Multivibrators

Sıralı Mantık Devreleri
Sıralı Mantık DevreleriShift RegisterT-tipi Flip Flop
JK Flip FlopJohnson Ring SayıcıD-tipi Flip Flop
MultivibratörlerFlip-Flop Dönüşümleri

Individual sequential logic circuits can be used to create more complex circuits such as Multiverters, counters, shift records, latches, and memories.
But for such circuits to work in a "sequential" way, they require the addition of some kind of clock pulse or timing signal to cause them to change their status. Clock pulses are usually continuous square or rectangular waveforms produced by a single pulse generator circuit, such as a Multivibrator.

A multivibrator circuit is oscillated between a "high" state and a "low" state and produces a continuous output. Astable multivibrators usually have a 50% task cycle, meaning that 50% of the cycle time is output "high" and the remaining 50% of the cycle time is output "off". In other words, the task cycle for the astable timing pulse is 1:1.

Sequential logic circuits that use a clock signal for synchronization depend on the frequency and therefore the clock pulse width to enable switching actions. Sequential circuits can also change switching states using the rising edge, falling edge or both sides of the clock signal, as we have seen before in basic flip-flop circuits. The following list are usually terms associated with a timing pulse or waveform.

Multivibrator

Active high –if the change of status occurs on the rising edge of the pulse of the watch or from "low" to "high" during the clock width.
Active low –if the status change occurs from "high" to "low" on the falling edge of the clock's pulses.
Clock Pulse width (Clcok Width) – this is when the value of the clock signal is equal to "1" or a high logic.
Clock Period – this is the time between consecutive transitions in the same direction, that is, between two rising or two falling edges.
Duty Cycle – this is the ratio of the clock width to the hour period.
Clock Frequency – the clock frequency is the inverse of the clock period, frequency = 1 / hour period. (ε = 1 / T )

Clock pulse production circuits can be a combination of analog and digital circuits that produce a continuous series of pulses (these are called Astable multivibrators) or a pulse over a certain period of time (these are called Monostable (single stable) multivibrators). Combining two or more multivibrator circuits allows the creation of a desired pulse pattern (including the pulse width, the duration between pulses and the frequency of pulses).

Basically, there are three types of clock pulse production circuits:

  • Astableis a self-running multivibrator circuit that does not have stable states but is constantly switching between the two states. This action produces a square wave pulse train at a known constant frequency.
  • Monostableis a one-time multivibrator with only one stable state, as it returns to its initial stable state when externally triggered.
  • Bistableis a flip-flop with two stable situations that produce a single impact high or low in value.

One way to produce a very simple clock signal (or pulse) is to connect the doors of digital logic. Nand doors include current amplification and can also be used to provide a suitable clock signal or timing pulse with the help of a single capacitor and resistance to provide the necessary feedback and timing functions.

These timing circuits are often used due to simplicity.

Monostable Multivibrator Circuits

Monostable Multivibrators or "single pulse" pulse generators are often used to convert short sharp pulses into much larger ones for timing applications. Monostable multivibrators produce a single output pulse of "high" or "low" when a suitable external trigger signal or initial pulse T is applied.

This trigger pulse signal initiates a timing cycle (t1) that causes the monostable output to change the state at the beginning of the timing cycle. Output, timing capacitor, CT and resistance remain in this second state until the end of the timing period (t2) determined by RT's time constant.

The monostable multivibrator now remains in this second timing state until the end of the RC time constant and automatically returns to its "zeros" or original (stable) state.

Simple NAND Gate Monostable Circuit

Multivibrator
Simple NAND Gate Monostable Circuit

Not Door Monostable Multivibrator

Multivibrator
Not Door Monostable Multivibrator

As with the NAND gate circuit above, initially the trigger input is high at the logic level "1", so that the output from the first NOTE gate U1 is low at the logic level "0". Timing resistance, RT and capacitor, CT, the second door are connected to each other parallel to the entrance of u2. Since the input to U2 is low, the output in Q will be high.

A NAND gate is given as T = 2.2 RC per second with the output frequency given as time constant ε = 1/T for the Astable Multivibrator.

For example: if the resistance is R2 = 10kΩ and the capacitor is C = 45nF, the oscillation frequency of the circuit will be given as follows:

Multivibrator
Oscillation Frequency Equation Solution

The Output frequency is then calculated as 1 kHz, which is equal to a time constant of 1 ms. so that the output waveform will look like this:

Multivibrator
WaveForm

Bistable Multivibrator Circuits

The Bistable Multivibrators circuit is basically an SR flip-flop that we looked at in previous tutorials with the addition of an inverter or door to provide the necessary switching function. As with Flip-Flops, both conditions of a bistable multivibrator are stable, and the circuit remains stable indefinitely. This type of multivibrator circuit passes "only" from one state to another when an appropriate external trigger pulse is applied, and therefore two trigger pulses are required to go through a complete "SET-RESET" cycle. This type of circuit is also known as "Bistable latch", "transition latch" or simply "T-latch".

NAND Gate Bistable Multivibrator

Multivibrator
NAND Gate Bistable Multivibrator Circuit

The simplest way to make a bistable latch is to connect a pair of Schmitt NAND doors together to create an SR latch, as shown above. Two NAND gates, U2 and U3, the input NAND gate, form the bistable triggered by U1. This U1 NAND door can be skipped and replaced with a single passkey to make a key debounce circuit that we have seen before in the SR Flip-flop tutorial. We also recommend that you read an article we have done about these topics. In this way, you will be much more comfortable in the transition from theory to practice.