- Welcome to the third 555 tutorial that JA put together
for us.
This time we are talking about the bistable configuration
in which we are going to learn about how,
with the proper setup, a 555 timer,
you can use a momentary button, momentary switch
like these tactile switches that I have here.
And if you press one, it goes high and stays high
until you press the other one and then it stays low.
And then it stays low
until you press the other one and it goes high again.
Now, why you would do this
and not just get a latching switch of some sort
I don't know who am I to judge, it's your life.
You do what you wanna do.
But there will be those situations where you want this
where you only have a tactile switch
or you want the feel or something along those lines.
And this is that bistable configuration where
it will be stable.
The 555 timer will be stable in either state
either outputting a one or a zero, unlike the mono stable
which it wants to drop back to one of the stable
configurations or the a stable where it doesn't,
it just can't make up its mind and it's unstable either way
which gives you a nice, beautiful square wave.
Now this is by far the easiest configuration
of the three, and we will go over it really quick
and then we will show you some very boring lines
on othelioscope as it goes
from zero to one and then back to zero as I push a button.
It just doesn't get any more exciting than that.
Good times.
All right, so let's look at the circuit diagram itself
to see how this is configured.
So here we have the 555 timer
and let's just go over the more boring aspects really quick.
You have pin eight
that's just the power that needs to be connected to VCC.
You have pin one that's just ground that just
needs to be connected to ground.
You have three, that's the output.
That's where we will of course be, in this case
hooking up our otheloscope.
And then we have pin five, that control pin
where you can override the two-thirds VCC input
to the upper comparator
which we don't want to do in any of these configurations.
And so again, we tie it to ground via a capacitor to get rid
of any spurious noise that may occur on the pin.
You'll notice that pin seven, we can just leave floating.
That is the discharge pin
that you can discharge a capacitor through
but we will not be using that.
And then pin six, which is our threshold
which is one of our basic input pins, is tied to ground.
And that's gonna be kind of interesting because we,
we don't even use the upper comparator at all
in this configuration.
We only use the reset on the SR flip-flop
and the trigger which controls the bottom comparator.
So we will briefly go over R one and R two.
And all these are, are serving dual purposes of one
acting as pull up resistors and two, acting
as a limiting resistor so that when you use switch one
or switch two, you're not tying VCC directly
to ground and blowing up a power supply.
And that is all we have
for the configuration of the 555 timer,
two resistors, two switches, a capacitor that is not truly
necessary for it to actually run,
but it's nice just to make sure you don't have any issues.
So where it gets a little bit more interesting is
internally. So let's look at the inside.
So in the normal state where you don't have any
of the buttons being pushed
you basically have your inverting input
to your lower comparator tied to VCC
which makes that higher than your non-inverting input
which makes your output zero.
And then on your upper comparator you can see
that your non-inverting input is tied to ground
which is gonna be obviously lower than two-thirds VCC.
So your output of the upper comparator
to your R is also gonna be low.
And if you remember from your truth table
of the SR flip-flop, if your inputs to RNS or SNR
are both zero, it is a memory state.
It basically just outputs whatever its last state was.
Now we know that the resistor, R one
and switch one are connected to the SR flip flops reset.
Now that tiny little dot at the top of the box
of the SR flip-flop means that it's an active low reset.
So it doesn't reset unless the voltage drops low.
And in this case it is high.
So we're ignoring reset.
Both of our inputs to RNS are zero,
which it keeps its previous state and we'll assume is zero.
But what happens if we push a button?
So if I am to push switch two and connect the input
to the inverting part of the comparator through switch two
I'm going to drop that voltage down to zero and suddenly,
that will be a lower voltage
than my non-inverting input on the comparator.
And I will output a high voltage going into S.
And again, looking at the truth table of our SR flip-flop
if you have a one on S, a zero on R
suddenly Q goes high, Q knot goes low, and then
you have this output that flips a signal and you get a high.
Now since you've done this, okay great
you can push that button,
hold it and you have a high output.
But as soon as you release, you go back to that last state
and we are back to our inputs to SNR still being zero
going to that truth table with SNR being zero.
Again, it's just whatever the memory is.
So since it was that one, it still remains that one.
So pretty straightforward so far.
Now the only change so far that we're going to see is
if we close switch one which ties ground
to the reset pin on SR.
And basically all that does is makes Q go low.
And that's it.
It just overrides everything else.
And so no matter what else happens, your Q of your SR
flip-flop goes low and your output's low.
And as soon as you release that, you jump back to
that original state of switch one and switch two being open.
And since it was already low
your output will remain low this time.
And that's literally it.
Basically, you either force the output high
by making that inverting input to the lower comparator drop
to ground, making the output of that comparator to go high
making the S go high, making Q go high
or you reset the SR flip-flop by dropping it
to a lower voltage and then once you release it
it just sits there in whatever state it was until it's over.
So extremely, extremely, extremely simple,
very straightforward.
Nothing is complicated as the monostable
and the Astable configurations.
Very, very straightforward.
And with that, let's actually set up the camera
and I'll show you.
Basically you're just gonna see the voltage switch up
and down as I push the buttons.
It's not gonna be too exciting, but let's do it anyway.
Okay, so we are currently outputting zero volts.
All I need to do is press this switch here
and it jumps to a high voltage and that's it.
I'm giving it seven volts and as you can see
have five volts down here.
So it gives you a nice clean seven volt signal.
And then if I were to push the other button
which ties reset to ground, it drops down.
That is literally it.
And unless something happens, this will stay
like that forever or it will stay like that forever.
Yep, yep, that's it.
And so, again, very, very basic straightforward operation
but depending on what you need could be very useful.
As I promised, wasn't too exciting
but still hopefully helped you understand exactly
the behavior that you would expect from doing this setup.
So thank you very much
for joining us on this third 555 tutorial.
Please join us on the next one where we go
over the Astable configuration
which is gonna be a little bit more complicated again.
If you liked this, give this video a like,
if you haven't already, subscribe to the channel
and we will catch you in the next one.
Take care.