Telephone Record Control Circuit Diagram

This circuit will allow you to connect any tape recorder that has a mic and remote input to a phone line and automatically record both sides of a conversation when ever the phone is in use. You will need to take a couple of voltage readings before connecting the circuit. First determine the polarity of your phone line and connect it to the circuit as shown and then determine the polarity of the remote input and connect it to the circuit. Circuit operation is as follows. When the phone is on hook the voltage across the phone line is about 48volts dc. When the phone is off hook the voltage will drop to below 10volts dc. When the line voltage is at 48volts the FET is off which causes Q2 and Q3 to be off. When the phone is picked up the FET turns on along with Q2 and Q3 which turns your recorder on. The tape recorder must be in the record mode at all times. As you can see the power source for the circuit is the phone line.

Circuit Diagram

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Mobile phone Circuits to Get Even smaller

Transceivers, appliances such as mobile phones that can send and receive messages, have become smaller and smaller over the last few years, but users are about to experience a new meaning in miniaturisation. 

Research at The Hong Kong University of Science & Technology (HKUST) has successfully combined a unique system architecture and new circuit design techniques to reduce them in size like never before. 

Principal Investigator Dr Howard Luong said the handset of a typical mobile phone today may contain between 150 and 300 separate electrical components.

His research group proposed and demonstrated circuit techniques that make it possible to combine many of these components to a single chip and therefore to significantly reduce the size of circuitry (see example in graphic). A US patent has been granted for one of the circuit techniques. 
 
The transformation applies to the CMOS (Complimentary Metal-Oxide Semiconductor) manufacturing process, which can produce integrated circuits and systems with the highest integration level at the lowest cost. Applying new techniques to the CMOS process, Dr Luong’s research enables many “off-chip” components to be combined to realize a system-on-chip. “But,” he said, “this integration created great challenges in circuit implementation.” Part of the research was to solve the problems by new circuit design techniques.
 
“The system architecture and circuitry go hand in hand, he added. “They must both work, or neither will be useful.”
The resulting design gives the highest component integration in the smallest chip area ever reported, said Dr Luong.
In his design, all off-chip components are fitted into a central chip measuring 36 mm with packaging, and 8mm without being packaged.
Dr Luong’s miniaturisation method means appliances will soon be made for even lower cost and lower power consumption in addition to being much smaller in size and lighter in weight.
“With the lowering of cost, size and power, many new and interesting applications will become possible and practical,” he said.
Low-power wireless transceivers, for example, could be integrated into implanted devices such as heart pacemakers to wirelessly transmit and receive information between patients and doctors or monitoring systems.
Wearable mobile phones as small as wrist watches at an affordable price could also become a reality.

Auther

Principal Investigator

Dr Howard Luong

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Telephone Tapping Indicator Circuit Diagram

This simple circuit can indicate a misuse or tapping of Telephone line through a loud alarm. The circuit is too simple and can be easily assembled on a common PCB. Line voltage of Telephone lines is around 48 volts DC in the On hook state. When the handset is lifted, this voltage reduces to 12 volt DC. This change in voltage level is used to activate the circuit.When the switch S1 is closed, circuit becomes active and the telephone enters into the armed state.

The high volt DC from the telephone line passes through R1 and VR1 and bias T1 into conduction. As a result, the collector of T1 goes to ground potential to inhibit T2 from conduction. Buzzer and LED thus remain off. When the handset is lifted, the DC voltage from the telephone lines drops to 12 volts. This turns off T1 and T2 conducts. Buzzer beeps and LED lights indicating that the telephone is using.

Telephone Tapping Indicator Circuit Diagram

Setting
Connect the circuit to Telephone lines using a telephone plug. The free socket of the telephone or Caller ID can be used. Close S1 and adjust VR1 till buzzer stops beeping. Lift the handset. Buzzer should sound. Otherwise, just adjust VR1 till buzzer beeps.

