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Device Control using telephone

Nagabhooshan S. Shet


Controlling devices using switches are common. From a few decades controlling devices using remote control switches like infrared remote control switch, wireless remote control switches, light activated switches are becoming popular. But these technologies have their own limitations. Laser beams are harmful to mankind.

          Some technologies like IR remote control are used for short distance applications. In such case if we have system which does not require any radiations or which is not harmful, long remote control switch!!  Yes here is the solution. Here I am introducing such a system which does not require any radiations, any laser beam which has no limitation of range, I mean it can be used from any distance from meters to thousand kilometers using a simple telephone line or mobile phone.

          Here I am using a telephone as a media, which serves main part of this system, by using home phone as a local phone and another phone, either landline or mobile phone as a remote phone.


1.     You can control up to 10 devices. It may be any electric or electronic appliances or devices with simple to heavy appliances. Each device is given a unique code.

2.     It makes accurate switching, any false switching of device are not done.

3.     There is no risk for false switching.

4.     Your local phone (i.e., home phone or office phone) can be used for normal use by using a DPDT switch. So you need not use a separate telephone line for this device controlling.

5.     To perform any operations through remote phone line, the user needs to dial to the local telephone (to which the interfacing circuit is connected) then the respective code of the device is dialed.

6.     This circuit does not require any complex IC, so any one with little knowledge of electronics can construct this circuit, because it does not need any programmable IC's or programming.

7.     This system detects the ringing signal from your exchange with the help of ring detector and automatically switches ON.

8.     This device saves your money. This circuit switches OFF after a time of 60 seconds (you can change this switch ON-Time which is discussed in detail in coming section).

9.     Before changing the state of the device we can confirm the present status of the device.

10.This circuit gives an acknowledgement tone after switching ON the devices to confirm the status of the device.

11.  You can control devices from local telephone. It can also be controlled by PCO.

Taking a tour of the project:

           This system uses Dual Tone Multi Frequency (DTMF) technology of our telephone set. Every telephone set will have this facility. We have two type of dialing facilities in our telephone system (i) Pulse dialing mode (ii) Tone dialing mode. Here this system works on tone dialing mode. The DTMF mode is shortly called as tone dialing mode. (Check for availability of tone dialing mode in your telephone set).          

This system is divided into two sections 1: Remote Section 2: Local Control Section.

1: Remote Section:

  It is nothing but remote telephone set which is present in the remote place. This may be your workspace (office / school) phone or mobile phone or a phone in PCO. Signals are sent through this telephone.

2: Local Control Section:

          This is a control system through which you can control your appliances. This contains one telephone line and a control unit. The appliances to be controlled must be connected to telephone line through control unit .Control unit is kept with a sufficient backup.


 WHAT IS DTMF?             

          When you press a button in the telephone set keypad, a connection is made that generates a resultant signal of two tones at the same time. These two tones are taken from a row frequency and a column frequency. The resultant frequency signal is called "Dual Tone Multiple Frequency". These tones are identical and unique.

          A DTMF signal is the algebraic sum of two different audio frequencies, and can be expressed as follows:           

f(t) = A0sin(2*П*fa*t) + B0sin(2*П*fb*t) + ...........    ------->(1)

Where fa and fb are two different audio frequencies with A and B as their peak amplitudes and f as the resultant DTMF signal. fa belongs to the low frequency group and fb belongs to the high frequency group.

          Each of the low and high frequency groups comprise four frequencies from the various keys present on the telephone keypad; two different frequencies, one from the high frequency group and another from the low frequency group are used to produce a DTMF signal to represent the pressed key.

The amplitudes of the two sine waves should be such that                  

                     (0.7 < (A/B) < 0.9)V               -------->(2)

          The frequencies are chosen such that they are not the harmonics of each other. The frequencies associated with various keys on the keypad are shown in figure (A).

          When you send  these DTMF signals to the telephone exchange through cables, the servers in the telephone exchange identifies these signals and makes the connection to the person you are calling.  

The row and column frequencies are given below: 


Fig (A)

 When you press the digit 5 in the keypad it generates a resultant tone signal which is made up of frequencies 770Hz and 1336Hz. Pressing digit 8 will produce the tone taken from tones 852Hz and 1336Hz. In both the cases, the column frequency 1336 Hz is the same. These signals are digital signals which are symmetrical with the sinusoidal wave.

 A Typical frequency is shown in the figure below:

Figure (B)

 Along with these DTMF generator in our telephone set provides a set of special purpose groups of tones, which is normally not used in our keypad. These tones are identified as 'A', 'B', 'C', 'D'. These frequencies have the same column frequency but uses row frequencies given in the table in figure (A). These tones are used for communication signaling.

