Monostable multivibrator

A monostable multivibrator , also a monostable multivibrator , monoflop or univibrator , is a digital circuit that only has one stable state. When triggered by an incoming trigger signal , the circuit changes its switching status for a certain time. The tipping stage then returns to the rest position.

classification

A distinction is made between retriggerable (also: retriggerable) and non-retriggerable monoflops. Retriggerable means that a trigger signal that arrives while the time has elapsed starts the internal time again and the active switching state is extended accordingly. In the case of a non-retriggerable monoflop, a trigger signal has no effect during the active phase.

Discrete monoflop

Monostable multivibrator with bipolar transistors

The example circuit shown here is a retriggerable monoflop. In the ground state, the bipolar transistor Q2 is conductive, while the transistor Q1 blocks. Resistor R2 supplies the required base current of transistor Q2. To trigger the monoflop, the trigger signal must be set to the L level . This blocks transistor Q2. The feedback through resistor R4 makes transistor Q1 conductive.

From this point on, the time for the monoflop begins to run. In this circuit, the time constant is set by resistor R2 and capacitor C1. The capacitor C1 is charged by the charging current via the resistor R2. After a defined period of time (the switching time of the monoflop) the capacitor is charged to such an extent that the capacitor voltage comes to the order of magnitude of the base-emitter voltage of transistor Q2. From this point in time, the transistor Q2 becomes conductive again. Resistor R4 couples the control of transistor Q1 to transistor Q2. The collector-emitter voltage of the conductive transistor Q2 is below the base-emitter voltage of the transistor Q1, so that it blocks.

When the transistor Q1 blocks again, the capacitor C1 is discharged via the two resistors R1 and R2.

The discrete monoflop has the disadvantage of having a certain dependency on the operating voltage, which is not reflected in the approximated formula for the holding time, with the curve for the simple and the improved version being roughly the same.

Example of the dependency of the holding time on the operating voltage

Hold time (approximated / approximated)

The output signal at the collector of Q1 is a negative pulse of duration in the circuit shown

{\ displaystyle t_ {H} \ approx \ ln (2) R_ {2} C_ {1}}

.

The product is also often abbreviated as size . It has the dimension of a time. ${\ displaystyle R_ {2} C_ {1}}$ ${\ displaystyle \ tau}$

The approach for this equation was taken. In order to get from this formula to the approximation, it is assumed for the termination condition, for example , that applies. ${\ displaystyle U_ {BE_ {Q_ {2}}} (t) = U_ {B} (1-2e ^ {- {\ frac {t} {\ tau}}})}$ ${\ displaystyle U_ {BE_ {Q_ {2}}} (t_ {H}) = U_ {BE_ {Q_ {2}} sat}}$ ${\ displaystyle U_ {BE_ {Q_ {2}} sat} = 0}$

Hold time

Again, taking into account the termination condition , the duration is calculated for an exact holding time ${\ displaystyle U_ {BE_ {Q_ {2}}} (t_ {H}) = U_ {BE_ {Q_ {2}} sat}}$ ${\ displaystyle U_ {BE_ {Q_ {2}}} (t) = U_ {B} - (2U_ {B} -U_ {BE_ {Q_ {2}} sat} -U_ {CE_ {Q_ {1}} sat }) e ^ {- {\ frac {t} {\ tau}}}}$

{\ displaystyle t_ {H} = R_ {2} C_ {1} \ ln \ left ({\ frac {2U_ {B} -U_ {BE_ {Q_ {2}} sat} -U_ {CE_ {Q_ {1} } sat}} {U_ {B} -U_ {BE_ {Q_ {2}} sat}}} \ right) \ approx \ ln (2) R_ {2} C_ {1}}

.

The time t is almost independent of the operating voltage, but the example circuit shown can only tolerate a supply voltage of up to 5 volts, as otherwise the base of Q2 at the start of the pulse takes on a value below -5 volts - a typical limit value for bipolar transistors.

Discrete monoflop (improved)

Improved monostable multivibrator with bipolar transistors

In order to avoid the above-mentioned problem, it is recommended to connect a diode in front of the BE path of T2, since the capacitor as a source then cannot allow any current to flow against the direction of the protective diode during the discharge process and thus no negative voltage across the BE -Track drops off from T2.

The exact hold time changes according to the additional protective diode

{\ displaystyle t_ {H} = R_ {2} C_ {1} \ ln \ left ({\ frac {2U_ {B} -U_ {BE_ {T_ {2}} sat} -U_ {CE_ {T_ {1} } sat} -U_ {F}} {U_ {B} -U_ {BE_ {T_ {2}} sat} -U_ {F}}} \ right) \ approx \ ln (2) R_ {2} C_ {1 }}

.

Furthermore, the diode has the advantage that if you simulate the negative impulse with a button, the right side is not pulled to ground immediately and thus the hold time does not deviate stochastically from the above formulas, since depending on the individual duty cycle over the direct contact to ground in contrast to the previous case. ${\ displaystyle C_ {1}}$ ${\ displaystyle C_ {1}}$

Another improvement is to connect a resistor between the base and emitter of , which blocks better, as according to the formula ${\ displaystyle R_ {5}}$ ${\ displaystyle T_ {1}}$

{\ displaystyle U_ {BE_ {T_ {1}}} = U_ {CE_ {T_ {2}} sat} \ cdot {\ frac {R_ {5}} {R_ {4} + R_ {5}}}}

only a part of the CE saturation voltage drops between base and emitter . ${\ displaystyle T_ {2}}$ ${\ displaystyle T_ {1}}$

Integrated monoflop

Monoflop symbol according to DIN

Pulse diagram
E: input
Q: output

Both non-retriggerable and retriggerable monoflops are available as integrated circuits (for example the standard type 74x121) that only require the time-determining components (a resistor and a capacitor) as external circuitry. These modules have a significantly lower operating voltage and temperature dependency of the pulse duration than the above example circuit. With them, times of around 50 ns to around 1 s can be achieved. For longer times, good, large and therefore expensive capacitors are required; integrated circuits with an internal frequency divider fed by a multivibrator are used instead for longer times . As with monoflop circuits, the frequency of the multivibrator can be selected with an external resistor and a capacitor. If the multivibrator reaches the number of pulses of the division ratio, the output pulse is terminated. The frequency divider is often programmable so that times from seconds to several hours can be achieved.

Application examples

The function of a monoflop is implemented, for example, in an automatic staircase lighting system , in which the light is switched on for a certain period of time at the push of a button. In this application, a retriggerable monoflop makes sense, as this allows the light phase to be extended by pressing again.

Further applications are:

Pulse generators
Part of frequency-voltage converters and tachometers
Extending a short pulse to a defined length
Timing relay

Web links

Commons : Monostable multivibrators - collection of images, videos and audio files

A special, resettable monoflop and a practical application The circuit is a functional combination of monoflop and flip-flop