MONOSTABLE CIRCUIT
A monostable multivibrator can be used as circuit delay. Such a circuit it can receive a very fast signal as input and produce a signal with a well-determined active time as output . Think of the lights on the stairs of an apartment building. After being triggered by a switch, the lights remain on for a period of time set by the timer. It is possible to create a monostable both in positive and negative logic, ie which responds with a logic level 0 or 1 at the output and activated by the same logic at the input. A monostable multivibrator, also called a one shot or monoflop, is a sequential logic electronic circuit that generates an output pulse. When activated, a pulse of predefined duration is produced. The circuit then returns to its stable state and produces no more output until it is turned back on again. Monostables can be thought of as a distorted form of multivibrator in which the state is stable following the activation of a pulse, then it will return to the unstable state spontaneously. If repeated application of the input pulse keeps the circuit in an unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable monostable.
MONOSTABLE MULTIVIBRATOR, THE TIMER CIRCUIT
In the monostable circuit the time constant T is always fixed by an RC circuit. In this case the frequency does not matter but only the delay generated as the circuit is not like an astable multivibrator capable of self-oscillating.
T=0.693xR2XC2
Suppose we want to generate a 10 second delay, setting the resistance at 150KΩ we have:
C2=10/0,693X150X10 3 = 100ɲF
OPERATION MONOSTABLE MULTIVIBRATOR
Assume the following: At the instant t=0 when the supply voltage is supplied, the inputs of the first NOR gate are both at logic level 0, presenting a logic level 1 at the output. This level, via the supply voltage, also appears at the input of the second NOR gate configured as an inverter. With its output at logic level 0 it disables the transistor thus turning off the LED. In these conditions the potential on the plates of the capacitor C1 supplied by the resistance R2 and by the logic level 1 at the output of the first NOR is identical. This causes the condenser to remain discharged. In this condition the circuit is in its stable state.
PRESSING THE P BUTTON
By pressing the P button, then providing a positive pulse, the first NOR gate will present a logic level of 0. Since C1 is unloaded, the input of the second NOR gate will receive the same level 0 for a short instant while the output will be at a high logic level (logic 1). The output of NOR gate number two is driven back and briefly to the input of NOR gate 1, thus holding the output of the first NOR gate at 1 even if the button is no longer pressed. At this point the potential difference present on the plates of the capacitor ( 0 at the output of the first NOR gate and 1 through the resistance R2), starts the charging phase of the capacitor like an RC circuit.
THE CHARGE OF THE CAPACITOR
As soon as the voltage on the capacitor reaches the threshold for switching the input of the second NOR gate (Inverter), its output goes to logic level 0. At this point the transistor is cut off, turning off the LED and bringing the logic level 0 back to the input of the first NOR (Feedback) gate. The feedback simulates pressing the button even if it is no longer pressed, the logic state 1 remains on the output of the second NOR gate until the capacitor charges. Further button presses in this phase have no effect.
BJT TRANSISTOR MONOSTABLE MULTIVIBRATOR
In the monostable multivibrator, the single resistive-capacitive network (C2-R3 in figure 1 of the multivibrator) is replaced by a resistive network (only one resistor). The circuit can be thought of as a 1/2 astable multivibrator. The collector voltage Q2 is the output of the circuit (unlike the astable circuit, it has a perfect square waveform since the output is not charged by the capacitor). The green line shows the input waveform to the monostable multivibrator. The red line shows the converted output. It can act as a frequency divider. When triggered by an incoming pulse, a monostable multivibrator will go to its unstable position for a period of time, then return to its stable state. The length of time the monostable multivibrator remains in an unstable state is given by t = 0.693*R2C1. For the circuit shown, in the stable state, Q1 is off and Q2 is on. It is activated by the zero or negative input signal applied to the Q2 base. As a result, the circuit goes into the unstable state. After the time has elapsed, it returns to its initial stable state. This behavior is retriggerable: the output will stay high as long as the input is active, then stay high for the relaxation period that begins when the input is released.
BJT TRANSISTOR CIRCUIT
WAVE FORMS
DEEPENING AI
The circuit provided is an example of a monostable multivibrator based on NOR logic gates. A monostable circuit is a type of circuit that has only one stable state and one metastable state; it automatically returns to the stable state after a certain time interval once it has been activated. Here is how the circuit works:
Circuit Components
-NOR ports : NOR ports produce a low-level output only when all inputs are high; otherwise, the output is high.
-Resistors(R1, R2, R3, R4): They determine the current in the various branches of the circuit.
-Capacitor(C2): Used to control the delay time of the circuit.
-Transistor(TR1): Acts as a switch controlled by the output of an NOR port.
-LED: Visual indicator showing the status of the circuit.
-Power supply(+5V and GND): Provides the necessary voltage for circuit operation.
Circuit Operation.
1. Initial Condition (Resting State):
-In normal condition (no trigger input), the output of the first NOR port (indicated as “1”) is high because both its inputs are low (R1 is connected to ground).
-This makes the output of the second NOR port (indicated as “2”) low because at least one of its inputs (the output of the first NOR port) is high.
-Transistor TR1 remains off because the low output of the second NOR port does not provide the necessary base voltage.
2. Trigger (Activation Pulse):
-When a high trigger pulse is supplied (at the point marked “+I p”), the output of the first NOR port becomes low.
-This causes the output of the second NOR port to be high, which turns on transistor TR1 and the LED.
-In addition, the change of state charges capacitor C2 through R2.
3. Return to Steady State:
-Capacitor C2 begins to slowly discharge through R2. When the voltage across the capacitor drops, the input of the first NOR port goes low again.
-This makes the output of the first NOR port high, causing the output of the second NOR port to return low, thus turning off transistor TR1 and the LED, returning the circuit to its steady state.
Conclusions
This circuit is useful for applications where a timed drive is needed, such as electronically controlled timers or delays. The duration of the active pulse (LED on) can be adjusted by changing the values of R2 and C2 to affect the charging time of the capacitor.
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