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Home > Mall Dynamic > Circuit Diagram And Application Example Of Monostable Multivibrator With 555 Timer
It is well known that a monostable multivibrator has only one stable state. When the input is triggered by the application, the output generates a pulse and returns to a stable state after some time.
The duration of the pulse being high will depend on the timing circuit consisting of a resistor (R) and a capacitor (C).
The following diagram shows the use of the 555 timer as a monostable multivibrator. This is the basic mode of operation of the IC 555, which requires only two additional components to make it work as a monostable multivibrator: a resistor and a capacitor.
Among them, pins 1 and 8 are connected to the ground and the power supply VCC) respectively, and the output is performed at pin 3. To avoid accidental circuit reset, pin 4 is connected to the VCC. Pin 5 is the control voltage
Input, should be grounded when not in use. To filter out noise, it is grounded through a small capacitor with a capacitance of 0.01uF.
The monostable mode is also called a "single" pulse generator. The sequence of events begins when a negative trigger pulse is applied to the trigger comparator. When this trigger comparator detects a short negative trigger pulse just below the reference voltage (1/3 VCC), the device triggers and the output becomes high
The discharge transistor is turned off and the capacitor C attached to its collector will start charging to its maximum through the resistor R. When the charge on the capacitor reaches 2/3 VCC, the HIGH output pulse ends. The internal connection of the IC 555 to the RC timing circuit in monostable mode is as follows:
Initially, the trigger is RESET, which will saturate the discharge transistor. A capacitor connected to a transistor with an open collector (drain in CMOS) is provided with a discharge path. Thus the capacitor is fully discharged and the voltage at both ends is 0. The output of pin 3 is low (0).
When a negative pulse input is applied to the trigger comparator (comparator 2), it is compared with a reference voltage of 1/3 VCC. The output remains low until the trigger input is greater than the reference voltage. When the trigger voltage is below 1/3 VCC, the output of the comparator becomes high, which sets the trigger. Therefore, the output of pin 3 will become high.
The discharge transistor is turned off and capacitor C will start charging, with the voltage at both ends rising exponentially. This is just the smell voltage on pin 6. This is supplied to comparator 1 along with a parallel, reference voltage of 2/3 VCC. The output of pin 3 will remain high until the voltage at both ends of the capacitor reaches 2/3 VCC
In the case that the value voltage (the voltage at both ends of the capacitor) becomes greater than the reference voltage, the output of comparator 1 becomes higher. This will reset the trigger, so the output of pin 3 will drop to a low level (logical 0), i.e. the output returns to its steady state. Due to the low output, the discharge transistor is driven to saturation and the capacitor will be fully discharged.
Therefore, the output of pin 3 is low at startup, when the trigger becomes less than 1/3 VCC, the output of pin 3 becomes high, when the smell value voltage is greater than 2/3 VCC, the output becomes low until the next trigger pulse occurs. A rectangular pulse is generated at the output. The length of time the output remains high or the width of the rectangular pulse is controlled by a timing circuit, i.e. the charging time of the capacitor depends on the time constant RC.
The voltage at both ends of capacitor C increases exponentially. Therefore, the equation of capacitor voltage VC can be written as: VCC = VCC(1-e-t /Rc), when the capacitor voltage is 2/3 VCC, then there is:
2/3 VCC = VCC (1 - e -t/Rc ,
2/3 = 1 - e -t/RC
e -t/RC = 1/3
- t/RC = In (1/3)
-t /RC = -1.098
t = 1.098 RC
That is, t~ 1.1RC
Therefore, the pulse width of the output rectangular pulse is W = 1.1RC. In addition, the waveform of the monostable operation is shown below!
When the C555 is used as a monostable multivibrator, the output will provide a positive rectangular pulse when a short-duration negative pulse is applied at the trigger input. By adjusting the time interval t of the charging or timing circuit, the device can be made to work as a frequency divider circuit.
If the time interval t is slightly greater than the time period of the input pulse (trigger pulse), the device can be used as a two-frequency circuit. The time interval can be controlled by appropriately washing the value of the positive R and capacitor C in the timing circuit. The input and output signal waveform corresponding to the binary frequency circuit is shown in the figure below.
The circuit will trigger the first negative pulse of the trigger input. As a result, the output will enter a high state. The output will remain high for the time interval t. During this interval, even if a second negative trigger pulse is applied, the output will not be affected and will continue to remain high because the timing interval is greater than the time period of the trigger pulse. On the third negative trigger pulse, the circuit is re-triggered.
