Telephone Ringtone Generator

This is a simple home telephone ringtone generator circuit which is built with applying only several electronic components / parts. It generates simulated telephone ringtone and requires only DC supply with 4.5V DC to 12V DC voltage.

One may possibly use this circuit in ordinary intercom or phone-type intercom. The sound is pretty loud when this circuit is operated on +12V DC power supply. Even so, the volume of ring sound can be adjusted.

Telephone Ringtone Generator Circuit Diagram

CD4060B is chosen to produce three kinds of pulses. Preset VR1 is fine-tuned to get 0.3125Hz pulses at pin 3 of IC1. At the same time, pulses obtainable from pin 1 will be of 1.25 Hz and 20 Hz at pin 14. The three output pins of IC1 are connected to base terminals of transistors T1, T2, and T3 through resistors R1, R2, and R3, respectively.

Working with a built-in oscillator-type piezobuzzer generates about 1kHz tone. In this particular circuit, the piezo-buzzer is turned ‘on’ and ‘off’ at 20 Hz for ring tone sound by transistor T3. 20Hz pulses are obtainable at the collector of transistor T3 for 0.4-second duration. Just after a time interval of 0.4 second, 20Hz pulses become again obtainable for another 0.4-second duration. This is followed by two seconds of nosound interval. Thereafter the pulse pattern repeats by itself.

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Electronic Telephone Ringer

This circuit produces a ringing sound similar to that made by more recent telephones. It consists of three almost identical oscillators connected in a chain, each generating a squarewave signal. The frequency of each oscillator depends on the RC combination: R4 and C1 around IC1.A, R8 and C2 around IC1.B and R12 and C3 around IC3.C. The pairs of 100 kΩresistors divide the asym-metric power supply voltage (between 5 V and 30 V) so that, in conjunction with the 100 kΩfeedback resistors (R3, R7 and R11) either one third or two thirds of the supply voltage will be present at the non-inverting inputs to the opamps. The voltage across the capacitor therefore oscillates in a triangle wave between these two values.

Circuit diagram :

Electronic Telephone Ringer Circuit Diagram

The first oscillator is free-running at a frequency of approximately 1/3 Hz. Only when its output is high, and D1 stops conducting, can the second oscillator run. The frequency of the second oscillator is about 13 Hz, and optional LED D3 flashes when it is running. When the output of the second oscillator is low, the third is allowed to run. The frequency of the third oscillator is around 1 kHz, and this is the tone that is produced. The second oscillator is not absolutely necessary:

its function is just to add a little modulation to the 1 kHz tone. A piezo sounder is connected to the output of the third oscillator to convert the electrical signal into an acoustic one. The current consumption of the circuit is just under 1 mA with a 5V power supply, rising to about 1.65 mA with a supply volt-age of 15 V.

 

Author : L.Libertin - Copyright : Elektor

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Telephone Ring Repeater

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.

Circuit diagram :

 Telephone Ring Repeater Circuit Diagram

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.

 

Author : Christian Tavernier  - Copyright : Elektor

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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

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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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.

Circuit diagram :

Telephone line Based Audio Muting and Light-On Circuit Diagram

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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Two-Line Intercom-Cum-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.

Circuit diagram:

2-Line Intercom-Cum-Telephone Line Changeover Circuit Diagram

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.

Author : J. Srinivasan - Copyright: Efymag

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Telephone Conversation recorder

This circuit 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 suit-ably  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.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. 

Circuit diagram :

Telephone Conversation recorder Circuit Diagram

This ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines.During on-hook condition, the output from the bridge (48V DC) passes through 12V zener D5 and is applied to the base of transistor T1 via the voltage divider comprising resistors R3 and R4. This switches on transistor T1 and its collector is pulled low. This, in turn, causes transistor T2 to cut off and relay RL1 is not energised. When the telephone handset is lifted, the voltage across points X and Y falls below 12 volts and so zener diode D5 does not conduct.

