Multivibrator

A multivibrator, astable multivibrator also called, is an electronic circuit that can be in two states between which they independently controlled or external switches back and forth. The term multivibrator but often used synonymously with the more general term flip-flop.

The astable multivibrator is basically composed of two mutually coupled electronic switches that mutually switch through a positive feedback. After a frequency-determining delay time is always automatically triggered a renewed mutual switching, so that a periodic behavior arises. Thus, the circuit is one of the Relaxation oscillators.

• 2.1 Astable multivibrator NE555
• 2.2 Astable multivibrator with logic gates

Astable multivibrator ( astable multivibrator )

Typical circuit with discrete components

R1 and R2 are resistors, K1 and K2 are the coupling elements with delay function, they may be resistors or capacitors. The two states of the multivibrator are:

• S1 and S2 are turned off,
• S1 and S2 are turned off.

Both coupling elements are capacitors reload faster or slower depending on the capacity and the resistance circuit. Thus, each of the two conditions is not stable, and the circuit between the two states are alternately tilted to and fro. Due to the size of R1, R2 and K1, K2, you can change the times that the circuit spends in each of the two states. If R1 = R2 · K1 · K2, so it is in both the same length.

The resulting nearly ideal square waves have a high content of harmonics. In this circuit, the frequency generated is strongly dependent on the operating voltage used.

In this representation, one can interpret the multivibrator as an interconnection of two monostable multivibrators.

Astable multivibrator using analog technology

In analogue delay circuits can be realized by means of capacitors. Here, the operation of an astable multivibrator is explained using an example with bipolar NPN transistors.

In the de-energized circuit, the transistors Q1 and Q2 are blocking their Duch resistance value (from collector to emitter ) is therefore almost infinite. The capacitors C1 and C2 are initially discharged. R2 and R3 are selected so that the bases of the transistors get enough power to control by can. R1 and R4 limit the current work. The switching frequency of this multivibrator is governed by the values ​​of R2, R3 and C1, C2. The resistance values ​​of R2 and R3 is substantially greater than R1, and R4.

Switch-

By applying the operating voltage UB first current flows through 1: R1, C1 parallel to R2 via Q2, and 2: R4, C2 parallel to R3 via Q1. One of the transistors is from a certain base current first conductive and draws on its collector side capacitor connected the base of the other transistor toward zero over the base then no current flows and is therefore prevented from passing taxes.

Which transistor is first conductive, depends on the specific component values ​​, especially from the transistors, which may have significant parameter tolerances for the part.

Now that a transistor has become conductive, receives its base current through the corresponding capacitor until it has charged, and also simultaneously on R2 and R3. These resistors are there, among other things, independent of the capacitor to supply the transistors in the closed state, the holding current, so that they can then control even when the capacitor has charged and thus by not conducting more current. This capacitor has then about the potential UB - 0.7 V between its plates.

The other capacitor is charged at this time across R2 and R3, so the voltage at the base of the blocking transistor slowly increases from approximately 0 V direction UB, to approximately 0.60 V, the threshold voltage is reached. This is the voltage at which the blocking transistor begins by heading.

This phase described so far occurs only once after each power- on and is of much shorter duration than the following described two states. After power up the circuit starts its periodic behavior. She tilts alternately between two temporary states back and forth, here arbitrarily called state A and state B, in state A, the transistor Q1 transistor Q2 is by navigating through controlling and in state B.

Condition A

Q1 here was controlling and therefore decreased its collector- emitter voltage of 0.2 V down to about UB. Result, the collector-side plate of C1 UB was pulled down to 0.2 V, ie by UB - 0.2 V, the other plate to the same difference. However, since the plate side towards the base Q2 to UB - 0.7 V lower potential than the other side, lie on her suddenly 0.2V - (UB - 0.7 V) UB So, 0.9 V. This is well below zero, and therefore Q2 is switched off until C1 through R2 has again transferred slowly and rest against the base of Q2 about 0.65 V, which begins by heading and therefore can flip the circuit in state B. In the meantime, C2 charges through R4 on a plate voltage of UB - 0.7 V (collector Q2 has UB, base Q1 0.7 V).

