Multivibrators

Multivibrators
The number of electronic circuits which can be built with basic components such as resistors, capacitors, transistors, coils, etc. is limited only by your imagination and needs.   Here is a circuit where two transistors operate as a pair:

This circuit has two stable states and so it is called a “bi” “stable” or “bistable” circuit.  It is important to understand the operation of this simple and useful circuit.

If press-button switch ‘A’ is pressed, it short-circuits the base/emitter junction of transistor TR1.   This prevents any current flowing in the base/emitter junction and so switches TR1 hard off.   This makes the voltage at point ‘C’ rise as high as it can. This leaves transistor TR2 powered by R1 and R2 which have 11.3 Volts across them and switches TR2 hard on.

This pulls point ‘D’ down to about 0.1 Volts.   This happens in less than a millionth of a second.   When the press-button switch ‘A’ is released, transistor TR1 does not switch on again because its base current flows through resistor R3 which is connected to point ‘D’ which is far, far below the 0.7 volts needed to make TR1 start conducting.

The result is that when press-button ‘A’ is pressed, transistor TR2 switches on and stays on even when press-button ‘A’ is released.   This switches transistor TR3 off and starves the Load of current.   This is the first ‘stable state’.

The same thing happens when press-button ‘B’ is pressed.   This forces transistor TR2 into its ‘off’ state, raising point ‘D’ to a high voltage, switching transistor TR3 hard on, powering the Load and holding transistor TR1 hard off.   This is the second of the two ‘stable states’.

In effect, this circuit ‘remembers’ which press-button was pressed last, so millions of these circuits are used in computers as Random Access Memory (‘RAM’).   The voltage at point ‘C’ is the inverse of the voltage at point ‘D’, so if ‘D’ goes high then ‘C’ goes low and if ‘D’ goes low, then ‘C’ goes high.   In passing, the output at ‘D’ is often called ‘Q’ and the output at ‘C’ is called ‘Q-bar’ which is shown as the letter Q with a horizontal line drawn above it.   This is shown on the next circuit diagram.

A minor variation of this circuit allows a load to be energised when the circuit is powered up:

When powered down, the capacitor ‘C1’ in this circuit is fully discharged through resistor ‘R6’.   When the 12 Volts supply is connected to the circuit, capacitor C1 does not charge instantly and so holds the base of TR2 down below 0.7 Volts for much longer than it takes for transistor TR1 to switch on (which, in turn, holds TR2 hard off).   Mind you, if it is not necessary to have the Load held powered on indefinitely, then an even more simple circuit can do this:

Here, when the switch is closed, both sides of the capacitor C1 are at +12 Volts and this causes the 1K8 resistor to conduct heavily, driving the transistor and powering the load.   The capacitor charges rapidly through the transistor and reaches the point at which it can no longer keep the transistor switched on.   When the battery is switched off, the 1M resistor discharges the capacitor, ready for the next time the battery is connected.

The Monostable Multivibrator.
The monostable has one stable state and one unstable state.   It can be flipped out of its stable state but it will ‘flop’ back into its stable state.   For that reason, it is also known as a ‘flip-flop’ circuit.   It is similar to a bistable circuit, but one of the cross-link resistors has been replaced by a capacitor which can pass current like a resistor, but only for a limited amount of time, after which, the capacitor becomes fully charged and the current flow stops, causing the ‘flop’ back to the stable state once more.

In this circuit, the ‘R’ resistor and the ‘C’ capacitor values determine how long the monostable will be in its unstable state.   The circuit operates like this:

    1. In the stable state, transistor TR1 is off. Its collector voltage is high, pushing the left hand side of capacitor ‘C’ to near +12 Volts.   As the right hand side of capacitor ‘C’ is connected to the base of TR2 which is at 0.7 Volts, the capacitor gets charged to about 11.3 Volts.
    1. The press-button switch is operated briefly.   This feeds current through its 10K resistor to the base of transistor TR1, switching it hard on.   This drops the collector voltage of TR1 to near 0 Volts, taking the left hand side of the capacitor with it.
    1. As the voltage across a capacitor can’t change instantly, the right hand side of the capacitor drives the base of transistor TR2 down below 0.7 Volts, causing TR2 to switch off.
  1. The circuit can’t hold TR2 in its ‘off’ state for ever.   The resistor ‘R’ feeds current into the capacitor, forcing the voltage at the base of TR2 steadily upwards until the voltage reaches 0.7 Volts and transistor TR2 switches on again, forcing TR1 off again (provided that the press-button switch has been released).   This is the stable state again.   If the press-button switch is held on, then both transistors will be on and the output voltage will still be low.   Another output pulse will not be generated until the press-button is let up and pressed again.

This circuit could be used to switch a microwave oven on for any chosen number of seconds, create a delay on your home-built burglar alarm, to give you time to switch it off after walking through your front door, operate a solenoid valve to feed a pre-determined quantity of beverage into a bottle on a production line, or whatever…

The Astable multivibrator.
The astable circuit is the monostable with a second capacitor added so that neither state is stable.   This results in the circuit flopping backwards and forwards continuously:

The rate of switching is controlled by the R1/C1 and R2/C2 combinations.   The load’s ON time to its OFF time is called the ‘mark-space’ ratio, where the ON period is the ‘mark’ and the OFF period is the ‘space’.   If you choose to use electrolytic capacitors which have their own polarity, then the +ve end of each capacitor is connected to the transistor collector.

While it is good to understand how these multivibrator circuits operate and can be built, nowadays there are pre-built circuits encased in a single package which you are much more likely to choose to use.   These are called Integrated Circuits or ‘ICs’ for short.   We will be discussing these shortly.   Before we do, notice that in the circuit above, transistor TR3 has been changed to a new variety called a Field Effect Transistor (‘FET’).   This type of transistor is newer than the ‘bipolar’ transistors shown in the earlier circuits.   FETs come in two varieties: ‘n-channel’ which are like NPN transistors and ‘p-channel’ which are like PNP transistors.

FETs are more difficult to make but have now reached a level of cost and reliability which makes them very useful indeed.   They require almost no base current (called ‘gate’ current with this type of transistor) which means that they have almost no effect on any circuit to which they are attached.   Also, many of them can handle large currents and boast major power handling capabilities.   Because of this, it is usual to see them packaged with a metal plate mounting, ready to be bolted to an aluminium heat-sink plate to help dissipate the heat generated by the large amount of power flowing through them.   The ‘RFP50N06’ shown above can handle up to 50 Volts and carry up to 60 Amps, which is serious power handling.

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