 The frequency table is as follows:

Figure (C)

    Due to its accuracy and uniqueness, these DTMF signals are used in controlling systems using telephones. By using some DTMF generating IC�s (UM91214, UM91214, etc) we can generate DTMF tones without depending on the telephone set.


This system is divided into two sections, 1: Remote Section 2: Local Control Section.


           This unit consists of telephone set which is present in the remote place. This may be your workspace (office / school) phone or mobile phone or a phone in PCO. Signals are sent through this telephone. The figure (E) shows the circuit diagram of the DTMF encoder which resembles the telephone set. It uses DTMF encoder integrated circuit, Chip UM 91214B (click here to download datasheet). This IC produces DTMF signals. It contains four row frequencies & three column frequencies. The pins of IC 91214 B from 12 to 14 produces high frequency column group and pins from 15 to 18 produces the low frequency row group. By pressing any key in the keyboard corresponding DTMF signal is available in its output pin at pin no.7. For producing the appropriate signals it is necessary that a crystal oscillator of 3.58MHz is connected across its pins 3 & 4 so that it makes a part of its internal oscillator.

Figure (E). Circuit diagram of the DTMF encoder

           This encoder IC requires a voltage of 3V. For that IC is wired around 4.5V battery. And 3V backup Vcc for this IC is supplied by using 3.2v zener diode.

The row and column frequency of this IC is as on the fig. "B". By pressing the number 5 in the key pad the output tone is produced which is the resultant of addition of two frequencies, at pin no. 13 & pin no.16 of the IC and respective tone which represents number '5' in key pad is produced at pin no.7 of the IC . This signal is sent to the local control system through telephone line via exchange.


This is a control unit through which you can control your appliances. This contains one telephone line and a Local Control Section. The appliances to be controlled must be connected to telephone line through control unit. Control unit is kept with a sufficient backup.

Local Control Section consists of a DTMF decoder, 4-16 line decoder/demultiplexer, D-flip-flops, and relay driver circuits. Before going into detail of the circuit, we will take a brief description about integrated circuits used in local control section.

MT 8870 DTMF decoder:

IC MT8870/KT3170 serves as DTMF decoder (click here to download datasheet). This IC takes DTMF signal coming via telephone line and converts that signal into respective BCD number. It uses same oscillator frequency used in the remote section so same crystal oscillator with frequency of 3.85M Hz is used in this IC.

Working of IC MT8870:

The MT-8870 is a full DTMF Receiver that integrates both band split filter and decoder functions into a single 18-pin DIP. Its filter section uses switched capacitor technology for both the high and low group filters and for dial tone rejection. Its decoder uses digital counting techniques to detect and decode all 16 DTMF tone pairs into a 4-bit code. External component count is minimized by provision of an on-chip differential input amplifier, clock generator, and latched tri-state interface bus. Minimal external components required include a low-cost 3.579545 MHz crystal, a timing resistor, and a timing capacitor. The MT-8870-02 can also inhibit the decoding of fourth column digits.

MT-8870 operating functions include a band split filter that  separates the high and low tones of the received pair, and a digital decoder that verifies both the frequency and duration of the received tones before passing the resulting 4-bit code to the output bus. 

The low and high group tones are separated by applying the dual-tone signal to the inputs of two 6th order switched capacitor band pass filters with bandwidths that correspond to the bands enclosing the low and high group tones.

Figure (F).Block diagram of IC MT8870

The filter also incorporates notches at 350 and 440 Hz, providing excellent dial tone rejection. Each filter output is followed by a single-order switched capacitor section that smoothes the signals prior to limiting. Signal limiting is performed by high gain comparators provided with hysteresis to prevent detection of unwanted low-level signals and noise. The MT-8870 decoder uses a digital counting technique to determine the frequencies of the limited tones and to verify that they correspond to standard DTMF frequencies. When the detector recognizes the simultaneous presence of two valid tones (known as signal condition), it raises the Early Steering flag (ESt). Any subsequent loss of signal condition will cause ESt to fall. Before a decoded tone pair is registered, the receiver checks for valid signal duration (referred to as character- recognition-condition). This check is performed by an external RC time constant driven by ESt. A short delay to allow the output latch to settle, the delayed steering output flag (StD) goes high, signaling that a received tone pair has been registered. The contents of the output latch are made available on the 4-bit output bus by raising the three state control input (OE) to logic high. Inhibit mode is enabled by a logic high input to pin 5 (INH). It inhibits the detection of 1633 Hz.

