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Step 2Build the circuit.
Some ciruit assembly skills are needed in this step, so I'll assume that and show you the schematic. Use your 7805 IC and 1000 uF capacitor to provide 5 V steady supply to it. (Ignore the zener and 100 Ohm resistor, the 7805 takes it place and is a lot better). Since we'll be supplying only 5 Volts to a single IC, the heatsink can be obmitted. From your hand free, take the cord to the earpice, cut it of (earpice). and solder the mesh or shield to ground, then the inner cable to the 100nF audio input capacitor.
This way your cellphone will pass the tone presses audio signals to the DTMF and it will decode them to its binary output.
On every binary output you will connect a buffering transitor to drive a 12 V relay. Now the relays are fed with 12 Volts, the transistor only grounds them so they operate, but the relays drive no current from the 5V supply (thus, we keep it simple).
BC548 is common, but you can try any low or mid power npn, like BC238, 2N2222, etc.
Reverse emf diodes should be mounted in parallel to the relays' coils but in reverse mode (so they catch back emf). I was lucky my relays came with diodes, so my boards doesn't show them. (snap!)
This way your cellphone will pass the tone presses audio signals to the DTMF and it will decode them to its binary output.
On every binary output you will connect a buffering transitor to drive a 12 V relay. Now the relays are fed with 12 Volts, the transistor only grounds them so they operate, but the relays drive no current from the 5V supply (thus, we keep it simple).
BC548 is common, but you can try any low or mid power npn, like BC238, 2N2222, etc.
Reverse emf diodes should be mounted in parallel to the relays' coils but in reverse mode (so they catch back emf). I was lucky my relays came with diodes, so my boards doesn't show them. (snap!)
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