21-08-2014, 10:29 AM
The project (Home Automation System using DTMF) consists of two sections:
1. Remote Section
2. Local Section
Remote section consists of DTMF generator. This IC produces DTMF signals. It contains four row frequencies & three column frequencies. By pressing any key in the keyboard corresponding DTMF signal is made available by IC UM 91214 B as output at pin no.7. The output of IC UM 91214 B is given to the input of this transmitter circuit which effectively frequency modulates the carrier and transmits it in the air. The carrier frequency is determined by coil L1 and trimmer capacitor VC1 (which may be adjusted for around 100 MHz operations). An antenna of 10 to 15 cms (4 to 6 inches) length will be sufficient to provide adequate range.
The frequency modulated DTMF signals are received by the FM receiver and the output of FM receiver (DTMF tones) is fed to the Local section. This section consists of a DTMF to BCD decoder, IC MT 8870, 4 to 16 line decoder IC 74154,hex inverter gate IC 7404,D flip flops and relays. The output of FM receiver is fed to IC MT 8870 which is a DTMF-to-BCD converter. The 4 bit BCD number output obtained at pins from 11 to 14.
This output is fed to the 4-16 line decoder IC 74154. This IC takes the BCD number and decodes. According to that BCD number it selects the active low output line from 1 to 16 which is decimal equivalent of the BCD number present at its input pins. Since the low output of this IC the output is inverted to get logic high output. This inversion is carried out by hex inverter IC 7404- built on TTL logic.
The output of IC 7404 is given to D flip flop. Here this D flip flop is used in the toggle mode. For each positive going edge of the clock pulse will trigger the flip flop. To carry out the switching of any appliances or devices we commonly use the relays. We use the transistors to energize the relay coil. When the base voltage of the transistor is above 0.7V the emitter-base (EB) junction of the transistor forward biased as a result transistor goes to saturation region it is nothing but the switching ON the transistor. This in turn switches on the relay. By this the device is switches ON. When the output of D flip-flop goes low the base voltage drops below 0.7V as a result the device also switches OFF.