Remote control by mobile phone

Many of you will have an old mobile phone gathering dust. The circuit shown in this article gives the old mobile a new lease of life, enabling it to switch an electrical appliance on and off in or around the home. The circuit will even talk to you on in reply to predefined commands. The overall complexity of the circuit is greatly reduced by the
 Remote control by mobile phone - schematic

use of a PIC microcontroller. The addition of a speech synthesizer chip allows the control unit to return spoken information to the user on the calling phone. The schematic of the Remote Control by Mobile Phone is given in Figure 1. The connector for the headset is plugged into the mobile phone and the wires to the earphone pieces are connected to the input of a DTMF decoder IC type 8870 (IC1). The received signals are amplified by T1 and then applied to pin RB2 of the central microcontroller, a PIC16F84 (IC2). DTMF stands for dual-tone multi-frequency, a tone coding/decoding system that`s been in use on wired telephone sets for almost 30 years now. Each key on the DTMF keypad is linked to a combination of two (nonharmonically related) tones in the audible range. At the receiving side, the tone pairs are decoded into the matching number (0-9, #, *). For our application, the mobile phone is set to auto-answer mode. In operation, the PIC micro will continuously monitor the level at RB7 (pin 13). A ring signal will wake up the PIC from its sleep state and cause it to activate the speech chip (IC3) which supplies a spoken` message to the mobile phone via the microphone connector of the headset. At the calling end, the user hears the welcome message and is prompted to enter the system password (MSG1). The level of the audio signal may be adjusted on preset P2. The selection of spoken text stored in the speech chip is determined...

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