Differentiate between bistable multivibrators and astable multivibrators with reference to the output states - NSC Electrical Technology Electronics - Question 3 - 2023 - Paper 1

When S2 is pressed, the output of the bistable multivibrator changes its state. Assuming it was in a low output state before pressing S2, it will switch to a high output state, providing a positive voltage at the output.

Pressing S1 triggers the bistable multivibrator to toggle its output state. Initially, if the output is low, pressing S1 applies voltage to the input of the operational amplifier, causing its output to switch high. This action sets one stable state, holding the output high until the circuit is reset by pressing S2.

When S1 is pressed, the output voltage of the circuit transitions to a high state. The specific value of the output voltage will be near the supply voltage, which is +9V in this configuration, assuming ideal conditions and minimal voltage drop across the components.

To generate the output waveform based on the given input, follow the characteristic response of the bistable multivibrator. The output waveform will resemble a square wave, indicating the high and low states alternated according to the input pulse timing.

The circuit in FIGURE 3.3 uses an active low trigger. The trigger pin of the 555 timer activates the output when it detects a low voltage at the trigger input.

The threshold voltage for the circuit is determined by the supply voltage and the capacitor charge state. It is typically set at 2/3 of the supply voltage, which for a +6V supply is approximately 4V.

To eliminate unwanted noise from the supply, use decoupling capacitors close to the power pins of the active components. Additionally, you may implement a low-pass filter to smooth out voltage spikes or fluctuations in the supply voltage.

The duty cycle is defined as the ratio of the high duration of the output signal to the total duration of one complete cycle, expressed as a percentage. A duty cycle of 50% means the output is high half the time and low the other half, indicating equal time spent in both states.

The charging time of the capacitor is typically longer than the discharging time in a monostable multivibrator. This is because the capacitor charges through a resistor which limits the current, while discharging occurs directly through a lower resistance path.

The voltage waveform across capacitor C1 will show a rising characteristic during the charging phase followed by a rapid drop during the discharging phase, forming a sawtooth pattern.