Unit 5

Timing circuit

Q1) Explain Multivibrators and its types.

A1)

Multivibrators are sequential logic circuits that operate continuously between HIGH and LOW.

There are three types of clock pulse generation circuits:

• Astable – It has NO stable states but switches continuously between two states.  This action results in a train of square wave pulses at a fixed frequency.
• Monostable – It has only ONE stable state and is triggered externally with it thus returning back to its first stable state.
• Bistable – It has TWO stable states that produces a single pulse either positive or negative.

Q2) Explain Monostable and Astable multivibrator.

A2)

Monostable Multivibrators or “one-shot” pulse generators are often used to convert short sharp pulses into wider ones for timing applications. They generate a single output pulse when a suitable external trigger signal or pulse T is applied.

Astable Multivibrators are the most commonly used multivibrator circuit. It is a free running oscillator that have no permanent state but are continually changing there output from (LOW) to (HIGH) and then back again. This continuous switching action produces a continuous and stable square wave output that switches abruptly between the two logic levels making it ideal for timing and clock pulse applications.

Q3) Write some features of LM555.

A3)

• It operates from +5 Volts to +18 Volts supply voltage.
• Sinking or sourcing 200 mA of load current.
• The duty cycle is adjustable.
• The maximum power dissipation per package is 600 mW.

Q4) Draw and explain pin diagram of LM555.

A4)

Fig.1:Pin Diag of LM555

1. TRIG: It is an input pin. Output of the timer depends upon the amplitude of the external trigger voltage.
2. OUT: This output is approximately 1.7 V below +Vcc.
3. RESET: The timer does not start until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides threshold.
4. THR: The timing (OUT high) interval ends when the voltage at threshold is greater than that at CTRL.
5. DIS: In phase with output, Open collector output may discharge a capacitor between intervals.

Q5) Explain working of LM555.

A5)

The 555 timer operates in 3 modes:

1. A-stable
2. Mono-stable
3. Bi-stable modes.

Block Diagram:X

Fig.2:Block Diag of LM555