of the different ideas.
In this one, we are going to be going over
that one of three normal configurations for a 555 timer.
Now, the monostable configuration
is where the 555 timer wants to settle into a certain state.
So you push a button, it goes high,
and then depending on your RC constant,
it stays high for a certain amount of time,
and then it drops back down to being low.
And that's where the monostability,
there's only one stable point in this circuit,
that's where it wants to be.
So today we are going to talk about that.
We're going to get quite in depth
talking about what's going on inside.
If you're just looking for a quick circuit diagram
so you can put something together
for your own experimentation,
go check out our website where we have the circuit diagram.
That'll be a lot faster and you can just put it together,
and it'll be really easy and straightforward.
This is to understand the why,
the why it switches from one to zero
and why it's dependent on that RC time constant,
and why it acts the way it does.
So it's gonna be quite exciting,
and we're gonna do that conceptually.
We're gonna go through the circuit itself
and the different states that it goes through.
And then once we're done with that,
we are going to do a practical circuit right here
where we turn on the oscilloscope
and we show actually what's happening with the output
and with that timer circuit.
And so it's always fun to see things in action.
With that, let's jump
straight into the circuit configuration.
We'll briefly show what it looks like,
and then we'll go into how it works.
So this is your typical diagram that you look at
when you have a 555 timer.
This is the monostable configuration.
As you can see, there are two resistors, two capacitors,
and a trigger, which is basically just a tactile switch.
This is pretty straightforward.
The output just goes to whatever your output is.
This 10 nanofarad capacitor is simply there
so that your control input doesn't have any noise on it.
It's basically absorbing any noise.
This R2, this resistor, all it's doing is making it
so that when you push in your tactile switch,
that you aren't connecting VCC directly to ground.
So the value there is semi-arbitrary,
just enough to make it so the circuit doesn't burn up.
And then that time constant that I mentioned
is actually R1 and C.
Now, I won't get into the details of how that's calculated,
but you basically, your time constant
of how high this circuit will be is 1.1 times R and C,
R in ohms and C in farads.
And so, for our practical circuit,
I actually chose 100 microfarads and 10,000 ohms.
So we get a time constant of around a second,
1.1, 1.2 seconds, which we will see,
but it's very, very straightforward.
That time constant is simply 1.1 times RC,
and it's this R1 and C that actually controls that.
So this is not a great way to understand what's going on.
So let's switch over to the other view
where we will see what happens internally.
So with the monostable configuration,
what we have here is we have VCC going through R2
and going to the inverting input
of this comparator down here.
And we have the capacitor here.
The voltage on this capacitor is going into
the non-inverting input of this comparator right here.
And those two things are absolutely crucial.
Now, here you'll notice that VCC
is going to this inverting input right here.
And since the switch is open, this gives us a low output
because the inverting input is higher
than our 1/3 VCC.
So right now, our S is zero.
Now, also because Q bar,
and this is pulling from a previous state,
due to the fact that Q bar is high,
we are actually closing this BJT.
So you have a high voltage to the base of this BJT,
closing this switch, which is discharging the capacitor
through to ground.
So we have a zero voltage
to the non-inverting input to this comparator,
which then gives us a zero on the reset as well.
And if you remember from your flip-flop truth tables,
that means it's just using the previous state.
And we'll get to how it gets to this state
and how it, again, gets to this stable point.
So with all of this, the output is zero
because this output right here is inverting the Q bar.
So Q bar is high, our output is low.
Our voltage on this inverting comparator input is high.
Our voltage on this non-inverting input is low.
So what happens when you push the button?
If you push the button,
suddenly you are grounding this inverting input.
And so, now that your inverting input drops to zero
and the 1/3 VCC is higher than the voltage here,
your output that goes into S is now one.
So now our truth table is S is one, R is still zero,
but because, if we look at our flip-flop truth table,
Q bar now drops to zero.
Now, what this does is this opens this BJT switch right here
and makes it so immediately you can start to get a charge
through R1 to C1.
It used to be that you would have
that current flowing straight through R1 through to ground,
but now it can go into the capacitor
and fill up this capacitor.
Now, looking at this comparator right here,
we are waiting for this voltage to hit 2/3 of VCC
compared to this voltage divider
right here going into the inverting.
We're waiting for it to hit 2/3 VCC for this to flip
in providing R a high voltage to invert
the output of the flip-flop.
