RF transmitter circuit:(friend or foe)

CHAPTER 2

COMPONENTS USED:

                  

2.1 RF transmitter circuit:

Fig. 2.1 shows the RF transmitter circuit.

Output of the microcontroller in the initiator unit is given as input to the gate pin of the microcontroller and the MOSFET regulates the output of the laser. Source pin is directly grounded. If gate pin is high then MOSFET and source pin will get short circuited else, open circuited.

 Example,

Consider a decimal number 14. Its corresponding byte in binary format is

0001 0100

Case 1:

From the above example, if the input to the gate is right most significant bit in a byte of given number is 0 (i.e. Low). Connection between drain and source will get disconnected. Hence the laser output is zero.

Case 2:

If the input to the gate is 1 (i.e. High). Drain and source will get connected. Hence the laser gets the supply. This process is very rapid, and continues until all the bits get sent.

2.1.1 MOSFET

The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. Although the MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B) terminals, the body (or substrate) of the MOSFET often is connected to the source terminal, making it a three-terminal device like other field effect transistor. Because these two terminals are normally connected to each other (short-circuited) internally, only three terminals appear in electrical diagrams. The MOSFET is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common transistor.

2.2 RF RECIEVER CIRCUIT

Fig. 2.2 shows the RF receiver circuit.

The input pin is connected to a 5V source via a pull-up resistor, and hence defaults to 5V when it is not being driven. Since the op-amp outputs a low signal when a laser beam is incident upon the connected phototransistor, this will cause the input pin of the MCU to go low as well due to the diode being in the forward active region. Conversely, if the pin is pulled low by another Schmitt trigger while this Schmitt trigger still has a high output, the diode will now be reverse-biased and allow the voltage differential to persist. This behavior forms the basis of our simple diode-based OR-gate.

1.    Photo transistor receives encrypted RF signal from initiator unit. It is given as voltage to base of the photo transistor (Q1). Hence emitter and collector of the photo transistor (Q1) will get short circuited. Because emitter is grounded. Hence base of the transistor (Q2) at zero voltage level. Hence the output is high.

2.    If the voltage at base of the photo transistor Q1 is low (i.e. there is no light incident on the base of the photo transistor). Base of the second transistor is at high level. Hence the output is low. And then the binary code is converted to decimal number by an TTL logic of the bipolar junction.

2.2.1     Phototransistor:

Silicon photodiodes are semiconductor devices responsive to high-energy particles and photons.  Photodiodes operate by absorption of photons or charged particles and generate a flow of current in an external circuit, proportional to the incident power.  Photodiodes can be used to detect the presence or absence of minute quantities of light and can be calibrated for extremely accurate measurements from intensities below 1 pw/cm2 to intensities above 100 mw/cm2.  Silicon photodiodes are utilized in such diverse applications as spectroscopy, photography, analytical instrumentation, optical position sensors, beam alignment, surface characterization, laser range finders, optical communications, and medical imaging instruments

 Introduces the main purpose of our project. This chapter gives a brief explanation of the aim of present study, study area, objective, methodology, sources of information and outlines the contents of the upcoming chapters.

Chapter 2 Deals with working of RF transmitter and RF receiver circuits used.

Chapter 3 Deals with details of hardware, different components used,    flowchart of the transmitter and receiver part of the PIC microcontroller and the experimental set up of the model.                .

Chapter 4 Deals with the discussion of the result.

Chapter5 Deals with advantages and limitations derived from the relevant Studies.

Chapter 6 Deals with the conclusion and future scope.