DESCRIPTION ABOUT PLCC

DESCRIPTION ABOUT PLCC (VIEW BLOCK DIAGRAM)

Power Supply Section

For the functioning of various sections of the PLT unit, pure and stable dc voltage is required. Pure and stable dc bias is received from the power supply section of PLT unit for the functioning of the various sections and circuit. The power supply section of the PLT unit receives ac supply from the ac main wires and covert into the dc and after regulating it gives dc supplies of 9 volts.

Approximately 220 volts (50) Hz ac supply enters into the PLT unit from ac power line through the ac main wires. The supply is directly fed to the primary winding of the main transformer of the PLT unit. Main transformer is a step down transformer.

Microphone Preamplifier

Impedance microphones have an extremely small output voltage, which in normal use is unlikely to ever exceed 1m-vrms. This means that the preamplifier must have high voltage gain and a very low noise input if a high signal to noise ratio is to be achieved. This circuit requires only 200 mv rms at the input for an output level 0.1 vrms and the unweighted signal to noise ratio (input open circuit) is a little under –66 dB.

Many dynamic microphones have an integral step up transformer which gives increased output voltage, but the available output current is of course decreased, and so a preamplifier having a higher input impedance (usually 50k) is required.

Audio Amplifier Section

This section consists of audio amplifier and speaker. Input of the audio amplifier is very low. It is required to amplify this signal and increase the signal to noise level. The output of the audio amplifier goes to speaker.

Line Trap

Line trap unit is fixed between coupling capacitor to the lines .it is a parallel tuned circuit containing inductance and capacitance. It has less than 0.1 V resistor.

The line trap unit provides a protection or prevention of high frequency signal from entering the neighboring line and carrier current.

Protection Circuit

There are many types of protection circuits used in the system. They prevent high voltage, noise, and interference. They are as follows: -

A) To Protect IFT

This protection circuit is used to protect IFT which is connected parallel with the 230 volt ac. when noise spikes exceed from some predetermined values, one of the diode gets short -circuited and protects the IFT.

B) To Provide Constant Voltage

To protect constant voltage for the protection of whole circuit, zener diode is used. Zener diode is connected in parallel with the bridge circuit. When the voltage across the bridge increases from the 9 volts, zener diode gets short-circuited and hence protects the whole circuit.

C) Other Protection

There are various protection circuits connected with the IC's according to IC's specifications.

Signaling Network

In this block, IC CD 4093 is used as a signaling network. This IC acts as schmitt trigger, which provides the train of pulses to generate beep sound.

Matching Network

This network is used to match the frequency of transmitter and receiver .it is also used to avoid the interference between the modulating signal and power line signal of 50 Hz.

TONE DECODER

The LM 567 and LM567C are general-purpose tone decoders designed to provide a saturation transistor switch to ground when an input signal is present within the pass band. The circuit consists of an I & Q detector driven by a voltage controlled oscillator, which determines the center frequency of the decoder. External components are used to independently set the central frequency, bandwidth and output delay.