The output code will remain the same as the previous detected code. On the M- 8870 models, this pin is tied to ground (logic low).

The input arrangement of the MT-8870 provides a differential input operational amplifier as well as a bias source (VREF) to bias the inputs at mid-rail. Provision is made for connection of a feedback resistor to the op-amp output (GS) for gain adjustment.

The internal clock circuit is completed with the addition of a standard 3.579545 MHz crystal.

The input arrangement of the MT-8870 provides a differential input operational amplifier as well as a bias source (VREF) to bias the inputs at mid-rail. Provision is made for connection of a feedback resistor to the op-amp output (GS) for gain adjustment.

        The internal clock circuit is completed with the addition of a standard 3.579545 MHz crystal.



Figure (D) shows the overall block diagram of "Device control using the telephone" construction.

 IC NE 555 timer:

The NE555 is an integrated circuit that capable of producing accurate timing pulses. This IC is used as a multivibrater (click here to download datasheet). By using this IC we can construct two types of multivibrater, monostable and astable. The monostable multivibrater produces a single pulse when a triggering pulse is applied to its triggering input. The astable multivibrater produces a train of pulses depending on the Resister-Capacitor combination wired around it.

With a monostable operation, the time delay is controlled by one external resistor and one capacitor connected between Vcc-Discharge (R), and Threshold-Ground (C). With an astable operation, the frequency and pulse width are produced by two external resistors and one capacitor connected between Vcc-Discharge (R), Discharge-Threshold (R), and Threshold-Ground (C).


Figure J. IC NE 555

 74154  4-16 line decoder/demultiplexer:

          IC 74154 is a 4-16 line decoder, it takes the 4 line BCD input and selects respective output one among the 16 output lines (click here to download datasheet). It is active low output IC so when any output line is selected it is indicated by active low signal, rest of the output lines will remain active high. This 4-line-to-16-line decoder utilizes TTL circuitry to decode four binary-coded inputs into one of sixteen mutually exclusive outputs when both the strobe inputs, G1 and G2, are low. The demultiplexing function is performed by using the 4 input lines to address the output line, passing data from one of the strobe inputs with the other strobe input low. When either strobe input is high, all outputs are high. These demultiplexer are ideally suited for implementing high-performance memory decoders.

Figure G. IC 74154 4-16 line decoder

All inputs are buffered and input clamping diodes are provided to minimize transmission-line effects and thereby simplify system design.


 74126 Tri - State Buffer:

This IC is a tri state buffer contains four independent gates each of which performs a non-inverting buffer function. The outputs have the 3-STATE feature (click here to download datasheet). When control signal is at high state, the outputs are nothing but the data present at its input terminals. When control signal is at low state, the outputs are held at high impedance state. So no output will be available at the output terminal.

Figure H. IC 74126

 IC 7474 D-flip-flop:

                        IC 7474 is a conventional D-flip-flop IC. This IC consists of two D flip-flops. These flip-flops are used to latch the data that present at its input terminal (click here to download datasheet). Each flip-flop has one data, one clock, one clear, one preset input terminals. 

(Above figure shows a single D-flip-flop)

 IC 7447 BCD - seven segment decoder:

The DM74LS47 accepts four lines of BCD (8421) input data, generates their complements internally and decodes the data with seven AND/OR gates having open-collector outputs to drive indicator segments directly (click here to download datasheet). Each segment output is guaranteed to sink 24mA in the ON (LOW) state and withstand 15V in the OFF (HIGH) state with a maximum leakage current of 250 mA. Auxiliary inputs provided blanking, lamp test and cascadable zero-suppression functions.

        Figure I. IC 7447 BCD - seven segment decoder


Local control section contains a telephone interface circuit, ring detector circuit, signal decoder circuit, device status check circuit, device controlling circuit, device status feedback circuit.  

 1. Telephone interface circuit:

When a signal is sent from the remote telephone, the telephone interface circuit comes to receive the signal. This circuit is directly connected to the telephone line. This circuit consists of some passive components like resisters, capacitors.

 2. Ring detector circuit:

This circuit is useful to receive the telephone in the absence of the person. This circuit identifies the ringing signal sent from the telephone exchange. On getting the ringing signal this circuit connects the master unit to the telephone line.

            When some one calls another person through telephone by dialing second persons number, on getting this number of the second person the system in the telephone exchange sends a short duration ringing signal, this signal is sent at 25-30 Hz pulse of 70-90 rms.

