ملتقى الفيزيائيين العرب - تكفووون ساعدوني

السلام عليكم ورحمة الله وبركاته

مسائكم /صباحكم فل وكادي

أخواني وأخواتي طالبتكم لا تردوني

عندي بحث فيزيائي بعنوان (مذبذب الترانزستور ) multivibrators

جمعت معلومات بس للأسف لم أجد إلا باللغه الإنجليزية وصعب علي الترجمه وكذلك التلخيص وتحديد المهم من هذه المعلومات

فكلي رجاء وأمل أن تساعدوني بهذا البحث أو من لديه أي معلومة عن هذا الموضوع فكلي أمل أن تمدون كفوفكم الكريمة لي ولا تبخلون علي بما لديكم

أعزائي هذا مأتم جمعه فمن يجد لديه ألقدره على الترجمة و التلخيص بتحديد المعلومات المهمة والتي تخص multivibrators إبلاغي وكلي رجاء أن لايطول انتظاري وأنا مستعدة لدفع أي مبلغ مالي مكافأة لهذا العمل على أن ينتهي منه خلال اليومين القادمة

تفضلوا :::

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as light emitting diodes, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors and capacitors.
There are three types of multivibrator circuit:
• astable, in which the circuit is not stable in either state—it continuously oscillates from one state to the other.
• monostable, in which one of the states is stable, but the other is not—the circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. A common application is in eliminating switch bounce.
• bistable, in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. Such a circuit is important as the fundamental building block of a register or memory device. This circuit is also known as a flip-flop.
In its simplest form the multivibrator circuit consists of two cross-coupled transistors. Using resistor-capacitor networks within the circuit to define the time periods of the unstable states, the various types may be implemented. Multivibrators find applications in a variety of systems where square waves or timed intervals are required. Simple circuits tend to be inaccurate since many factors affect their timing, so they are rarely used where very high precision is required.
Before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.
Astable multivibrator circuit

Figure 1: Basic BJT astable multivibrator
This circuit shows a typical simple astable circuit, with an output from the collector of Q1, and an inverted output from the collector of Q2.
Suggested values which will yield a frequency of about f = 0.24Hz.
Basic mode of operation
The circuit keeps one transistor switched on and the other switched off. Suppose that initially, Q1 is switched on and Q2 is switched off.
State 1:
• Q1 holds the bottom of R1 (and the left side of C1) near ground (0V).
• The right side of C1 (and the base of Q2) is being charged by R2 from below ground to 0.6V.
• R3 is pulling the base of Q1 up, but its base-emitter diode prevents the voltage from rising above 0.6V.
• R4 is charging the right side of C2 up to the power supply voltage (+V). Because R4 is less than R2, C2 charges faster than C1.
When the base of Q2 reaches 0.6V, Q2 turns on, and the following positive feedback loop occurs:
• Q2 abruptly pulls the right side of C2 down to near 0V.
• Because the voltage across a capacitor cannot suddenly change, this causes the left side of C2 to suddenly fall to almost -V, well below 0V.
• Q1 switches off due to the sudden disappearance of its base voltage.
• R1 and R2 work to pull both ends of C1 toward +V, completing Q2's turn on. The process is stopped by the B-E diode of Q2, which will not let the right side of C1 rise very far.
This now takes us to State 2, the mirror image of the initial state, where Q1 is switched off and Q2 is switched on. Then R1 rapidly pulls C1's left side toward +V, while R3 more slowly pulls C2's left side toward +0.6V. When C2's left side reaches 0.6V, the cycle repeats.
Multivibrator frequency
The period of each half of the multivibrator is given by t = ln(2)RC. The total period of oscillation is given by:

T = t1 + t2 = ln(2)R2 C1 + ln(2)R3 C2

where...
• f is frequency in Hertz.
• R2 and R3 are resistor values in ohms.
• C1 and C2 are capacitor values in farads.
• T is period time (In this case, the sum of two period durations).
For the special case where
• t1 = t2 (50% duty cycle)
• R2 = R3
• C1 = C2

