25-08-2017, 09:32 PM
CIRCUIT OPERATION
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ABSTRACT
In the transmitter section, NE555 (IC1) is wired as an astable multivibrator whose oscillating frequency is decided by resis¬tors Rl and R2, preset VR1 and capacitor Cl. Capacitor C3 bypasses the noise to ground, preventing any change in the cal¬culated pulse-width. The output of IC1 is fed to the base of transistor Tl, which drives an infrared light-emitting diode (IR LED) to transmit the modulated IR signal. Resistor R4 lim¬its the current flowing through the IR LED. Preset VR1 is used to vary the modulating frequency.
The transmitter and the receiver are arranged such that the transmitted IR rays fall directly onto phototransistor L14G1 (T2) of the receiver. The signal received by T2 is amplified by transistor T3 and operational amplifier A741 (IC2). Series input resistor R8 and feedback resistor R9 determine the gain of op-amp IC2. The amplified signal is applied to pin 3 of PLL LM567 [IC3] through capacitor C4.
IC LM567 is a highly stable PLL with synchronous AM lock detection and power output circuitry. It is primarily used as a frequency decoder, which drives the load whenever a sustained frequency falling within its detection band is present in its self-biased input. The centre frequency of the detection band and output delay is independently determined by the external components.
In the absence of any input signal, the centre frequency of the PLL's internal free-running current-con¬trolled oscillator is de¬termined by resistor R12 and capacitor C8. Preset VR2 is used for tuning IC3 to the de¬sired centre frequency in the 6-10 KHz range, which should match the modulating fre¬quency of the trans¬mitter. Capacitors C6 and C7 are used as low-pass filter (LPF) and output filter, respectively.
When the received signal is locked to the frequency of the transmitted signal, pin 8 of IC3 goes low and LED1 glows. Since pin 8 is connected to the base of the transistor T4 through resistor R13, it is cut off and its collector voltage rises. As a result, transis¬tor T5 is forward biased to energize relay RL5. The pole and normally-closed (N/C) contact of relay RL5 are connected to + 5 V and pin 4 (PA0) of IC7, respectively.
Normally, the transmitted IR signal falls on phototransistor T2, so relay RL5 is energized and there is no input to the processor via IC7. When the IR signal is interrupted, relay RL5 de-energizes to pro¬vide a high (TTL-level) signal to the pro-cessor via port A of the programmable peripheral interface (PPI).
The processing section consists of an 8-bit 8085 microprocessor (1C4), EPROM IC 2732A [IC5), octal transparent latch 1C 74LS373 (IC6) and programmable periph¬eral interface IC 8255A (IC7). When the microprocessor gets a high signal from port A of IC7, it starts working as per the code loaded in the EPROM (IC5).
MEMORY:
EPROM IC 2732A is a UV erasable and electrically programmable memory. It is organized as 4096 words x 8 bits. The transparent window allows the user to ex¬pose the chip to ultraviolet light to erase the chip. After erasing the chip, a new program can be burnt into it.
MICROPROCESSOR:
IC 8085 [IC4] is an 8-bit, general-pur¬pose microprocessor capable of address¬ing 64K of memory. It includes most of the logic circuitry required for performing computing tasks and communicating with the peripherals. The low-order multiplexed address and data lines AD0 through AD7 of IC4 are connected to the EPROM [IC5] through the octal latch (IC6), while its high-order address lines A8 through A10 are directly connected to the EPROM. Address lines A0 through A7 are separated from data lines D0 through D7 by latch-enable sig¬nal (ALE). Address latch-enable (ALE) pin 30 of the microprocessor is connected to latch-enable pin 11 of IC6. When ALE is high, the latch is transparent, i.e. the output changes according to the input data. When ALE goes low, the low-order address is latched at the output of IC6. Data lines D0 through D7 of the micro¬processor are connected to the data lines of IC5 and IC7 each. Chip-select signal (CS) for IC5 is generated by RD and IO/M lines with the help of a NAND gate. The inverted IO/ M signal provides CS signal to IC7.