This AC signal is bypassed by resister RE and capacitor CE and applied to the optocoupler MCT2E. This optocoupler is 6 pin IC. This is made up of internally built one Light Emitting Diode (LED) and a transistor. When the internal LED glows, the light falls on the emitter-collector junction the transistor. By this transistor is forward biased and the output is obtained at the emitter of the transistor.

On applying the signal to anode of the optocoupler, grounding the cathode, on the positive cycle of the signal LED glows as a result +5volt output is obtained at the emitter of the optocoupler at pin no 4.

          The ring detector circuit is built around a monostable multivibrater constructed around timer IC 555.

When a negative going pulse is applied to its triggering input at pin 2, the output of the IC goes high. This output is available at pin 3 of this IC. This will remain high for the time period designed by the RC combination. Depending on values of resister RA and capacitor CA.

High on the pin 3 of this timer IC biases the transistor T2 in the relay driver circuit which in tern switches ON the relay. This relay puts a resistance loop of 220Ω across the telephone line. By this resistance loop the line voltage in the telephone line drops from 50v to 12v. This is same as lifting the receiver of telephone handset (hook-off state).

          Here this circuit is designed for a period of 60 seconds. This period is calculated by the formula,

td = 1.1 RA CA

After this period the output of this IC goes low which intern switches OFF the transistor T2. By varying the values of the RA and CA the ON period of the monostable multivibrater is changed according to the formula given above. In the relay driver circuit resister is used to provide the necessary base current to the transistor so that it can bias properly.

                Now our circuit is ready to receive any coded signal of the devices connected to the local control section from the remote control section. 


 Fig J. Circuit Diagram of Local Control Section.

 3. Signal Decoding Unit:

          This is the main unit of this system. This unit consists of a DTMF to BCD decoder IC MT 8870, 4 to 16 line decoder IC 74154 and hex inverter gate IC 7404 (click here to download datasheet). The working of all the above IC's are mentioned here before.

The DTMF to BCD decoder IC MT8870 takes a valid tone signal from the telephone line. Then the tone signal is converted in to 4 bit BCD number output obtained at pins from 11 to 14. This output is fed to the 4-16 line decoder IC74154. 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. This IC inverts the data on its input terminal and gives inverted output.

 4. Number display unit:

This unit displays the received device code from the telephone line dialed from remote section. This unit consists of a BCD to seven segment decoder IC7447 and a seven segment display.

A seven segment display has seven LEDs connected in a sequence to give a regular shape and a LED to display the dot for decimal point.

It has 10 pins. Out of this two pins are common for all LEDs and remaining are another polarity terminals of the LED. When common anode seven segment display is used, two common terminal pins are connected to +5v or logic high state and another terminal are kept at logic low state. Then respective LED glows.

Here common anode seven segment display is used.  Because of this here we need a BCD to seven segment decoder which gives logic low output for the respective BCD input. Therefore I used a TTL IC 7447.

          The device selected from the Remote Section for control purpose, its code is displayed in this seven segment display.

         Next we discuss about the device control unit .This is an important unit in this project.

Device control unit consist of device status check unit, device switching unit, device status feedback unit, relay driver circuit and beep tone generator unit.

Now we see these units in detail.

 5. Device status check unit:

        Before switching On/Off any device, we always have confusion about its present status. If we are not sure about its status and if we false switch the appliance, then this will lead to some problems like damaging the device. Here we provide a facility that tells about present status of the device.

The inverted output of 4-16 line decoder and the output of respective flip-flop are fed to the independent block of AND gate of  IC 8- IC12 using IC 7408 (click here to download datasheet). If the device is already in the ON state, then we will here a beep sound. The output of each AND gate are connected to the beep tone generator unit by using a transistor. This beep generator unit produces a short duration beep indicating than the device is already in switched ON state.

If device is in OFF state then no beep will be heard.

 6. Device switching unit:

          This unit consists of a tri state buffer and a D flip flop. After making confirmation of current status of the device to alter the status of that device, you have to change the mode of the tri state buffer by making the control input high. This is done by pressing the �#� key. When this key is pressed the output of the 4-16 line decoder goes low. This gives a triggering pulse to monostable multivibrater which is build around the IC 6. This will keeps the output high for a 5seconds. Working of the monostable multivibrater already discussed. In this time interval the output of the tri state buffer will be the signal at its input terminal.

          So now the device code of the respective device is again pressed whose status is to be altered.

          The output of tri state buffer is latched by using a 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.