Initial power-up
When the circuit is first powered up, neither transistor will be switched on. However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on. This will quickly put the circuit into one of the above states, and oscillation will ensue. In practice, oscillation always occurs for practical values of R and C.
However, if the circuit is temporarily held with both bases high, for longer than it takes for both capacitors to charge fully, then the circuit will remain in this stable state, with both bases at 0.6V, both collectors at 0V, and both capacitors charged backwards to -0.6V. This can occur at startup without external intervention, if R and C are both very small. For example, a 10 MHz oscillator of this type will often be unreliable. (Different oscillator designs, such as relaxation oscillators, are required at high frequencies.)
[edit] Period of oscillation
Very roughly, the duration of state 1 (low output) will be related to the time constant R2*C1 as it depends on the charging of C1, and the duration of state 2 (high output) will be related to the time constant R3*C2 as it depends on the charging of C2. Because they do not need to be the same, an asymmetric duty cycle is easily achieved.
However, the duration of each state also depends on the initial state of charge of the capacitor in question, and this in turn will depend on the amount of discharge during the previous state, which will also depend on the resistors used during discharge (R1 and R4) and also on the duration of the previous state, etc. The result is that when first powered up, the period will be quite long as the capacitors are initially fully discharged, but the period will quickly shorten and stabilise.
The period will also depend on any current drawn from the output and on the supply voltage.
Protective components
While not fundamental to circuit operation, diodes connected in series with the base or emitter of the transistors are required to prevent the base-emitter junction being driven into reverse breakdown when the supply voltage is in excess of the Veb breakdown voltage, typically around 5-10 volts for general purpose silicon transistors. In the monostable configuration, only one of the transistors requires protection.

Figure 2: Basic BJT monostable multivibrator.

Figure 3: Basic BJT bistable multivibrator.

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Monostable multivibrator circuit
When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state. The time period monostable multivibrator remains in unstable state is given by t = ln(2)*R2*C1. If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator.
Bistable multivibrator circuit
Suggested values:
• R1, R2 = 10K
• R3, R4 = 10K
This circuit is similar to an astable multivibrator, except that there is no charge or discharge time, due to the absence of capacitors. Hence, when the circuit is switched on, if Q1 is on, its collector is at 0 V. As a result, Q2 gets switched off. This results in more than half +V volts being applied to R4 causing current into the base of Q1, thus keeping it on. Thus, the circuit remains stable in a single state continuously. Similarly, Q2 remains on continuously, if it happens to get switched on first.
Switching of state can be done via Set and Reset terminals connected to the bases. For example, if Q2 is on and Set is grounded momentarily, this switches Q2 off, and makes Q1 on. Thus, Set is used to "set" Q1 on, and Reset is used to "reset" it to off state.

TRANSISTORS
V. Ryan © 2002 - 09
Transistors can be regarded as a type of switch, as can many electronic components. They are used in a variety of circuits and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors which work at different voltages but all of them fall into these two categories.
TWO EXAMPLES OF DIFFERENT SHAPES OF TRANSISTOR
Transistors are manufactured in different shapes but they have three leads (legs).
The BASE - which is the lead responsible for activating the transistor.
The COLLECTOR - which is the positive lead.
The EMITTER - which is the negative lead.
The diagram below shows the symbol of an NPN transistor. They are not always set out as shown in the diagrams to the left and right, although the ‘tab’ on the type shown to the left is usually next to the ‘emitter’.
The leads on a transistor may not always be in this arrangement. When buying a transistor, directions will normally state clearly which lead is the BASE, EMITTER or COLLECTOR.

SIMPLE USE OF A TRANSISTOR

DIAGRAM 'A'
DIAGRAM 'B'
Diagram 'A' shows an NPN transistor which is often used as a type of switch. A small current or voltage at the base allows a larger voltage to flow through the other two leads (from the collector to the emitter).

