555 IC — Bistable Mode

Playing with 555 Timer — Part II — Bistable #basicTronics 06

Here is the internal layout of a 555 timer in its bistable configuration.


. A negative pulse to pin 2 turns the output positive
. A negative pulse to pin 4 turns the output negative
. The timer is stable in each of these states. Its run-time has become infinite

The difference for the mono and astable appears at first sight: there are no components on the right side of the chip.

Instead, it is applied a constant negative voltage to pin 6. Can you see the consequences?

Well, pin 6 in low causes comparator B to be disabled, there is no way to force the flip-flop to the up position, except through a negative pulse in the reset. Well this is the essence of how the 555 works in its bistable mode.

Let’s trigger it and see what happens!
We start from the up position. R9 is a pull-up resistor. This keeps the comparator A inert. The output will stay negative indefinitely, as it usually does, until you trigger the timer by dropping the voltage to pin 2. As soon as a negative pulse is generated …

It runs the chip internally …
which sends a pulse to the flip-flop forcing it to the down position.
And flips the timer back to generating its…

…positive output.

We alredy release the button…

Note that a brief negative pulse is enough. The setup is stable, remember?
How to reverse the situation if comparator B is disabled?
The answer is a negative pulse to pin 4 turns the output negative.

Let us see!

Realize that internally the resistors are charging normally (actually, voltage dividers; yep, it’s a metaphor! Works well, try!).
But we are in a peculiar state. The pulses of pins 5 and 7 are innocuous.

​Let’s run the reset button:

A negative pulse travels towards the chip…
…rips the front door …
…and bang, reset the timer!

It overrides everything and flips the output to negative, starting all the cycle again.

Components You will need for this breadboard:

1 x NE 555N IC
1 x 8-pin socket
1 x capacitor 100 uF
2 x capacitor 0.1uF (104)
2 x resistor 10k
1 x resistor 470R
1 x Arduino Uno
1 x breadboard
2 x button
1 x LED

Here is the experimental circuit:

Fritzing project are available too! Google Drive below…

That it for a bistable 555 timer mode!
Bringing your attention to pins 2 and 4.

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Here the slides! Modify if you will! It is Free!

Download All The Project Files

References & Credits

555 IC-Monostable Mode

555 IC-Bistable Mode

555 IC-Astable Mode

The 555 Timer in Monostable Mode

Book: Make: Electronics (Learning by Discovery) 1st Edition

Proteus — IC 4026 — Reaction Timer Circuit

Proteus — IC 555 Timer — Debounce Circuit

— — — — First published at 24/05/2017 at 8:35 PM — — — —