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Telephone for VoIP gate Circuit Diagram

A project of Telephone for VoIP gate Circuit Diagram. Calls over the Internet can be in several different ways. For households with a permanent connection to the Internet, it is advantageous to use different IP phones and VoIP gateways that need to function on the computer. Because IP phones are still relatively expensive, they are used more often different VoIP gateway. VoIP gateway is "one end" connected to a network with access to the internet, the "other end" connects a standard analog phone, which is then used in the normal way and that has given a telephone number available from any fixed or mobile phone. Some VoIP gateway router in addition to that we need as well, if we want to access via a modem to connect multiple computers. To my VoIP gateway, we can connect nehomologovaný, self made ​​phone. Instructions for simple phone is the subject of this article.

How to the phone works

Normally the phone line is a DC voltage, usually 50 to 70 V. During the ringtone to this voltage superposed yet AC 50-100 V with a frequency of usually 20 or 25 Hz. "Pick up the 'phone line will pass through the current 20-40 mA. On the phone while the voltage drop occurs several volts (Fig. 1). This voltage can be used to power the circuitry phone. The polarity of the DC voltage is not defined, the phone must function in both polarities. The phone line is when answering the call of a defined impedance, which should be adapted telephone. Line impedance in different countries vary so as the phone purchased in Germany (impedance 220 ​​ohms + 820 ohms in parallel with 220 nF) not in Bohemia (600 ohms) to work optimally. Sometimes differ impedance different networks within the same country, depending on the age and type of telephone exchanges, length and characteristics of the cables.

Fig. 1 The voltage on the raised phone would, depending on the passing stream should be in the area indicated by the letters A to G

The most important part of the phone is a colloquial circuit. The call is transmitted over the line modulation current that flows through the line. The current is modulated on the side panel (VoIP Gateway), which transfers the call to the user, and in the device, which is transmitted from the user. The speech circuit is a hybrid circuit that suppresses the signal from the microphone in the earpiece. The hybrid circuit is on the opposite side (PBX, VoIP gateway). The hybrid circuit is usually some sort of bridge, and therefore it is necessary to impedance phone and VoIP gateway (or exchange) have been adapted. It is important especially for VoIP devices, where the influence of digitization and transmission of data over the Internet arises signal delay in the order of tens of milliseconds. Mismatch, especially on the VoIP gateway, there is a very annoying echo.

Fig. 2 Colloquial hybrid transformer circuit. If L1 and L2 have the same number of threads and balancing impedance ZM is equal to the line impedance ZL, the microphone signal in the handset suppressed


Another important part of the circuit for dialing. All modern devices support tone dialing. Ringing circuit to alert the user to an incoming call. Acoustic (bell) or optical signals indicates the presence of ringing voltage on the line. Hook switch is a switch that turns on the circuit colloquial answer the phone.

Specifications

Telephone impedance:	aprox. 600 Ohm.
Recomended line impedance:	real. 600 Ohm.
Operating (off-hook) current:	20 mA (10 to 40 mA.)
Operating voltage:	aprox. 7 V @ 20 mA.
Dialing:	DTMF
"In-use" indication:	3 to 15 V/ aprox. 1 mA
On-hook current:	aprox. 0,01 mA @ 50 V

Phone Circuit Schematic

Fig 3rd Telephone unit schematic

Connection phone in Figure 3 Colloquial circuit with transistors T1 to T3 is connected via a hook switch and bridge DB1 to the telephone line . Diode bridge ensures that electronic circuits are supplied with the correct polarity. If the hook switch is closed, current passes through the transistor T1 lines , which is used as microphone amplifier and transistors T2 and T3 , which amplifies the signal to the receiver. Headphone amplifier operates in class A. The amplitude of the output voltage at the collector of the transistor is about 1 V, which for the purpose of completely sufficient . Headphones or speakers can be used with an impedance of 8 ohms , the amplifier but works better (has lower distortion) with higher impedance load . Trimmer P2 can adjust the gain as needed.

The amplifier is continuously drop of about 1.3 V, the sum of the voltage UBE transistors T2 and T3 and is only slightly dependent on the flowing current. This voltage across the resistor R1 is powered electret microphone . The signal from the microphone passes through C1 to the base of transistor T1 . It amplifies the signal level required to excite the line. Originally, the microphone amplifier much more difficult . But he had unnecessarily high gain and output impedance inappropriate . This simple connection works better . Working point T1 is set resistors R2 to R4 . The microphone amplifier there is a voltage drop of about 3.5 V , the voltage drop can be under current amplification T1 changed 3 to 4 V. Resistor R5 introduced at this stage, a negative feedback regulating the amplification and decreasing distortion. Balancing circuit C6, R6 and P1 suppresses the signal from the microphone in headphone amplifier .

