# Submission: 555 Bistable Multivibrator

#### Wendy

Joined Mar 24, 2008
22,704
Status: Schematic Verified, Drawings Complete, Proofreading in progress.

555 BISTABLE MULTIVIBRATOR

PARTS AND MATERIALS

• 9V Battery and Clip
• Battery Clip (Radio Shack catalog # 270-325)
• Mini Hook Clips (soldered to Battery Clip, Radio Shack catalog # 270-372)
• One 555 timer IC (Radio Shack catalog # 276-1723)
• D1 - Red light-emitting diode (Radio Shack catalog # 276-041 or equivalent)
• D2 - Green light-emitting diode (Radio Shack catalog # 276-022 or equivalent)
• R1,R2 - 1 KΩ Resistors 5% ¼W
• R3,R4 10 KΩ Resistors 5% ¼W
• R5 100 KΩ Resistors 5% ¼W
• C1 - 1 µF Tantalum Capacitor (Radio Shack catalog 272-1025 or equivalent)
• 2 inch (5 cm) piece of wire with the ends stripped and folded in half, red wire shown in Illustration.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 1, chapter: 16 Solving for unknown time
Lessons In Electric Circuits, Volume 4, chapter 10: Multivibrators

LEARNING OBJECTIVES

• Learn another use for Schmitt Triggers
• Learn one of many Bistable Multivibrator configurations

SCHEMATIC DIAGRAMS

ILLUSTRATION

INSTRUCTIONS

The most common 555 configurations are that of a Astable and Monostable Multivibrator. This shows how a 555 may be used as a Bistable Multivibrator. While a 555 has a Set/Reset flip flop built in (another type of Bistable Multivibrator) this circuit uses the concept of hysteresis to accomplish the same thing. If you use the red wire shown on the illustration as a toggle the LEDs will flip states, and stay that way until the circuit is toggled again. It will work over the entire power supply voltage range of the 555, which is 4.5V to 15V.

A CMOS 555 will also work well for this circuit, although it may have trouble driving the LEDs directly.

THEORY OF OPERATION

Because R3 and R4 creates a voltage that is exactly in the middle of the dead zone of the Schmitt Trigger hysteresis the output of the 555 is stable. It will hold the last state it was set in indefinitely while there is power. The capacitor C1 is at the same voltage as the output of the timer. When the toggle button is pushed the capacitor will put the same voltage on the input, causing the 555 (an inverter), to flip states. The capacitor will quickly charge or discharge to the voltage level that the network of R3, R4, R5 now presents. Since R5 is X10 larger than R3 and R4 this voltage will still be in the dead zone of the Schmitt Trigger and the output of the 555 is stable in its new state. When the toggle button is released the capacitor will again charge or discharge to its new voltage. The capacitor is being used as a memory to compliment the 555. It also makes a fairly convenient debounce for the button.

This circuit concept will work for all inverting Schmidt Triggers, though R5 may have to be increase to keep the transitions in the dead zone of the hysteresis.

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#### Georacer

Joined Nov 25, 2009
5,182
A charming little bugger. By a practical point of view, a 555 is smaller than a Flip Flop (they usually come in dual packages), but all these extra components and connections compensate for it. So all that remains is the pure pleasure of finding yet another use for the 555.

Only two remarks:
At the current capacitor value of 1μF, if you press the switch very fast (more that 2Hz) the state won't change. This is because the RC time constant of the R5,C circuit isn't small enough to charge the capacitor in time. Replace it with a 0.1μF cap, and you can wear your fingers out without worries.
And secondly a typo, found out by a metric-using country inhabitant: 2 inches is 5 cms and not 50. Not a big deal, but since you have such a neat presentation there, it would be a shame to leave it hanging.

#### Wendy

Joined Mar 24, 2008
22,704
Thanks, the length will be amended. 5 cm it is.

Given the highly technical sophisticated switch I'm using I opted for more debounce that less.

Thanks for the feedback.

#### Georacer

Joined Nov 25, 2009
5,182
The depth of the debounce technique really depends on the switch used. I used a common IC board black pushbutton and the 0.1μF cap worked just fine.
By the way, here is a really interesting article on debouncing, written by an "embedded guy", as he calls himself.
http://www.ganssle.com/debouncing.htm

#### Wendy

Joined Mar 24, 2008
22,704
I'm very old school, I was building computers and digital electronics before there were any computers, and state of the art was TTL. Debounce isn't really hard, though some folks can make it so.

#### Georacer

Joined Nov 25, 2009
5,182
Of course I didn't post the link to hint towards a software solution. This is a site that favors discrete components. This guy, though, has done a tremendous statistical analysis on the time for wich various switches bounce, and offers some discrete solutions too, with the most reliable, the SR latch.

#### Wendy

Joined Mar 24, 2008
22,704
nerdegutta said:
Hi Bill.

And I see this schematic:

I'm trying to build this circuit in KTechLab (a Linux electronic workbench) and I have your blog and the 555 datasheet on the screen. When I compare the pins I see that in your schematic, pin 6 is labeled Trig, and pin 2 is labeled Thresh, while it is the other way around in the datasheet. Has there been a mix-up, or is there something I'm not getting?
There is an error, as you have pointed out. In terms of the schematic it is easy to fix, and since the circuit hasn't been published yet I appreciate the input. Actually I appreciate the input in any case, but it makes life easier if I get it right before it is in the book.

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#### magnet18

Joined Dec 22, 2010
1,227
Looks good, the only thing that catches my eye is that you used an electrolytic capacitor in the schematics, but the pictorial has a ceramic image.
Not a big deal, good article!

#### Wendy

Joined Mar 24, 2008
22,704
Reread the parts list, it is a tantalum, which is what is shown. I buy Radio Shack parts and then draw them to scale.

#### magnet18

Joined Dec 22, 2010
1,227

#### ali abd

Joined Aug 29, 2011
1
thank you are any pieces equivalent 555 timer

#### Wendy

Joined Mar 24, 2008
22,704
Sorry, that didn't come through translation well.

There are many Schmitt Trigger ICs out there that would work with this and any other 555 inverting Schmitt Trigger scheme.

Examples include the 7414, 4093, 4584, 40106, 3 of the 4 are hex Schmitt Trigger inverters, the other a quad NAND gate that is also a Schmitt Trigger. Hysteresis is the key.

#### Dcrunkilton

Joined Jul 31, 2004
422
Status: Schematic Verified, Drawings Complete, Proofreading in progress.

555 BISTABLE MULTIVIBRATOR
I find that this was never added to Experiments volume. I have entered it into my local copy. It will be up at ibiblio.org in a couple of weeks unless someone requests it sooner.