Circuit consists of following stages.
1. Microcontroller
The heart of the system is Atmel AT89S51 microcontroller which is running on 12 MHz clock speed.

2. Power Supply
The main power source is 220V AC, which is step-down into 24V and 72V. The18V DC is used for telephone system and 72V AC supply is used for ringing telephone bells which is controlled by RL10.

3. Ring Detector
This stage converts the incoming bell signals into pulses. These pulses are counted by Timer1 in the micro controller. The system hold the (Outer) call after two bells, two bells have 37 pulses.

4. Tone Generator
This stage consists of CD4093. The gate A oscillate on 440Hz and gate B oscillate on low frequencies then gate C  “AND” the output of A and B gate. This arrangement is done for generating dial tone and busy tone. Gate D generate dial tone, dial tone is controlled by P3.6 and busy tone is controlled by P3.4.Dial tone is provided to speech bus and busy tone is provide on standby bus.
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The ATA8203/ATA8204/ATA8205 is a multi-chip PLL receiver device supplied in an SSO20
package. It has been specially developed for the demands of low-cost RF transmission systems
at data rates of 1 kbit / s to 10 kBbit / s in Manchester or Bi-phase code. Its main
applications in the areas of aftermarket keyless entry systems and tire pressure monitoring
telemetry systems, / remote control applications for industrial consumers, home entertainment
systems, access control and security technology systems. Can be used on the receive frequency of
range of f0 = 312.5 MHz to 317.5 MHz, f0 = 431.5 MHz to 436.5 MHz or 868 MHz to f0 =
870 MHz for ASK or FSK data transmission. All statements below refer to 315 MHz,
433 MHz and 868.3 MHz applications.

The PLL generates the frequency of the FLO unit for mixer using a fully integrated synthesizer with
low-noise integrated LC-VCO (Voltage Controlled Oscillator) and PLL-loop filter. The XTO (crystal
oscillator) generates the reference frequency FREF = fXTO / 2 (868 MHz and 433 MHz versions)
or FREF = fXTO / 3 (version 315 MHz). The integrated LC VCO generates two or four times the
frequency mixer unit fVCO. The I / Q signals for the mixer are generated with a divide by two or
four circuit (OLF = fVCO / 2 for the version of 868 MHz, FLO = fVCO / 4 to 433 MHz and 315 MHz versions).
fVCO is divided by a factor of 128 and 64 and is fed into a phase frequency detector and compared
with FREF. Output of phase frequency detector is introduced into an integrated circuit filter
and therefore generates the control voltage to the VCO. If it is determined FLO fXTO can be calculated
using the following formula:
FLO/128 FREF = 868 MHz, FREF = fLO/64 for 433 MHz fLO/64 FREF = 315 MHz
bands.

download data sheet [link]

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Using the PIC18F2550 GLCD Text Test as a basis for further experimentation, I put together a simple and accurate graphical oscilloscope using a PIC18F2550 microcontroller and a AGM1264F graphical LCD. The oscilloscope measures the average voltage, the maximum voltage, the minimum voltage, the peak-to-peak voltage, and the zero-crossing frequency for a DC signal over 100 samples. The oscilloscope has a built in edge trigger function that can be set to capture on rise or fall (or disabled altogether). The time scale for the display is variable and can be easily redefined using the changeTimeDivision function. Likewise, the voltage range can be change to 0-5V, 0-2.5V, and 0-1.25V. The main limitations of this oscilloscope include relatively slow acquisition time and sampling rate (~60kHz) and the fact that the inputs are limited by the constraints of the internal ADC. However, it is a very nice display and I highly suggest you view the videos to see it in action. [Via]

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The circuit is based on low-cost AtTiny2313 ATMEL microcontroller. It is able to read 125KHz RFID tags. Every tag is read then decoded, and its code is transmitted as ascii chars on the serial output. If the RFID code read is the same that is recorded in the micro non-volatile memory, then a relay pulses for 1,5 seconds, acting as electronic lock. The code stored in non-volatile memory is taken from the FIRST tag that is read from the circuit after burning the micro. The downloadable zip file contains the schematic diagram, the printed circuit board layout (single face, PDF 1:1 format) and the object file to burn in the micro. you can download document  RFID 125KHz

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