The signal from the microphone is on the collector of T1 opposite phase than the emitter , but the amplitude of the signal at the collector is much greater. Potentiometer P1 can set a condition where the signals from the collector and emitter of the capacitor C7 T1 readings and audio from the microphone in the handset is suppressed. Simplified diagram balancing the bridge is in Figure 4 The setting is very dependent on the impedance of the line. Because VoIP gateway creates a link with the active resistance of 600 ohm and short supply does not create a significant reactive component , the setting is simple and very effective. The signal coming from the gateway or PBX is the collector and emitter of T1 phase balancing circuit and is therefore not suppressed.

Fig. 4th Simplified wiring hybrid (balancing) circuit in the phone

In order to suppress its signal worked well on the VoIP gateway, you need the phone to be defined impedance close to the impedance of the gate, ie 600 ohm. The output impedance of the collector T1 for AC signals, large, almost current source. Impedance of the phone for voice signals are essentially a parallel combination of R6, R1 and about one order larger impedance collector T1. Rectifier bridge through which the signal passes, it will affect the impedance to a minimum.

 Fig 5 Dialer circuit arrangement

For DTMF I did not find any suitable integrated circuit , and generate DTMF signal discrete oscillators would be difficult and impractical . Finally, I modified the DTMF phone dialer . Dialer is a small box with buttons and speaker , generating tones when you press the button corresponding to the relevant figures. It is used for remote telephone answering and for the selective choice of the civil radio stations . The dialer (Fig. 5 ), the signal goes from the integrated circuit via the 22k resistor on the transistor, which drives a small speaker . Transistor speaker and I vymontoval a DTMF signal is connected to the transistor T4 on the phone. After pressing the button, the IO output voltage appears about 1 V with superimposed DTMF signal . Connect the telephone line is very simple, just enough transistor T4 and resistor R7. The change in resistance R7 can adjust the intensity of tone . Speaker of the phone dialer I used it as a handset . Dialer , according to the type of memory to 11 or 13 numbers , but unfortunately it lacks useful the " Redial" to repeat the last number called. I could not deal with power dialer from the phone line , so your phone is a small battery , which is likely to last for many years. The battery has an advantage in that it forgets the stored phone numbers if you disconnect it from the line.

Also, the ringing circuit I did not get special IO . That's why I like bell oscillator is used samovybuzujícím a piezoelectric transducer . The oscillator is fed stream that passes through the ringer capacitor C8 , bridges DB2 , LED2 to LED4 and resistor R14 . The oscillator is operated only in one half , the sound is more significant . In the other half cycle current through diode D1 . The bell must have a dead zone . That there is provided connecting the LED. Ringing voltage amplitude must have at least 10 V to Buzzer began publishing a sound. The phone I used a blue LED with high luminance. These intensely lit even at current several milliamps , which is ringing flow. LED also serves as a visual alarm bell , which could be useful in noisy environments .

The last telephone circuits are busy lamp . Phone I was seeking a second phone in parallel connected to the device at the other end of the apartment . When the phone is " suspended " , the hook switch is switched to the circuit with transistors T5 and T7 . If all telephones on the line hung on the line voltage of 50 to 60 V. The transistor T5 is opened, closed, and T7 LED1 off. The circuit is taken from the line is very little current through resistors R8 and R9 . Shrinks if the line voltage is below 15 V, T5 is closed , opens T7 and LED1 lights up. Transistor T6 T7 gate bias control by the loss of the R11 so that LED1 is energized about 1 mA , which is quite sufficient for display . In the voltage range of 3-15 V line lights LED1 virtually the same. BSS123 transistor has a maximum voltage UDS only 100 V. If the ringtone is this voltage is exceeded. In my phone's built higher voltage transistor has lasted several months . If you're worried about him , you can type BS108 with UDSmax 200 V, but it is not in SMD . The "in use" circuit not be planted, if you indicate you do not need , or can be used alone for another phone .