PROGRAMMABLE PERIPHERAL INTERFACE: IC 8255A (IC7) is a general-purpose programmable device compatible with most microprocessors. It has three pro-grammable ports, any of which can be used for bidirectional data transfer. The 24 I/O pins can be grouped in two 8-bit ports (ports A and B) and the remaining eight bits as port C. The eight bits of port C can be used as individual bits or grouped in two 4-bit ports, namely, CUPPER and CL0WER. Ports A and C are configured as the input ports, and port B is configured as the output port. Port A is used for in¬truder detection, port B for activating the siren, cassette player, telephone cradle switch and redial button, and port C for polarity-reversal detection.
PB0 (pin 18), PB1 (pin 19), PB2 (pin 20) and PB7 (pin 25) of IC7 are connected to the bases of transistors T6 through T9 via resistors R19 through R22, respectively. A high signal on these pins energizes re¬lays RL1 through RL4. Switch S1 is used to reset IC4. As you may be aware, telephone ex¬changes provide DC voltage reversal facil¬ity to PCOs (and other subscribers for a fee) to indicate call maturity. The same is assumed to have been incorporated in our telephone. The circuit for detecting the polarity reversal in the telephone line is built around opto couplers IC8 and IC9. Nor¬mally, TIP is positive with respect to the RING lead of the telephone line. With the handset in off-hook position, a nominal loop current of 10 mA is assumed to flow through the telephone lines. Resistor R23 is selected as 120 ohms to develop a volt¬age of 1.2V (which is adequate for an LED to turn on fully). When DC line volt¬age polarity reversal occurs, optocoupler IC8's internal LED conducts and LED3 glows to indicate polarity reversal. Simul¬taneously, opto coupler IC8's internal LED goes off and its pin 5 (collector) goes high to provide line-reversal sense signal to 8085 via pin 14 of 8255 PPI.
RELAY CONNECTIONS:
The cradle switch in the telephone instrument is a double pole, two-way switch. Replace this cradle switch with the contacts of DPDT relay RL3 as shown in the circuit diagram. Now relay RL3 is used to implement the action of lifting the telephone handset. There are four pads on the PCB of the telephone instrument where cradle switch is connected. The two pads which are shorted when the telephone handset is placed on the cradle are connected to the normally closed (N/C) contacts of relay RL3, while the other two pads which are shorted when the handset is off-hook are connected to the normally open (N/O) contacts of relay RL3. Relay RL2 is connected in parallel to the redial button of the telephone instru¬ment. When relay RL3 energizes to emu¬late lifting of the handset, relay RL2 is energized to switch on the redial button and the already loaded telephone number of the police station or any other help provider is automatically dialled. Relay RL4 activates the siren when¬ever the IR signal being received is inter¬rupted. The siren sounds continuously un¬til the user presses the reset button. Relay RL1 is used to switch on the audio cassette player, in which the user's residential address and alert message to be conveyed to the police station are pre¬recorded. The speaker output of the cas-sette player is connected to the telephone's microphone to convey the alert message to the police station. The player gets switched off when the message is over.
WORKING OF THE CIRCUIT
The transmitting IR LED1 and phototransistor T2 of the receiver are fitted to the opposite pillars of the gate such that the IR rays emitted by the LED di¬rectly fall on the phototransistor. The IR LED transmits a train of IR pulses. These pulses are received by the receiver and amplified by IC2. Output pin 8 of the PLL (IC3) is low when the PLL network is locked to the transmitter fre¬quency and relay RL5 energizes to make PA0 line of IC7 low. When someone walks through the gate to enter your home, the transmitted sig¬nal is interrupted. Output pin 8 of the PLL network goes high and relay RL5 de-energizes to make PA0 line of IC7 high. Now the microprocessor starts working as per the program loaded in the EPROM.
SOFTWARE
Fig. 4 shows the flowchart of the Assem¬bly language program. The device inter¬face IC (IC7) is initialized with control word 99H. Ports A and C of IC7 act as input ports, while port B becomes the output port. After initialization, the 8085 micropro¬cessor reads the status of port A. If port A is high, siren is activated. The telephone goes in off-hook condition and the emer¬gency number is dialled through the redial button. Redial button gets switched off after the number is dialled. Now the microprocessor reads the status of port C and checks for the polarity reversal of the telephone line. When polarity reversal is detected, the audio player turns on to play the message. Otherwise, the process re¬peats from activation of the siren followed by emergency number dialling and so on. After delivering the message, the player automatically gets turned off. The siren sounds until the reset switch is pressed.