As a result, base of transistor  T1  is  pulled  to  ground  potential  via resistor R4 and thus is cut off. Thus, base of  transistor  T2  gets  forward  biased  via resistor R5, which results in the energisation  of  relay  RL1. The  tape  recorder  is switched 'on' and recording begins. The tape recorder should be kept loaded with a cassette and the record button of the tape recorder should remain pressed to enable it to record  the conversation as soon as the handset is lifted. Capacitor  C2  ensures  that  the  re-lay is not switched on-and-off repeatedly when a number is being dialled in pulse dialling mode.

Author : PRADEEP  VASUDEVA - Copyright : EFY

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Multipurpose Circuit For Telephones

This add-on device for telephones can be connected in parallel to the telephone instrument. The circuit provides audio-visual indication of on-hook, off-hook, and ringing modes. It can also be used to connect the telephone to a  CID (caller identification device) through a re-lay and also to indicate tapping or misuse of telephone lines by sounding a buzzer.

In on-hook mode, 48V DC supply is maintained across the telephone lines. In this case, the bi-colour LED glows in green, indicating the idle state of the telephone. The value of resistor  R1 can be changed some-what to adjust the  LED glow, with-out loading the telephone lines (by trial and error).  In on-hook mode of the hand-set, potentiometer VR1 is so adjusted that base of  T1 (BC547) is forward biased, which, in turn, cuts off transistor T2 (BC108). While adjusting  potmeter  VR1, en-sure that the  LED glows only in green and not in red.

Circuit diagram:

Multipurpose Circuit For Telephones circuit Diagram

When the handset is lifted, the volt-age drops to around 12V  DC. When this happens, the voltage across transistor T1’s base-emitter junction falls below its conduction level to cut it off. As a result transistor pair T2-T3 starts oscillating and the piezo-buzzer starts beeping (with switch S1 in on position). At the same time, the bi-colour LED glows in red. In ringing mode, the bi-colour LED flashes in green in synchronization with the telephone ring. A  CID can be connected using a relay.

The relay  driver  transistor can be connected via point  A as shown in the circuit. To use the circuit for warning against misuse,  switch  S1 can be left in on position to activate the piezo buzzer when anyone tries to tap the telephone line. (When the telephone  line is tapped, it’s  like the off-hook mode of the telephone hand-set.)  Two 1.5V pencil cells can provide Vcc1 power supply, while a separate power supply for Vcc2 is recommended to avoid draining the battery. However, a single 6-volt supply source can be used in con-junction with a 3.3V zener diode to cater to both Vcc2 and Vcc1 supplies.

Author : Ranjith G. Poduval - Copyright : EFY

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Telephone Line Indicator

With the aid of an (old) moving coil instrument it is very little effort to make a simple voltmeter that, at a glance, indicates the status of a telephone line. Because the input impedance of this circuit is very high, there is no problem in having it permanently connected to the line, since it only draws a tiny amount of current. The schematic shows that the circuit consists of no more than a series resistor, a bridge rectifier and a moving coil meter. The value of the resistor depends on the sensitivity of the moving coil meter. In his prototypes, the author used old VU meters that require 250µA for full-scale deflection. A resistor value of 390 kΩ appeared to be optimal for these meters.

Telephone Line Indicator Circuit Diagram

For a 100-µA-instrument, this resistor value will have to be increased to about 680 kΩ. The starting point, when selecting a resistor value is that when the telephone is not in use, the meter should deflect about 2/3rd of full scale. The amount of meter deflection indicates the three different states of the telephone line: 1. The deflection is very small: the line is in use (voltage 5 to 12 V). 2. The deflection is 2/3rd of full scale: the line is not in use (voltage typically 48 V). 3. Full-scale deflection: ring signal (60 to 90 V AC). Because the idle voltage and certainly the ring voltage are high enough to be dangerous, it is recommended that the circuit is constructed in such a way that it presents no hazard when touched.

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