The circuit shown in diagram B is based on an NPN transistor. When the switch is pressed a current passes through the resistor into the base of the transistor. The transistor then allows current to flow from the +9 volts to the 0vs, and the lamp comes on.

The transistor has to receive a voltage at its ‘base’ and until this happens the lamp does not light.

The resistor is present to protect the transistor as they can be damaged easily by too high a voltage / current. Transistors are an essential component in many circuits and are sometimes used to amplify a signal.

THE DARLINGTON PAIR
V. Ryan © 2002 - 2009
Transistors are an essential component in a sensor circuit. Usually transistors are arranged as a pair, known as a ‘DARLINGTON PAIR’. It is very important that you can identify this arrangement of transistors and state clearly why they are used.
A darlington pair is used to amplify weak signals so that they can be clearly detected by another circuit or a computer/microprocessor.
The circuit below is a temperature sensor. When the temperature drops below zero the LED lights. This type of system is often seen in a car and warns the driver of the possibility of icy conditions. The two transistors are known as a darlington pair. Without a darlington pair the circuit would probably fail. The two transistors amplify the weak current in the circuit allowing the LED to light.
The circuit opposite is a ‘Darlington Pair’ driver. The first transistor’s emitter feeds into the second transistor’s base and as a result the input signal is amplified by the time it reaches the output.

The important point to remember is that the Darlington Pair is made up of two transistors and when they are arranged as shown in the circuit they are used to amplify weak signals.

The circuit to the right shows a single transistor. When the switch is pressed current flows from the 9v to the 0v and also to the base of the transistor. This allows the transistor to switch and in turn, current / voltage flows through the bulb, which lights.

However, there is a potential problem with this circuit. The signal / current at the base of the transistor may be too weak to switch the transistor and allow the bulb to light or it may flicker on and off.
A possible solution is seen to the right. A second transistor is added to the circuit, the circuit is now likely to work as the original signal / current is amplified.

The amount by which the weak signal is amplified is called the ‘GAIN’.

Below is a system designed to monitor the temperature of a car radiator. When the radiator temperature becomes too high the voltage from the temperature sensor and sensor unit changes. The comparator detects this change in voltage and activates the darlington pair. The darlington pair driver provides enough amplified current for the motor to operate, cooling the car radiator. .

If the darlington pair is replaced with a transistor module (composed of one transistor) - what would you expect to happen?

The single transistor does not amplify the current to the motor. As a result the motor does not ‘spin’. Control Studio software allows experimentation, without the need to build a real circuit using actual components. It saves time and money as components are not wasted.

TRANSISTOR EXAM QUESTION

V. Ryan © 2002

Above is a temperature sensor made up of different circuits called modules. When the thermistor is warmed its resistance falls, allowing current to flow from positive 9 volts to 0 volts. In turn, current flows from the temperature module to the transistor module triggering the transistor. The bulb module then lights.

However, there is a problem - the bulb flickers on and off. Redesign the transistor module to ensure that the bulb is constantly alight.
Explain how the three modules work together.
THE ANSWER IS SEEN BELOW:

TRANSISTOR BREADBOARD PROJECT

V. Ryan © 2002 - 2009

Components:
680 ohm resistor to protect the LED.
1K resistor from LDR to the base of the NPN transistor.
One BFY50 npn transistor (try any alternative).
One 10K preset resistor.
One LDR.
PIN LAYOUT OF NPN TRANSISTOR

LAYOUT OF SECOND TRANSISTOR
The legs/pins on the second transistor have been twisted slightly to allow them to be pushed into the breadboard in the correct positions:

When a single transistor is used in the circuit, as seen earlier, the LDR has to be completely covered before the LED lights. This is because the circuit lacks sensitivity as the current into the base of the transistor is quite weak. A darlington pair is needed to amplify the current and this is achieved by the first transistors emitter feeding into the base of the second transistor. The current is amplified to a greater level and the LDR has only to be covered partially before the LED lights.