The potential jumps of the capacitors during the tilting effect positive feedback, shortening the time tipping, that is, increase the switching speed.

The period of state A is determined by C1 and R2, since C1 has to load to about 0.65 V across R2 of UB about 0.9 V, so that Q2 can tip the circuit.

Condition B

C1 is charged via R2 so far that the voltage at the base of Q2 is about 0.60 V and Q2 exceed therefore tilted in the through -controlled state. Thus the right side of C2 from UB to about 0.2 V is pulled down. Due to the potential difference of the plates ( the left plate had about UB - 0.7 V less than the right ), the left plate of C2 is now about 0.2 V - (UB - 0.7 V) UB ie, 0.9 V. This is well below zero, and thus Q1 is OFF now until this record page again about 0.65 V exceeds via R3 and Q1 by the circuit flips in state A. By tilting of Q1 in the off-state charges C1 through R1, and the base of Q2 to a plate potential UB - 0.7 V (the left side has UB, the right 0.7 V). At the same time flows on R2, the holding current to keep Q2 still open even if there is not enough current flows through C1 and the more time needs to be bridged to the tilting vonQ1.

Again, shorten the capacitors by tilting the positive feedback.

The duration of state B depends on the values ​​of C2 and R3 and lasts until C2 through R3 UB - of 0.9 V was reloaded to approximately 0.65 V.

Calculating the time periods

The left side of C2 is at the beginning of state B to UB about and want to UB to be reloaded; the state tilts at about 0 V (more precisely, 0.7 V), ie at about the half of this trans-shipment. The Auf-/Ent- or transfer of a capacitor through a resistor is based on an exponential growth law. The time for half of transhipment just corresponds to the half-life, also see a time constant.

Switching the frequency of the astable multivibrator

The switching frequency of an astable multivibrator is calculated as follows:

This applies

• F is the frequency (in Hertz)
• R2 and R3 are resistance values ​​(in ohms)
• C1 and C2 are capacitance values ​​( in farads )
• T is the period of time ( in this case, the sum of the two phase durations, in seconds)

Circuits with integrated components

Astable multivibrator with NE555

And the following circuit generates a square wave voltage, but is simpler than the multi-vibrator shown above and has the advantage that the frequency hardly depends on the operating voltage. This can be in the range 0.1 Hz to 500 kHz and can be used with only a single potentiometer varies greatly. The function of the block used NE555 can be described as: As long as the voltage across capacitor C is less than 66 % of the operating voltage, it is charged through R ( series connection of potentiometers and 1 k-ohm resistor). The output voltage on pin 3 is around the operating voltage during this time. This is 66 - % - value is exceeded, tilts internally to a flip-flop, the output voltage drops to 0 volts and the capacitor will discharge through R. Once 33 % of the operating voltage are exceeded, the flip-flop flips back to the original position and the game begins again. The voltage on the capacitor has approximately the shape of a triangle, but may be lightly loaded.

With a 20 kohm potentiometer can change the frequency generated at a ratio of 1:20. Doubling the capacity halves the frequency generated. A small change in the voltage at pin 5 (setpoint: 66% of operating voltage) can electronically change the frequency (Voltage Controlled Oscillator). Through an alternating voltage to this connector, you can achieve a frequency modulation ( " Kojak Siren ").

Astable multivibrator with logic gates

An astable multivibrator can be constructed with logic gates. Can NAND, NOR gates, or inverters are used. This circuit works only with CMOS circuits, for example 74HC or 4000 series.

The frequency is calculated as follows:

Applications

• A signal generator for generating square-wave oscillations: By changing the time-determining elements, the frequency and / or duty cycle can be changed.
• In telemetry: Will the frequency-determining resistors and capacitors types, whose value depends on a physical parameter, you can create in this way pulse trains whose pulse length or pulse interval length depends on this size. Such pulses can be modulated, for example, to a high frequency signal to transmit it. In the receiver, the physical quantities (e.g., temperature, air pressure ) can be determined from the pulse parameters.
• As a flashing signal generator in lamps or as a tone generator in horns (eg, piezoelectric sounders )

Special shapes

Astable multivibrators is also available in special forms, in which three or more active components are in play ( multiphase multivibrators ).