Mechanical Design

Width of the printed circuit board was designed dimensions dialer , then the length of a "reasonable " distance between the microphone and earpiece. Although the board is a lot of space , most of SMD components . They take up less space and is easier mounting plate is not so much tinkering . Figure motive circuit and layout of components , following table .

Before you assemble components, prepare the first dialer and attach it to desce.Z dialer remove the battery and remove the hinged cover. Dialer carefully "crack". It should go fairly easily if you start in the corner, to which is attached a chain. Odpájíme the speaker wires from the board dialer and speaker carefully shucks from the bottom of the box. The board dialer vypájíme more transistor in TO92. The board dialer solder short wires to negative (Pad1) and positive (PAD3) terminal electrolytic capacitor and terminal for transistor-based vypájeného (PAD2). Editing is perhaps evident from Figure 9 photos The lower part of the housing dialer yet only tentatively secured to the plate phone plastic rivets (Fig. 10). We remove the box and the board will be filled by phone components.

 Fig. 6th Editing dialer and connection terminals

Phone was originally in a transparent plastic box, which was inside the LED backlight. Its production, however, I not successful, so I made ​​a wooden box. To her I used plywood manufacture of soft wood, obtained from the fruit of hazel. That will free devoted almost everyone greengrocer. From one high hazels get material for a few boxes. Plywood is easily machined and well bonded dispersion adhesive. I eventually recut the box, namořil and přelakoval polyurethane varnish.

 Fig. 7th Mounting boxes dialer

With the change in boxes I had to adjust some components on the board. LEDs were originally all SMD. Finally, there were only SMD LED 2. But it is not visible from the outside, and therefore does not need much light. You can replace the zener diode (3-12 V) or short-circuit, but it increases the sensitivity of the ringing circuit. Other LED in case of 3 mm diameter. It turns out that it is better to use LED with high luminous than type 2 mA of current, because the small stream shines more. Also switch SW1 on the phone "lifting", I moved to the side boards. Deferred phone is on the table laid down the keyboard. LED on the back because they are easy to see and switch is when you pick up the phone available with one touch. Of the joints it can not be soldered to the board just because they do not get tipped soldering iron to solder pad. On the solder contacts because I rozklepl small rivets, which I soldered joints by.

I then links to the rivets inserted the switch and soldered it from the opposite side of the board. Between the switch and the plate I put a piece of insulating paper. The switch can also be placed at any other suitable location in the box and board interconnect wires. Switches P-B143 of the newer series ("silver") are not too good and We turn the lever sometimes lose contact. To fix it so that "packs" pertinax holding plate with pins a little more same hopper. The defect had all the pieces that I had at home. Dimensionally the same switch, but painted black were fine.

 Fig. 8th The top side of the printed circuit board

The power dialer instead of the original articles I used lithium batteries. Do you solder boards phone as housing purchased as the GM and housing that vypájíte of defective motherboards PC that has otherwise placed pins. Piezoelectric transducer is bolted small screws through the PCB to the bottom of the box dialer.
On the microphone I slid the piece of tubing so that the tubing fit snugly inside the box. Thus reducing the acoustic coupling between the earpiece and microphone inside the device.

 Fig. 9th The underside of the plate and box

Fig. 10th The back of the phone with LED switch and a hole for the bell

Animation

Phone use in conjunction with the gateway Linksys Sipura SP2100. In the firewall settings to make sure that in menu / admin / voice / advanced tab you have set up Regional impedance lines FXS Port Impedance: 600 Phone can revive a product as shown in Figure 14 (SW1 closed). You can test with a different phone when line current, measure the voltage on the line and estimate the impedance of the phone - AC voltage test point should be after the switch opens twice. The trimmer P2 dial to maximum resistance (max volume). To attach the microphone sound source (eg mp3 player headphones) and adjust the trimmer P1 so that the sound from the microphone not hear the telephone handset. Try tone. Ringing circuit you can try connecting the phone via a suspended resistor 1 kOhm to AC voltage 24-50 V.