So again,
when you push the button,
what you're doing is you are dropping this voltage here,
making this comparator output a high voltage,
making this S1 Q bar zero, opening up this switch,
making it so that this capacitor is charging through R1.
And the amount of time it takes for C1 to get filled
via R1 to 2/3 VCC is the amount of time
that it will remain low.
Because what will happen next is
as soon as you hit 2/3 VCC,
the output of this comparator going into R is now one.
And suddenly, our truth table very briefly is S equals zero,
because we opened up the switch, we released the switch,
and so this now is at VCC.
So this is outputting to zero again.
R is equal to one, Q bar becomes one.
And when it does so, it closes this,
and once again, connects C1 directly to ground
and discharges that capacitor immediately.
So once that capacitor is discharged immediately,
all of this flips again
and R becomes zero again, S becomes zero again.
But since, for that brief moment, it was Q bar equal to one,
once you have S equals zero and R equals zero,
Q bar is just dependent on whatever
the previous state was, which was one.
And so, now we're back in our stable configuration,
and our output drops back down
because it is, again, the inverse of Q bar.
So that is what's happening is when you push that button,
it charges the capacitor,
and as soon as that capacitor reaches that certain level,
it basically discharges itself
and puts everything back to it's original state
of just having a low output.
And it's pretty crazy how simple it is,
yet how complex it is.
And again, everything getting inverted
and changing here in the flip-flop,
changing states sometimes
and not changing states other times
can be quite complicated.
But once you walk through this a couple of times,
it should start to make sense,
and hopefully having the truth tables
of the flip-flop visible
during the different states will help out a little bit.
Okay, with that,
let's actually get the oscilloscope set up
and show this in action.
So this is the physical setup that we're working with.
Basically, we have our RC right here.
This is the 10 kiloohm resistor that I mentioned earlier.
And the 100 microfarad capacitor right here.
This smaller ceramic capacitor on the other side
is simply that one on the control input
so that we don't have the noise.
And then over on this side,
we have just a simple resistor
that protects us from shorting VCC to ground.
And I have my tactile switch right here.
Everything else is simply to connect it
to the power supply and to the oscilloscope,
and also just so you can provide ground and power.
And then pin four is also on power,
which, if I don't remember off the top of my head,
but pin four, I think it's pin four,
the reset to the flip-flop,
we just wanna tie that to VCC
so that we don't accidentally get a reset.
So that is the actual physical circuit
that we're working with.
Let's get this trained on the oscilloscope
and see it in action.
So on this, I have channel one set up as the output,
and I have channel two set up
as that voltage over the capacitor.
So I put the oscilloscope into single shot,
and we will see what happens as I push the button.
Oh no, there it goes quite nicely.
All right, since we had a 100, or excuse me,
yes, a 100 microfarad capacitor and a 10,000 ohm resistor,
we were expecting about 1.1, 1.2 seconds.
So as we look here,
we have our 500 milliseconds per division.
So we got half second, one second, and then yeah,
we're at about 1.1, 1.2 seconds.
So this is exactly what we expected.
Our output goes high, and then as we hit 2/3 VCC,
the whole thing just resets, drops, and keeps on going.
And if I push it again, we'll see if it triggers again.
No, it won't because I didn't restart my oscilloscope.
So there you go.
And I can even do it again and again,
and you can see 'em over and over.
I can just even have this thing run in auto mode.
But yeah, that's basically it.
That's all we have.
It's something where you push the button,
you get an output for a set amount of time,
and if you want a longer time,
you increase the resistance or the capacitance
of your circuit and you can make it so it lasts,
honestly, as long as you reasonably want.
There are limitations.
You don't really wanna get a resistor below 1K.
And if you get anything up in the megaohms,
you're gonna start having problems,
and kind of the same thing in the capacitance side,
but it's generally pretty dang flexible.
So that is what the monostable configuration looks like
in its output in real life.
Okay, so now we have gone over the monostable configuration.
We went over what's going on inside
and kind of the complexities
of the different states and all of that.
And I hope you found that fascinating.
And we are able to see exactly how it works in real life,
which is always very, very exciting.
If you found this helpful, give it a like,
subscribe to the channel, all that good stuff.
Next time, we will be doing the bistable configuration,
where we're gonna be learning more about
what goes on internally with that,
and also have a good example
of how it actually looks in real life.
I hope you enjoyed it.
Have a great day.
We'll see you in the next one.
(toaster clicking)