          After a period of 5 seconds the output of the IC 6 goes low and puts the tri state buffer in the high impedance state. Therefore to change the status of any other device is to be done after the output of IC 6 goes low, again �#� key is pressed to make the tri state buffer act as input �output state and the respective code of the device is pressed.

 6. Device status feedback unit:

          After changing the present status of the device confirm the operation you did, here comes the unit which gives the feedback tone after switching ON any device. This device status feedback unit uses a dual input AND gate, the output of the flip flop and the tri state buffer are to as the input. When the both inputs are high that indicates that device is switched ON, then the output of the AND gate goes logic high state. This output is fed to the beep generator unit through switching a transistor. Until you press the key the feedback tone is heard

          This feedback tone is heard only when the device is switched ON. While switching OFF the device this tone is not heard.

 7. Beep tone generator unit:

Beep tone generator unit produces a beep tone of audible frequency. This unit is constructed using a 555 timer chip. Here it is wired as an astable multivibrater with a few external components like resister and capacitor are required along with the timer 555 chip set.

          This frequency comes in the audible range between 40Hz to 650Hz. It should be less than 650Hz otherwise it will mix up with the DTMF tone. When it is less than 650Hz the frequency which causes the false triggering is filtered-off by the external structure of DTMF decoder IC MT8870.

 8. Power supply unit:

          For the proper working of this local control section except the local telephone set it needs a permanent back up which gives a 5V back up continuously. This is achieved by using a 5V regulated power supply from a voltage regulated IC 7805. This 5V source is connected to all ICs and relays. This IC gets a backup from a 9V battery.

 9. Relay driver circuit:

To carry out the switching of any appliances or devices we commonly use the relays. Since the output of the D flip flop is normally +5V or it is the voltage of logic high state. So we cannot use this output to run the device or appliances. Therefore here we use relays which can handle a high voltage of 230V or more, and a high current in the rate of 10Amps to energize the electromagnetic coil of the relays +5V is sufficient. Here we use the transistors to energize the relay coil. The output of the D flip-flop is applied to the base of the transistor T5 � T15 via a resister. 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 intern 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.


        The whole local control section except local telephone set is assembled in a single board, which is available in the market as common PCB. The whole circuit except the devices is assembled in a single cabinet in which the board gets fairly fitted along with power supply unit.


Table 2

 A parallel connection from the telephone line is taken and connected it to telephone interfacing circuit using a DPDT switch. When you wish this section is to be work then you switch ON the DPDT switch otherwise your telephone is used for normal calling receiving purpose.

          The connecters are provided to all the relay switches so that the devices are connected easily. After connecting, the devices, devices are given the number according to table 2.

Now power supply is connected and devices are also connected so that the whole section is ready for control device using a remote telephone set.


Prototype of Device Control Using Telephone Project.


 Make sure that the installation of the local control section is perfect; every thing is ready and local control section is ready to receive signal from remote section. Now ON the DPDT switch so that the telephone line is connected to telephone interfacing unit.

Now make a call to your telephone set using a remote telephone set or mobile phone. The signal goes to telephone exchange and the exchange sends a ringing signal to your set through phone line.

The ring detector unit detects  the ringing signal and makes the output of the IC5 to high state so that local control section of then connected to the telephone for a time interval of 60 seconds.

Now we follow the steps to test the proper switching of devices given below:

i). Press the respective code of the device whose status is to be checked. The dialed number of the device is displayed on seven segment display. If the device is already switched ON then you will hear a short duration beep tone from beep tone generator unit.

ii). Now press the �#� button on the keypad and again press the device number, the device number is displayed and now the device is switched OFF and you won�t hear the feedback tone. Indicating that the device is switched OFF.

iii). Repeat the above step (i) once again. In step (i) you won�t hear the beep tone because the device is switched OFF during above step (ii). Repeat the step (ii). Now you will hear the feedback tone because the device is switched ON.

          After 60 seconds the local control unit will disconnected from the telephone line so that your money is saved. 


The main theme of this project is to control the devices using telephone. By using the basic idea of this project we can also construct many useful systems. I like to share some of those here.


By using this project we can construct the personally branched telephone exchange. In many of the PBEX we seen in the offices require one operator to divert the incoming calls to the respective internal telephone line. Here it does not require any such operator to operate this exchange. The person from a remote section is only to press the extension number to get connected to the respective number. In this type of PBEX only 12 extensions can be used.

 Control the light of advertising board:

In advertising board or sign board used for display the company product purpose at the road sides requires the focus light on the dusk time. To control the light of this sign board a person is needed to switch on the lights in the evening and switch off it in the morning time. By using this system we can control the light from a control room or by the cell phone of the marketing person.

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