 Fig. 11th test preparation

Conclusion

The phone is in operation for several months and yet it was conceived as a second auxiliary telephone is often used. I have tested it successfully on the public switched telephone network (PSTN), but because I'm not sure whether it meets all required standards, there should not be connected. PSTN to VoIP gateway over a larger voltage when hanging up a more current when the phone is picked up.

List of components
 

R1	4,7 kOhm, SMD 1206
R2	1 kOhm, SMD 1206
R3, R4	22 kOhm, SMD 1206
R5	33 Ohm, SMD 1206
R6	2,2 kOhm, SMD 1206
R7	220 Ohm, SMD 1206
R8, R9	10 MOhm, SMD 1206
R10	330 kOhm, SMD 1206
R11	470 Ohm, SMD 1206
R12	220 kOhm, SMD 1206
R13, R14	10 kOhm, SMD 1206
P1	250 Ohm, trimr PT6V (Piher)
P2	5 kOhm, trimr PT6V (Piher)
C1	220 nF, SMD 1206
C2	10 mikroF/16 V, tantalum., SMD B
C3	1000 mikroF/6,3 V, electrolytic
C4, C5	100 mikroF/6,3 V, tantalum, SMD D
C6	47 mikroF/16 V, tantalum., SMD D
C7	4,7 mikroF/10 V, tantalum, SMD A
C8	220 nF/250 V, foil MKT RM5
	PCB board bcs59

D1	1N4148SMD, SOD80
DB1, DB2	S250,mini DIP bridge
ZD1	BZV55C15SMD, Zener. diode 15 V SOD80
T1, T3	BC817-40, SOT-23
T2	BC807-40, SOT-23
T4 až T6, T8	BC848C, SOT-23
T7	BSS123, SOT-23 (BS108, TO92)
LED1	high intensity 3 mm red LED (it's better than 2 mA LED)
LED2	any 3 mm SMD LED or Zener diode 3 to 12 V
LED3, LED4	high intensity 3 mm blue LED (it's better than 2 mA LED)
MIC1	elektret. mikrofon MCE100
SP1	tiny speaker or headphone 100 Ohm (8 to 100 Ohm), removed from dialer
B1	CR2032 battery and holder
SW1	P-B143, SPST switch
K1	slim RJ11 plug WEBP 6-4 LP
SP2	piezo KPT2038FW
dialer	dialer DTMF 13 or 11

Sourced By :  Jaroslay and Circuitsstream

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Music-On-Hold for Telephones

Here is a simple circuit for music-on-hold with automatic shut off facility. During telephone conversation if you are reminded of some urgent work, momentarily push switch S1 until red LED1 glows, keep the telephone handset on the cradle, and attend to the work on hand. A soft music is generated and passed into the telephone lines while the other-end subscriber holds. When you return, you can simply pick up the handset again and continue with the conversation. The glowing of LED1, while the music is generated, indicates that the telephone is in hold position. As soon as the handset is picked up, LED1 is turned off and the music stops.

Circuit diagram :

Music-On-Hold for Telephones Circuit Diagram

Normally, the voltage across telephone lines is about 50 volts. When we pick up the receiver (handset), it drops to about 9 volts. The minimum voltage required to activate this circuit is about 15 volts. If the voltage is less than 15 volts, the circuit automatically switches off. However, initially both transistors T1 and T2 are cut off. The transistor pair of T1 and T2 performs switching and latching action when switch S1 is momentarily pressed, provided the line voltage is more than 15 volts, i.e. when the handset is placed on the cradle. Once the transistor pair of TI and T2 starts conducting, melody generator IC1 gets the supply and is activated. The mu-sic is coupled to the telephone lines via capacitor C2, resistor R1, and the bridge rectifier.

With the handset off-hook after a ring, momentary depression of switch S1 causes forward biasing of transistor T2. Mean-while, if the handset is placed on the cradle, the current passing through R1 (connected across the emitter and base terminals of pnp transistor T1) develops enough voltage to forward bias transistor T1 and it starts conducting. As a consequence, output voltage at the collector of transistor T1 sustains for-ward biasing of transistor T2, even if switch S1 is released. This latching action keeps both transistors T1 and T2 in conduction as long as the output of the bridge rectifier is greater than 15 volts. If the handset is now lifted off-hook, the rectifier output drops to about 9 volts and hence latching action ceases and the circuit automatically switches off.

