AM Transmitter 7Mhz

The circuit of a 7MHz C W / A M QRP transmitter described here can be used to transmit either CW or audio frequency modulated signal over a 7MHz carrier. The carrier frequency oscillator is crystal controlled using 7MHz crystal in its fundamental mode. The tank circuit comprises a shortwave oscillator coil which can be tuned to 7MHz frequency with the help of ½J gang capacitor VC1. 

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 Transistor T2 (with identical tank circuit connected    its collector as in case of transistor T1) serves as a power amplifier. The RF output from oscillator stage is inductively coupled to the power amplifier stage. The output from power amplifier is routed via capacitor C3 and inductor L3 to a half-wave dipole using a 75-ohm coaxial cable. ½J gang capacitor VC3 along with inductor L3 forms an antenna tuning and matching network between the output of power amplifier stage and coaxial transmission line for maximum power transfer. Suitable heatsink should be used for transistor T2.

  Tuning adjustments may be accomplished using a 6-volt torch bulb. Connect the bulb to the collector of transistor T1 first through a coupling capacitor and tune ½J gang VC1 for maximum brilliance. (Note: the bulb would light according to intensity of RF energy.) Same procedure may be repeated for power amplifier stage and antenna tuning network for ensuring maximum power transfer.

 For CW operation, switch S1 is to be kept on for bypassing the audio driver transformer and Morse key is used for on/off-type modulation. CW would be generated during key depressions. For AF modulation, Morse key points should be closed and switch S1 should be flipped to ‘off ’ position. Any suitable mic. amplifier may be used to feed audio input to the audio driver transformer X1. (For transformer X1 you may use the transistor-radio type AF driver transformer.)

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Radio Frequency Based Attendance Register

Traditionally the attendance at an establishment is usually done in a book register. It is time consuming. It is very difficult to verify the attendance over long periods. This method is very time consuming and very difficult to verify the attendance over a week or above. Keeping attendance registers is space consuming. Here chances of doing malpractice in marking attendance are high.

 RFID based attendance system uses RFID tags for each person. A person marks the attendance by swiping the tag near RFID reader module. The attendance is temporarily saved in the EEPROM. At any time the circuit can be connected to a computer and the attendance is moved from EEPROM to a text file in computer. It has following advantages. (1) Simplicity and reliability, (2) Saving attendance in a computer allows easy verification and longer record keeping.

It works on radio frequency transmitters and receivers. Each person is given a RF ID card which is having a unique code. When it is swiped on a RF ID card reader, it reads the code and is stored. The attendance is temporarily saved in the EEPROM. At any time the circuit can be connected to a computer and the attendance is moved from EEPROM to a text file in computer. The advantage of this system is simplicity and reliability. Saving attendance in a computer allows easy verification and longer record keeping.

The DT125R series RFID Proximity OEM Reader Module has a built-in antenna in minimized form factor. It is designed to work on the industry standard carrier frequency of 125 kHz. This LF reader module with an internal or an external antenna facilitates communication with Read-Only transponders type UNIQUE or TK5530 via the air interface. The tag data is sent to the host systems via the wired communication interface with a protocol selected from the module pinout. The LF DT125R module is best suited for applications in Access Control, Time and Attendance, Asset Management, Handheld Readers, Immobilizers, and other RFID enabled applications.The AUTOMATIC DATA COLLECTION Technology used in th RFID reader.

WORKING

 Microcontroller reads time from real time chip DS12887. Whenever a RFID tag is brought near RFID module, it sends the card number in ASCII fomat to the microcontroller.  The microcontroller compares this number with those numbers in the  EEPROM. If match is found, it first check whether it is a master card, then it displays menu for master card. If not, it displays the ID number and then sends the time and date details along with ID number to EEPROM for marking daily attendance. If no matches found, the microcontroller displays ‘Card Not Programmed’ in LCD.

            Master card menu consists of set time, add, edit, delete, transfer. The various options are selected using the keypad keys- up, down, enter, cancel. Keypad encoder converts the key pressed into corresponding hex file. The output from RFID is given as the serial  input. The card ID number is 8 bytes long and two extra bytes serve as start and stop bits.

Set time: It is used to edit the time of RTC. The time is entered using the keypad.

Add: It is used to add a new tag and store the new tag number and ID number to EEPROM.

Edit: It is used to edit the card ID number. The new ID number is entered through keypad.

Delete: It is used to delete a card from record.

Transfer: This option is used to transfer the marked attendance stored in EEPROM to the computer.

MAX232 is used to convert the TTL voltage to RS232 compatible voltage. Real time clock is used to provide the date, month, and year details for marking the attendance.

Almost all electronic devices used in electronic circuits need a dc source of power supply to operate .The source of dc power is used to establish the dc operating points for the passive and active electronic devices incorporated in the system. The combination of a transformer, a rectifier, and a filter constitutes an ordinary dc supply, also called an unregulated power supply. For many applications in electronics unregulated power supply is not good because of the following reasons.

·                    Poor regulation.

·                    Variations in the ac supply main.

·                    Variations in temperature.

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