📄 device control using telephone electrosofts.com.htm
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provided blanking, lamp test and cascadable zero-suppression functions.</font></span></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in; text-align: center;" align="center">
<b><font size="2" face="Verdana">
<img src="Device%20Control%20Using%20Telephone%20%20ElectroSofts.com_files/blockdiagram7447.jpg" border="0" width="319" height="249"></font></b></p>
<p style="margin-bottom: 0pt; text-align: center;" align="center">
<span style="font-weight: 700;"><font size="2" face="Verdana">
Figure I. IC 7447 BCD - seven segment decoder</font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<font size="2" face="Verdana"> </font><font color="#ff0000" size="2"><span style="font-weight: 700; font-family: Verdana;">WORKING
OF LOCAL CONTROL SECTION:</span></font></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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. </font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<font size="2" face="Verdana"> </font><b><i><font color="#ff00ff" size="2" face="Verdana">1.
Telephone interface circuit:</font></i></b></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<font size="2" face="Verdana"> </font><b><i><font color="#ff00ff" size="2" face="Verdana">2.
Ring detector circuit:</font></i></b></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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. </font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">
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.</font></span></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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. </font></span></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana"> The
ring detector circuit is built around a monostable multivibrater
constructed around timer IC 555. </font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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 R<sub>A</sub> and capacitor C<sub>A</sub>.</font></span></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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).</font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">
Here this circuit is designed for a period of 60 seconds. This
period is calculated by the formula,</font></span></p>
<p style="margin-bottom: 0pt;" align="center"><font face="Verdana">
<span style="font-weight: 700;"><font size="2">t<sub>d</sub> = 1.1 R<sub>A</sub>
C</font><sub><font size="2">A</font></sub></span></font></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">After this
period the output of this IC goes low which intern switches OFF the
transistor T2. By varying the values of the R<sub>A</sub> and C<sub>A</sub>
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.</font></span></p>
<p style="margin-bottom: 0pt;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">
Now our circuit is ready to receive any coded signal of the devices
connected to the local control section from the remote control
section. </font></span></p>
<p style="margin-bottom: 0pt;" align="center">
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<p style="margin-bottom: 6pt;" align="center"><i>
<span style="font-weight: 700;"><font size="2" face="Verdana"> Fig J.
Circuit Diagram of Local Control Section.</font></span></i></p>
<p style="margin-bottom: 6pt;" align="justify">
<font size="2" face="Verdana"> </font><b><i><font color="#ff00ff" size="2" face="Verdana">3.
Signal Decoding Unit:</font></i></b></p>
<p style="margin-bottom: 6pt;" align="justify"><b>
<font size="2" face="Verdana"> 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 <font color="#ff0000">
<a href="http://www.alldatasheet.com/datasheet-pdf/pdf/82662/ETC/7404/datasheet.pdf">
(click here to download datasheet)</a></font>. The working of all the
above IC's are mentioned here before. </font></b></p>
<p style="margin-bottom: 6pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p style="margin-bottom: 6pt;" align="justify">
<font size="2" face="Verdana"> </font><b><i><font color="#ff00ff" size="2" face="Verdana">4.
Number display unit:</font></i></b></p>
<p style="margin-bottom: 6pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p style="margin-bottom: 0pt; text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p class="MsoNormal" style="text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p class="MsoNormal" style="text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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.</font></span></p>
<p class="MsoNormal" align="justify"><span style="font-weight: 700;">
<font size="2" face="Verdana"> The device selected from the
Remote Section for control purpose, its code is displayed in this seven
segment display.</font></span></p>
<p class="MsoNormal" align="justify"><span style="font-weight: 700;">
<font size="2" face="Verdana"> Next we discuss about the device
control unit .This is an important unit in this project.</font></span></p>
<p class="MsoNormal" style="text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">Device
control unit consist of device status check unit, device switching unit,
device status feedback unit, relay driver circuit and beep tone generator
unit.</font></span></p>
<p class="MsoNormal" style="text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">Now we see
these units in detail.</font></span></p>
<p class="MsoNormal" align="justify"><font size="2" face="Verdana"> </font><b><i><font color="#ff00ff" size="2" face="Verdana">5.
Device status check unit:</font></i></b></p>
<p class="MsoNormal" align="justify"><b><font size="2" face="Verdana">
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.</font></b></p>
<p class="MsoNormal" style="text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">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
<font color="#ff0000">
<a href="http://www.alldatasheet.com/datasheet-pdf/pdf/50892/FAIRCHILD/DM7408N/datasheet.pdf">
(click here to download datasheet)</a></font>. 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.</font></span></p>
<p class="MsoNormal" style="text-indent: 0.5in;" align="justify">
<span style="font-weight: 700;"><font size="2" face="Verdana">If device is
in OFF state then no beep will be heard.</font></span></p>
<p class="MsoNormal" align="justify"><font size="2" face="Verdana"> </font><b><i><font color="#ff00ff" size="2" face="Verdana">6.
Device switching unit:</font></i></b></p>
<p class="MsoNormal" align="justify"><b><font size="2" face="Verdana">
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 ⌨️ 快捷键说明
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