EFY lab note. The value of resistor R2 determines the current through resistor R1 to develop adequate voltage (greater than 0.65 volts) for conduction of transistor T1. Hence it may be test selected between 33 kilo-ohms and 100 kilo-ohms to obtain instant latching.) The total cost of this circuit is around Rs 50.

Author : SIBIN K. ZACHARIAH - Copyright : Electronicsforu

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Video Switch for Intercom System Circuit Diagram

This is simple project of Video Switch for Intercom System circuit Diagram. Nowadays a lot of intercom units are  equipped with video cameras so that you can  see as well as hear who is at the door. Unfortunately, the camera lens is perfectly placed  to serve as a sort of support point for people  during the conversation, with the result that  there’s hardly anything left see in the video  imagery.  One way to solve this problem is to install two cameras on the street side instead only  one, preferably some distance apart. If you  display the imagery from the two cameras  alternately, then at least half of the time you  will be able to see what is happening in front  of the door. Thanks to the video switch module described  here, which should be installed on the street  side not too far away from the two cameras,  you need only one monitor inside the house and you don’t need to install any additional video cables.

 

Circuit diagram :

Video Switch for Intercom System Circuit Diagram

Along with a video switch, the circuit includes  a video amplifier that has been used with  good results in many other Elektor projects,  which allows the brightness and the contrast  to be adjusted separately. This amplifier is  included because the distance between the  street and the house may be rather large, so it is helpful to be able to compensate for cable attenuation in this manner.  The switch stage is built around the well  known 4060 IC, in which switches IC2a and  IC2d alternately pass one of the two signals to  the output. They are driven by switches IC2b and IC2c, which generate control signals that  are 180 degrees out of phase. The switching rate for the video signals is  determined by a clock signal from an ‘old  standby’ 555 IC, which causes the signals to  swap every 2 seconds with the specified com ponent values.

 

Naturally, this circuit can also used in many other situations, such as where two cameras are needed for surveillance but only one video cable is available.

 

Author :Jacob Gestman Geradts - Copyright : Elektor

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2 .Line IntercomTelephone Line Changeover

2 .Line Intercom Telephone Line Changeover.The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line. If another telephone is required at some distance, a parallel line is taken for connecting the other telephone. In this simple parallel line operation, the main problem is loss of privacy besides interference from the other phone. This problem is obviated in the circuit presented here. Under normal condition, two telephones (telephone 1 and 2) can be used as intercom while telephone 3 is connected to the lines from exchange. In changeover mode, exchange line is disconnected from telephone 3 and gets connected to telephone 2.
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Circuit diagram:

2-Line Intercom-Cum-Telephone Line Changeover Circuit Diagram
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For operation in intercom mode, one has to just lift the handset of phone 1 and then press switch S1. As a result, buzzer PZ2 sounds. Simultaneously, the side tone is heard in the speaker of handset of phone 1. The person at phone 2 could then lift the handset and start conversation. Similar procedure is to be followed for initiation of the conversation from phone 2 using switch S2. In this mode of operation, a 3-pole, 2-way slide-switch S3 is to be used as shown in the figure. In the changeover mode of operation, switch S3 is used to changeover the telephone line for use by telephone 2. The switch is normally in the intercom mode and telephone 3 is connected to the exchange line.

Before changing over the exchange line to telephone 2, the person at telephone 1 may inform the person at telephone 2 (in the intercom mode) that he is going to changeover the line for use by him (the person at telephone 2). As soon as changeover switch S3 is flipped to the other position, 12V supply is cut off and telephones 1 and 3 do not get any voltage or ring via the ring-tone-sensing unit. Once switch S3 is flipped over for use of exchange line by the person at telephone 2, and the same (switch S3) is not flipped back to normal position after a telephone call is over, the next telephone call via exchange lines will go to telephone 2 only and the ringtone-sensing circuit will still work. This enables the person at phone 3 to know that a call has gone through. If the handset of telephone 3 is lifted, it is found to be dead. To make telephone 3 again active, switch S3 should be changed over to its normal position.

