7.1 The 555 Timer

The 555 Timer is a highly versatile and popular integrated circuit used for generating accurate time delays and oscillations. It derives its name from the three internal 5kΩ resistors that form a voltage divider. It is primarily used for generating non-sinusoidal waveforms like pulses and square waves.

Pin Diagram

The 555 Timer is commonly available in an 8-pin Dual-Inline Package (DIP), with each pin serving a specific function for control, power, and output.

Functional Diagram

The internal architecture of the 555 Timer consists of several key functional blocks:

• Voltage Divider Network: Three 5kΩ resistors connected in series between the supply voltage (V_{cc}) and ground create two stable reference voltages: \frac{2V_{cc}}{3} and \frac{V_{cc}}{3}.
• Comparators: An Upper Comparator and a Lower Comparator compare external input voltages against the internal reference voltages. The Upper Comparator’s reference is \frac{2V_{cc}}{3}, and the Lower Comparator’s reference is \frac{V_{cc}}{3}.
• SR Flip-Flop: The outputs of the two comparators are connected to the Set (S) and Reset (R) inputs of an SR flip-flop. This bistable logic element controls the state of the timer’s output.
• Transistors and Inverter: A discharge transistor (Q_1) is controlled by the flip-flop’s output and is used to discharge an external timing capacitor. A buffer transistor (Q_2) isolates the reset input, and an inverter amplifies the power level of the final output signal.

The 555 Timer has two primary modes of operation: monostable mode, used for generating a single output pulse of a specific duration, and astable mode, used for generating a continuous square wave.

7.2 The Phase Locked Loop (PLL) IC 565

A Phase Locked Loop (PLL) is a crucial feedback control system used extensively in communication systems for tasks like frequency synthesis, modulation, and demodulation.

Block Diagram of a PLL

A PLL consists of three core components connected in a feedback loop:

1. Phase Detector: This block compares the phase of an incoming signal with the phase of a feedback signal from the oscillator. It produces a DC voltage that is proportional to the phase difference between the two signals.
2. Active Low Pass Filter: This filter smooths and amplifies the DC voltage from the phase detector, removing any high-frequency components and providing a stable control voltage.
3. Voltage Controlled Oscillator (VCO): This oscillator generates a periodic signal whose frequency is directly controlled by the DC voltage from the low pass filter. The VCO’s output is fed back to the phase detector.

The PLL operates in three distinct modes:

• Free running mode: When no input signal is applied, the VCO oscillates at its natural, or free-running, frequency.
• Capture mode: When an input signal is applied, the VCO frequency starts to shift towards the input frequency as the loop tries to minimize the phase error.
• Lock mode: Once the VCO frequency exactly matches the input signal frequency, the loop is “locked,” and the VCO tracks any small changes in the input frequency.

IC 565

The IC 565 is a common general-purpose PLL chip. Its free-running VCO frequency can be set by external components according to the formula:

f_{out}=\frac{0.25}{R_VC_V}

where R_V and C_V are an external resistor and capacitor connected to pins 8 and 9, respectively. For basic operation, the VCO output (pin 4) is fed back to the phase detector input (pin 5). The input signal is applied to pins 2 and 3, the filtered demodulated output is available at pin 7, and the supply voltage is applied to pin 10.

7.3 Voltage Regulators

A voltage regulator is an essential circuit that maintains a constant DC output voltage, regardless of fluctuations in the input supply voltage or variations in the load current. They are widely available as integrated circuits.

Voltage regulators are classified into two main types:

• Fixed Voltage Regulators: These ICs produce a fixed, non-adjustable DC output voltage, which can be either positive or negative.
  • The 78xx series provides fixed positive voltages. The ‘xx’ denotes the output voltage (e.g., a 7805 produces +5V). Pin 1 is for input, pin 2 is for ground, and pin 3 is for the output.
  • The 79xx series provides fixed negative voltages. The ‘xx’ denotes the magnitude of the negative output (e.g., a 7905 produces -5V). Pin 1 is for ground, pin 2 is for input, and pin 3 is for the output.
• Adjustable Voltage Regulators: These ICs produce a variable DC output voltage that can be set within a specified range using external resistors.
  • The LM317 is a popular positive adjustable voltage regulator with three pins: Pin 1 (Adjust), Pin 2 (Output), and Pin 3 (Input). It can supply up to 1.5A over an adjustable output range of 1.25V to 37V.

These specialized ICs bridge the gap to our final topic: the conversion between the analog and digital data domains.