5.1 Name the type of multivibrator that: 5.1.1 Produces one pulse cycle of 'high' and 'low' when a trigger pulse is applied to its input - NSC Electrical Technology Electronics - Question 5 - 2022 - Paper 1

The type of multivibrator that changes state when a trigger pulse is applied and remains in that state is a bistable multivibrator.

R1 serves as a pull-up resistor, ensuring that the voltage at the input pin is high when the switch is open.

R3 is used to limit the current flowing to the LED, preventing it from being damaged.

When the set switch is pressed, it pulls pin 2 low (0 V), causing the output to go high, which turns the LED on.

When trigger pin 2 is high, the LED remains off, as the output of the multivibrator stays low.

Connecting pin 6 to ground allows the IC to reset itself, which prevents the output from remaining high when the switch is released.

R2 serves as a reference resistor, setting the threshold voltage at the inverting input of the op-amp.

As the level of light increases, the resistance of the LDR decreases, which in turn increases the voltage on the non-inverting input.

The op-amp compares the voltages on its two input terminals. When the voltage on the non-inverting input is higher than that on the inverting input, the op-amp outputs a high voltage, illuminating LED 1. Conversely, if the inverting input has a higher voltage, LED 1 is off and LED 2 is illuminated.

When the voltage on the non-inverting terminal is higher, LED1 will be on and LED2 will be off.

This is an inverting Schmitt trigger circuit.

The output voltage will be -10 V when the voltage on the non-inverting input is -1 V.

This circuit uses positive feedback.

When the input voltage rises above the lower trigger voltage, the output goes high, and it stays high until the input falls below the upper trigger level. This creates hysteresis in the circuit.

An increase in R1 will cause the trigger voltage levels to increase, moving the trigger boundaries higher.

Rf serves to control the gain of the amplifier and to set the reference voltage.

The output voltage can be calculated using the formula:

$V_{out} = \left( V_{in} \frac{R_f}{R_1 + R_f + R_2} \right)$

Substituting the values gives:

$V_{out} = \left( 0.4 \times \frac{78.26}{78.26 + 10 + 0.25} \right)$

An increase in Rf increases the gain of the amplifier, leading to a larger output voltage for the same input voltage.

Increasing Rf beyond this value is not recommended as it may lead to distortion in the output signal, as the op-amp may be driven into saturation.

This limitation can be addressed by setting the supply voltage to a higher value, thereby allowing the output voltage to achieve maximum allowable levels.

Output should show the waveform representing the charging and discharging behavior of the capacitor based on the input signal.

The output waveform will reflect a slower charge and discharge cycle due to the increased capacitance.