Sourced By: Streampowers.blogspot.com

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Telephone Ringing Circuit Diagram

Telephone Ringing circuit Diagram. Even though cordless phones have invaded our homes and offices, you don’t always have them at hand, and as their ringtones are usually very much quieter than the old rotary-dial- type analogue phones, it can happen that you miss a call you’ve been waiting for while you’ve been going about your daily business. 

Until quite recently, you could still find remote ringers that could be plugged into any standard phone socket in order to have an additional ringer, but it seems as if these accessories are currently being phased out as everyone is ‘going cordless’. So we decided to suggest something better, with this phone ring repeater that makes it possible to control any device connected to the AC power outlet using the ringtone available on any subscriber line, and naturally, with all the guarantees of safety and isolation that are of course rightly expected. So it’s capable of driving a ringer, or indeed even a high-powered sounder to alert you when you are in the  garden, for example; but it is equally able to light a lamp for a ‘silent ring’ so as to avoid waking a sleeping baby or elderly person.
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Circuit diagram :

 Telephone Ring Repeater Circuit Diagram
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This circuit has been designed to be compatible with all phone systems the author is  aware of and also to be totally stand-alone. What’s more, the circuit can be connected to the phone system without any danger though in some countries, it is forbidden to connect non-approved devices to the public  switched telephone network (PSTN). Check local regulations in this respect.

In order to understand the principle of it, we just need to remember that the ringtone present on a phone installation is an alternating voltage, whose amplitude and frequency vary somewhat between countries, but always with comparable orders of magnitude except in the case of exchange systems  used in large companies. However, when the  line is quiescent or a call is in progress, it carries only a direct voltage. Capacitor C1 makes  it possible to pick off just the AC ringing volt-age, which is then rectified by D2 and amplitude-limited by D1. The resulting DC voltage charges capacitor C2, which makes it possible to light LED D3 as well as the LED in the  optocoupler IC1. This is no ordinary optocoupler, but is in fact an AC power zero-crossing  detecting optotriac, which allows us to con-trol the chosen load while generating no, or  less, interference, which would not be the  case using a standard optotriac. 

The output triac it contains is not powerful  enough to drive a load directly connected  to the mains, so it is used to drive the trigger of triac TRI1, which is a totally standard  400 V device, rated at x amps, where x is chosen to suit the maximum power of the load  you want to control using this circuit. Resistors and capacitors R5 and C3 on the one  hand, and R6 and C4 on the other help, serve  to suppress the switching transients, which  are already inherently low because of the AC  zero-crossing switching provided by IC1.

Construction is not at all difficult, but does  require a few precautions in choosing some of the components. First of all, capacitor C1  must be an MKT type, mylar or equivalent,  with a 250 V operating voltage because of  the relatively high amplitude of the ringing  voltage. For safety reasons, it is essential that  capacitors C3 and C4 are self-healing types  intended for AC power use at 250 VAC. These  capacitors are generally known as Class X or  X2 capacitors.

As for the triac, it should have a 400 V operating voltage (but see below for users on 120  VAC power) and maximum current slightly  greater than the maximum current drawn by the load being driven. As this will usually be a sounder or a common lamp, a 2 A type will  usually be more than adequate in most situations. As the circuit can be expected to operate for short periods only, there is no need to  mount the triac on a heatsink. One final important point: as the right hand  part of the circuit is connected directly to AC  power, it is vital to fit this inside a fully-insulated housing, for obvious safety reasons.  Make sure you cannot touch any part when  the circuit is in use. 

The circuit should work at once and without  any problems, but if you notice that D3 doesn’t  light up fully, and hence incorrect or erratic  triggering of the triac, because of too low a  ringing voltage, all you need to put things to rights is reduce the value of resistor R1.  The circuit as shown was dimensioned for  operation from 230 VAC power. Readers on  120 VAC power should modify the following  component values: R4 = 180 Ω; R5 = 220 Ω;  TRI = 200 V model; IC1 = MOC3031. Option-ally, C3 and C4 may be rated at 120 VAC.