DUAL TRANSISTOR MULTIVIBRATOR CIRCUIT

V. Ryan © 2004 - 2009

A multivibrator circuit is a circuit that has identical components arranged on the left and right hand sides. In the case of the example below, the two PNP transistors, the capacitors and the LEDs are the key components. This circuit will trigger itself repeatedly and in this way the LEDs flash alternately. Increasing the value of the two electrolytic capacitors increases the time each LED remains on/off. The transistors are general PNP type. It is important to protect the LEDs and this is achieved by adding the 680R (or lower if necessary) fixed resistors.

As the switch is pressed, the capacitors charge up and then discharge. As one capacitor charges the other discharges. As the capacitors discharge, each triggers the base of the transistor it is connected to. This allows current to pass from the collector to the emitter and the LEDs light, alternately.

PCB VERSION OF THE MULTIVIBRATOR CIRCUIT, WITH ALL COMPONENTS IN POSITION
PCB VERSION OF THE MULTIVIBRATOR CIRCUIT, WITH THE OUTLINE OF ALL THE COMPONENTS
QUESTIONS:
1. Name the two most important components in the multivibrator circuit shown above.
2. What would be the effect of increasing the values of both capacitors?
THE THYRISTOR
V. Ryan © 2002 - 2009
A Thyristor (silicon controlled rectifier or SCR) is a little like a transistor. When a small current flows into the GATE (G), this allows a larger current to flow from the ANODE (A) to the CATHODE (C). Even when the current into the gate stops the thyristor continues to allow current to flow from anode to cathode. It latches on.

The circuit opposite represents a steady hand game which consists of a wire loop that has to be moved around a wire course without touching it. If the wire course is touched by the loop, current flows into the 'gate' of the thyristor and the buzzer sounds.
The buzzer will continue to sound after the loop has touched the wire course. This is due to the thyristor which once activated cannot be deactivated until all power is turned off.
This type of circuit is also known as a ‘latching circuit’
ALARM CIRCUIT
The circuit below is an alarm circuit and it incorporates a thyristor. When the house holder leaves he/she turns on the master power switch and the exit switch. If an intruder steps on the pressure pad the alarm sounds and ‘latches’ on (stays on) because of the thyristor.

1. Draw the symbol for a thyristor. 2. Explain how a thyristor works.
3. Draw a circuit which includes a thyristor and explain how the circuit works

THE THYRISTOR - BREADBOARD CIRCUIT

V. Ryan © 2005
A Steady Hand Game is shown below. The aim is to move the handle around the wire shape without touching it. If the handle touches the wire a buzzer sounds. This is the type of game that contains a thyristor circuit. When the handle touches the wire the buzzer will sound until the reset push switch is pressed, even if the handle is moved away from the wire.
The circuit for this type of game is shown below. The main component is called a thyristor. This is a special type of switch. When it is activated it cannot be turned off unless electrical power is removed from the whole circuit.

The symbol that represents the thyristor is shown opposite. It has three pins. The ANODE, CATHODE and GATE.

1. Using circuit simulation software draw the thyristor circuit.
2. Close switch ‘A’ to supply power to the whole circuit.
3. Close switch ‘B’ to allow current flow into the thyristor’s gate.
The buzzer should sound.
4. Open switch ‘B’ - the buzzer should still sound because the thyristor cannot be deactivated until all power to the circuit is removed.
5. Construct the thyristor circuit using a breadboard and the components listed below. Be careful to line up the components accurately. The black dots show the position of wires and components.
COMPONENTS
Three 1K resistors.
One thyristor.
One 6 volt buzzer.
One battery snap.
One 9 volt battery.
Red and Black wire.
When the battery is connected power is supplied to the circuit. Touching the red and black wires for a split second activates the thyristor which allows the buzzer to sound. The buzzer will continue to sound even though the red and black wires are not touching. The ‘buzzing’ can only be stopped if the battery is removed.

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