Therefore, the circuit will trigger on each alternate negative trigger pulse, that is, there is one output pulse for every two input pulses, so it is a binary frequency circuit. By adjusting the time interval, a monostable circuit can be made to produce an integer portion of the input frequency.
The monostable operating mode of the C555 can be converted to a pulse-width modulator by applying a modulated signal on pin 5 as a control voltage. The circuit of a pulse-width modulator using a monostable multivibrator is shown below.
The control signal will modulate the smell voltage and thus the output pulse width. With the change of control voltage, the rush voltage and the input of comparator 1 are also different. As a result, the time it takes to charge the capacitor to the smell voltage level will vary, resulting in a pulse-width modulated wave at the output. The waveforms of the input, output and modulated signals are shown below.
Due to the application of the control signal, the upper rush voltage level of the capacitor will vary. The new upper limit run level UTL is given by the following formula :UTL = 2/3 VCC + VMOD
Where VMOD is the voltage of the modulating signal
Due to the new rush level, the output pulse width is given by the following formula :W = -RC In (1-UTL /CC)
The output time period is the same as the input time period
The monostable multivibrator will act as a linear ramp generator and add a constant current source. A current mirror consisting of a diode and a PNP transistor is used as a constant current source, which is placed in the position of a timing resistor. A linear ramp generator circuit with IC 555 in monostable mode is shown below.
The current IC from the constant current source will charge to the peak voltage (VCC) at a constant rate, resulting in a linear upward ramp. When the voltage at both ends of the capacitor reaches 2/3 VCC, comparator 1 will drive the discharge transistor to saturation. As a result, the capacitor begins to discharge. When discharging, the comparator 2 will turn off the discharge capacitor as the voltage at both ends of the capacitor drops to 1/3 VCC.
So the capacitor will start charging again. The discharge time of the capacitor is very short compared to the charging time. As a result, the downward slope is very steep (discharge almost immediately). Therefore, the time period of the ramp output is actually equal to the charging time of the capacitor. The time period of the ramp output is approximately given by the following formula:
T =(2/(3) Vcc Re (R1+R2)C)/(R1 VCC-VBE (1+R2)) The ramp output and pulse output waveforms of the ramp generator are shown below:
A monostable multivibrator can be used to drive a relay, and its circuit is shown below:
These circuits are called time-delay relays. In this circuit, the relay, once activated, remains on for a period of time. The time the relay is on can be anywhere from 0 to 20 seconds depending on the values of R and C in the timing circuit.
For example, if the relay is to be turned on for 10 seconds in order to power an external device, the values of the resistor and capacitor can be calculated using the formula t = 1.1rc as follows assuming that the value passing through the capacitor is its lowest possible value, i.e. 10uF, and the value of the resistor is 10 = 1.1* R*10uF
That is, R = 909090.9090 = 909KQ
Potentiometers can be used to adjust the resistance and thus the time delay
The circuit of the missing pulse detector is shown below, with the PNP transistor connected to the capacitor and the input trigger pulse train provided to the base end of the transistor as well as the pin 2 trigger input of the IC 555:
The trigger pulse train will continuously reset the timing period, so the output is always high. If any trigger pulse is missing, the device detects this missing pulse and the output becomes low, which works as follows:
When the input is 0, the PNP transistor is switched on, the voltage at both ends of the capacitor is clamped to 0.7V, and the output is HIGH. When the input trigger voltage is high, the triode shuts off and the capacitor starts charging.
If the input trigger signal becomes low again before the timing cycle is complete, the voltage at both ends of the capacitor drops to 0.7V before reaching the sniff voltage (2/3 VCC) and the output remains high. If the input trigger signal does not become low before the timing cycle is complete due to a lack of pulse, it allows the capacitor to charge to the smell value voltage and the output will become low.
In order for the circuit to function as a missing pulse detector, the time period of the input trigger signal should be slightly less than the timing interval. Therefore, a continuous negative input pulse will not allow the capacitor to charge to the smell value voltage, and the output continues to remain high. In the case of a change in input frequency or loss of pulse, the capacitor will be charged to the national voltage and the output will drop. The input pulse waveform, capacitor voltage waveform and output signal waveform are shown in the figure below.
As we all know, a monostable oscillator, also known as a self-excited multivibrator, is a circuit part. In a circuit, a monostable multivibrator has only one stable state. When the input is triggered by the application, the output generates a pulse and returns to a stable state after some time. In practical applications, 555 timers are used to build a monostable oscillator
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