DTMF  dual tone is multiple frequency tone which is generated by  phone when the key is pressed. This tone can be used at various applications to control the hardware. DTMF based home appliances is the one of the application of them. When the two phones are connected for the call at that time when the keys is pressed then DTMF tone is also generated. So in this method we can control it from the anywhere in the world.

Block Diagram of DTMF Control AC Appliances

The DTMF based home appliances are divided into the following parts.

  1. DTMF Decoder
  2. Microcontroller
  3. Relay driver
  4. Relay and Load (bulb)

DTMF Decoder

DTMF Decoder is decoding the analog frequency signal which is generating by the each key when they  are pressed. It decodes   each key signal into the BCD numbers. The BCD number is processed for the further circuitry.

DTMF frequency is generated from the phone and can be heard through the speaker and earphone. So to decode this DTMF sound we have to provide these signals to the input of the DTMF decoder. To pass sound signal which is analog and block the other signal we use a series of  capacitors. These capacitors passes only DTMF signal and blocks the all other signals. The DTMF decoder IC decodes this signal and converts it into the BCD number at the output. Now the output of the DTMF decoder is fed to the microcontroller section.


The microcontroller receives the input from the DTMF decoder and sends it to  the other port. The different decision making program is fed to the microcontroller for security purpose. For example if we make the combination of  different codes  only then a   person knowing the code  can operate the system and  no other person will be able to  operate it after that  the outputs of the microcontroller  is send to the relay driver.

Relay Driver

The relay drivers block  gets the input from the microcontroller and operates the relay to control the AC appliances. The voltage and current from the microcontroller is not sufficient to drive the relay. So relay driver is required to control the AC appliances device.

Relay and Load

The relay is used to control the high voltage and protect AC appliance from the low voltage (digital logic).  Here AC appliances used a load. For demo purpose we can use and 230 Volts devices like AC bulbs.

Circuit Diagram of DTMF Control AC appliances

The circuit diagram consists the following section

  1. DTMF Decoder (8870)
  2. Microcontroller (AT889S52)
  3. Relay driver (ULN2003)
  4. Relay and Load (bulb)

DTMF Decoder (8870)

The DTMF Decoder IC 8870 is used to decode the DTMF signals which are generated by the phone. These DTMF signal is provided to this IC from the speaker of the mobile phones at pin number 2. A series capacitor is used to pass only AC signals that is DTMF signals and resistance (i.e. 100 KΩ for the limiting the voltage level up to desirable level. The high value resistance that is 1 MΩ is used for the isolation between pin number 2 and pin number 3. A 330 KΩ resistance and .1µf capacitor is used for the biasing circuit which is connected at pin number 17 and supply voltage. The whole circuit is operated at 3.579MHz frequency. So a crystal oscillator is used between pin number 7 and pin number 8. To check the BCD output four LED’S are  connected between  D0 TO D4 pin (i.e. pin number 11 to 14). When the DTMF signal is successfully decodes this decoder at that time STD pin number 15 goes high at that movement, to check this states a LED is also connected at that pin. Now the BCD output is provided to the microcontroller for the further processing.

Microcontroller (AT889S52)

The microcontroller AT89S52 is used for the decision making. The various combination of password is set through this microcontroller so that only authorized person can operate this system. According to the program the microcontroller operates the load. The input signal is provided at the lower port of the microcontroller i.e. pin number 1 to the pin number 4. The output is given to the lower port of the port 2 i.e pin number 21 to 24. The output of the microcontroller is fed to relay driver circuit.

Relay Driver (ULN2003)

The relay driver IC ULN2003 consists seven Darlington pair so it can drive seven loads. But for the demo purpose we are using four loads (bulb) so we required four relay. The output of the microcontroller is provides to the four Darlington pair which is pin number 1 to pin number 4. The output is taken from the pin number 16 to 13 simultaneously. The output of the relay driver is provides the relay.

Relay and Load

The relay is used to control the AC appliances from the low voltage digital logic signal as well as it also provides the isolation between input and output. The relay driver circuit provides the supply according to the relay rating. Here we are using 12 volts relay so the supply voltage of the relay driver circuit is provided with  12V between pin number 8 and pin number 9. When the high signal is applied to the relay driver circuit, it gives the low output that is logic 0. The 12V supply is directly applied to the relay coil and another pin is connected to the relay driver circuit. The AC Load (220V AC bulb) is connected between pole and normally open terminal of the relay. Hence when the relay gets on load is also ON.

Hence the DTMF control AC appliances project works.

Program of 8051 based DTMF Control AC Appliance