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Telephone line Based Audio Muting and Light-On Circuit

Telephone line Based Audio Muting and Light-On Circuit. Very often when enjoying music or watching TV at high audio level, we may not be able to hear a telephone ring and thus miss an important incoming phone call. To overcome this situation, the circuit presented here can be used. The circuit would automatically light a bulb on arrival of a telephone ring and simultaneously mute the music system/TV audio for the duration the telephone handset is off-hook. Lighting of the bulb would not only indicate an incoming call but also help in locating the telephone during darkness.
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Telephone line Based Audio Muting and Light-On Circuit diagram :

 

Telephone line Based Audio Muting and Light-On Circuit Diagram 
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On arrival of a ring, or when the handset is off-hook, the inbuilt transistor of IC1 (opto-coupler) conducts and capacitor C1 gets charged and, in turn, transistor T1 gets forward biased. As a result, transistor T1 conducts, causing energisation of relays RL1, RL2, and RL3. Diode D1 connected in antiparallel to inbuilt diode of IC1, in shunt with resistor R1, provides an easy path for AC current and helps in limiting the voltage across inbuilt diode to a safe value during the ringing. (The RMS value of ring voltage lies between 70 and 90 volts RMS.) Capacitor C1 maintains necessary voltage for continuously forward biasing  transistor T1 so that the relays are not energised during the negative half cycles and off-period of ring signal. Once the handset is picked up, the relays will still remain energised because of low impedance DC path available (via cradle switch and handset) for the in-built diode of IC1. 

After completion of call when handset is placed back on its cradle, the low-impedance path through handset is no more available and thus relays RL1 through RL3 are deactivated. As shown in the figure, the energised relay RL1 switches on the light, while energisation of relay RL2 causes the path of TV speaker lead to be opened. (For dual-speaker TV, replace relay RL2 with a DPDT relay of 6V, 200 ohm.) Similarly, energisation of DPDT relay RL3 opens the leads going to the speakers and thus mutes both audio speakers. Use ‘NC’ contacts of relay RL3 in series with speakers of music system and ‘NC’ contacts of RL2 in series with TV speaker. Use  ‘NO’ con-tact of relay RL1 in series with a bulb to get the visual indication. 

Author : Dhurjati Sinha - Copyright : EFYmag

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MT8870 DTMF Telephone Dial Tone Decoder Circuit Diagram

 Build a MT8870 DTMF Telephone Dial Tone Decoder Circuit Diagram. This is a  simple MT8870 DTMF Telephone Dial Tone Decoder Circuit Diagram. In this circuit one common DTMF receiver IC is the Motorola MT8870 that is widely used in electronic communications circuits. The MT8870 isan 18-pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately. 

A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments. Here’s a small and handy tester circuit for the DTMF IC. It can be assembled on a multipurpose PCB with an 18-pin IC base. One can also test the IC on a simple breadboard. For optimum working of telephone equipment, the DTMF receiver must be designed to recognize a valid tone pair greater than 40 ms in duration and to accept successive digit tone-pairs that are greater than 40 ms apart. 

 MT8870 DTMF Telephone Dial Tone Decoder Circuit Diagram

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Telephone Conversation Recorder Circuit Diagram

Build a Telephone Conversation Recorder Circuit Diagram. This Telephone Conversation Recorder Circuit Diagram enables automatic switching-on of the tape recorder when the handset is lifted. The tape recorder gets switched off when the handset is replaced. The signals are suitably attenuated to a level at which they can be recorded using the ‘MICIN’ socket of the tape recorder. Points X and Y in the circuit are connected to the telephone lines. 

Resistors R1 and R2 act as a voltage divider. The voltage appearing across R2 is fed to the ‘MIC-IN’ socket of the tape recorder. The values of R1 and R2 may be changed depending on the input impedance of the tape recorder’s ‘MIC-IN’ terminals. Capacitor C1 is used for blocking the flow of DC. The second part of the circuit controls relay RL1, which is used to switch on/off the tape recorder. 

 Telephone Conversation Recorder Circuit Diagram

A voltage of 48 volts appears across the telephone lines in on-hook condition. This voltage drops to about 9 volts when the handset is lifted. Diodes D1 through D4 constitute a bridge rectifier/polarity guard. This ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines.

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