Working of Local Control Section

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. 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. 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.

Assembling the Local Control Section

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.

Control Table

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

Testing of the Local Control Section:

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.

Applications:

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.

PBEX:

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.