Multivibrator - astable, monostable, bistable

18th August 2011, a little fix.

I first built a multivibrator in the autumn of 78 or maybe a little later. I think I used BC-107 transistors, those small in TO-18 metal. I had leds so I thought I build three multivibrators with different times and different led colors. I put them in a green box with an aluminium panel. I made 6 holes for the led housings. It was powered with a 9v battery and two cables on the rear as on/off. It was cool looking at in the evenings!

Sadly, the leds were later stolen by a so called "friend" of mine. I still have the box somewhere, without the leds. Maybe I someday try to get leds with the type of panel housings I once had, to put those back again. I hope so!

If you who read are an electronics beginner, I advice you to build one or a couple of transistor/led multivibrators. It's easy and like I wrote, cool looking at in the dark :-)
Dual transistor multivibrator circuit (If you change the 100k-resistors the light speed will change too. Instead, use two 220k potentiometers in series with a 10k resistor and you can set the time for each stage! )

A multivibrator is a relaxation oscillator

A multivibrator can be viewed as a form of relaxation oscillator. A multivibrator circuit consists of a pair of inverting amplifiers. These amplifiers are connected in series, and a direct feedback loop runs from the output of the second amplifier to the input of the first.

Block diagram of the basic scheme :

Multivibrators are extensively used in digital circuit design, especially bistable multivibrators, also known as "flip-flops".

There are three main types of multivibrator circuit;
Astable multivibrator.
No state with two capacitors. Free-running oscillator.
Monostable multivibrator.
one state with one capacitor. Maintains a single condition except when triggering pulse is applied. Then the state changes for a predeterminated length of time.
Bistable multivibrator.
two state with no capacitors. Also called flip-flop, can attain either of two stable conditions.

Below is a look at these three multivibrator types.

Astable multivibrator

An astable circuit is a form of oscillator. the word astable means unstable. An astable multivibrator consists of two active elements arranged in such way that the output of one is fed directly to the input of the other. Two identical resistance-capacitance networks determine the frequency at which oscillation will occur. The amplifying devices (tubes or transistors) are connected in a common-source or common-emitter configuration, as shown.

In the common-source or common-emitter circuit, the output of each transistor is 180 degrees out of phase with the input. An oscillating pulse might begin, for example, at the base of Q1 in the illustration. It is inverted at the collector of Q1, and goes to the base of Q2. It is again inverted at the collector of Q2, and therefore returns to the base of Q1 in its original phase. This produces positive feedback, resulting in sustained oscillation.

The astable multivibrator is frequently used as an audio oscilator, but it is not often seen in RF applications because its output is extremely rich in harmonic products due to the shape of the waveform.

Monostable multivibrator

A monostable multivibrator is a circuit with only one stable condition. The circuit can be removed from this condition temporarily, but it always returns to that condition after a certain period of time. The monostable multivibrator is sometimes called a one-shotmultivibrator.

Normally the output is high, at the level of supply voltage. When a positive triggeriung pulse is applied to the imput, the output goes low(0V) for a length of time that depends on the values of the timing resistor R and the timing capacitor C. If R is given in Ohms and C is given in microseconds, can be found by the equation:

T = 0.69RC

After the pulse duration time T has elapsed, the monostable multivibrator returns to the high state.

Monostable multivibrators have been used as pulse generators, timing-wave generators and sweep generators for cathode-ray-tube devices.

Bistable multivibrator (flip-flop)

The bistable multivibrator is much more known as flip-flop because of its two stable switching states, hence flip, flop etc. It occurs when an input pulse of the right polarity and sufficient level is introduced. A two transistor the bistable multivibrator acts as a one bit memory.

Digital counter stage

A single bistable circuit, a flip-flop, is counting to one. [Ref. flip-flop - counter]
If more stages are are added in series, higher levels of counting are possible as in a digital counter. The SN4LS93 4-bit counter consists of 4 flip-flops.

Flip-flops have been used in computers since the early tube computers. In the mid 50's, the first transistor computers were developed. In the years after, a number of other transistorized computers were developed. The first commercial microprocessor was released 4004 by Intel in 1971.