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Telephone Circuits

2 Line Intercom Cum Telephone Line Changeover Circuit
2 Line Intercom With Telephone Line Changeover Circuit:  The circuit presented here can be used for connecting two telephones in parallel and also as a2-line intercom.
56K RF Modem
8 Line Intercommunication using 89c51
9 Line Telephone Sharer:  This circuit is able to handle nine independent telephones (using a single Telephone line pair) located at nine different locations, say, up to a distance of100m from each other, for receiving and making outgoing calls, while maintaining conversation secrecy. This circuit is useful when a single Telephone line is to be shared by more members residing in different rooms/apartments.
900Mhz 9600bps Link
9-Line Telephone Sharer
Caller ID
Caller ID / DTMF Audio Decoder
Caller ID Circuits:  information and circuits for different systems used USA and Finland
Caller ID Decoder
Caller ID Decoder Part 1
Caller ID Decoder Part 2
Caller ID Display Service:  Caller ID decoder based on 68HC11, text in French
Cell-Phone-Controlled Audio/Video Mute Switch
Chip recorder customizes phone ringer:  07/25/2002  EDN - Design Ideas / Companies usually purchase one type of telephone for all employees, which is understandable, especially if they obtain a discount for buying in quantity. One ringer sound for everyone can pose a problem, though, if the sound of a ringing phone makes a dozen people pause and look at their telephones. Equally annoying is the absence of a visual message indicator..
Circuit Detects When A Shared Telephone Line Is In Use:  01/08/01 Electronic Design - Ideas for Design / The circuit shown can detect when a shared telephone line is in use (see the figure). This is particularly useful when an automated device wishes to place an outgoing call without disturbing an. . .
Circuit Provides Ground-Start Supervision For Phone Switching:  11/20/00 Electronic Design - Ideas for Design / Ground start is the primary analog signaling method used between telephone switches such as the central office (CO) and a private branch exchange (PBX). It offers a way to indicate on-hook and off-hook conditions in a voice network. There’s one. . .
Circuit Provides Message on Disabled Phone Line:  01/20/00 EDN-Design Ideas / PDF contains multiple circuits - scroll to find the one of interest / This circuits plays a prerecorded message into any phone on the line when its receiver goes off-hook
Circuit Translates a Law to µ Law:  05/07/98 EDN-Design Ideas / Two common methods exist to command voice for transmission through a PCM channel
Circuit Vocalizes Telephone Number:  02/03/94 EDN-Design Ideas
Clip extracts signal from phone line:  07/24/03  EDN-Design Ideas / Using a capacitive-coupled clip, you can pick up the signal from a twisted-pair or wire telephone line or from other unshielded analog lines without piercing the insulation. No line test can detect the clip's presence, and it leaves no evidence of having been attached. It needs no ground return. You can fasten the small, insulated pickup plates to the opposing jaws of an alligator clip …
Contactless Ringer for Telephones
Cordless Phone Backup
Creating your Own Telephone Ne2 Rk:  you can easily create your own intercom system using two telephones, a 9-volt battery (or some other simple power supply) and a300-ohm resistor
Cut Phone Line Detector
Cut Phone Line Detector #1:  Can be used to alarm security system
Cut Phone Line Detector #2
DC / DC Converter operates From Phone Line:  01/15/98 EDN-Design Ideas
Detect Loop-Reverse Battery Conditions On A Telephone Line:  02/18/02 Electronic Design - Ideas for Design / Equipment connected to the public switched-telephone network (PSTN) sometimes needs to detect polarity reversals of the battery feed. One common application is the Direct-Inward-Dialing (DID) service that uses what is termed as "loop-reverse battery. . .
Detecting a Telephone Ring
Digisound Ring Modulator
Digital / Standard Phone Line Tester:  make sure that the line is suitable for modem use and your PCMCIA does not damage if you plug it in
Doorphone Intercom
DSL Filter (phone-line)
DTMF Coder Makes Low Cost Sine Wave Generator:  10/13/94 EDN-Design Ideas  
DTMF Decoder / Logger
DTMF Generation With 3.58 MHz Crystal:  National Semiconductor Application Notes first published05-Aug-1995
DTMF Products for you:  DTMF decoder can be powered from a 9V battery or from your parallel printer, schematic and windows demo program, design by Schmidt Technologies
DTMF Proximity Detector
DTMF Receiver IC MT8870 Tester
DTMF Tone Decoder:  hears touch tones and displays them as telephone numbers.
Dual Tone Multiple Frequency DTMF:  National Semiconductor Application Notes first published04-Nov-1995
Electronic Dazer
ELF Telephone Audio Pickup
FM Radio Telephone Bug
FM Radio Telephone Transmitter
FM Telephone Bug
Free Line Indicator Stops Interruptions:  12/07/00 EDN-Design Ideas / PDF contains multiple circuits - scroll to find the one of interest A simple circuit lights an LED, which indicates whether the line is free (Figure1a) / PDF contains multiple circuits - scroll to find the one of interest. Batteries are unnecessary; the phone line powers the circuit, and an accumulator saves energy for an "in use" indication.
Generate CID / CIDCW Analog Signals:  12/07/00 EDN-Design Ideas / PDF contains multiple circuits - scroll to find the one of interest / A high-speed µP can generate CID (caller-ID) / PDF contains multiple circuits - scroll to find the one of interest and CIDCW (caller-ID-on-call- waiting) / PDF contains multiple circuits - scroll to find the one of interest analog signals at1200 baud FSK between the first and second20-Hz ring of an incoming call
Hold Function for Telephone
Home Made PBX / PABX
Infrared Head Phones:  Using this low-cost project1-can reproduce audio from TV without disturbing others. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from TV to headphones. FAIR WARNING -- THIS SITE HAS ANNOYING POP UP ADS.
Interface Cables for Various Cell Phones
Isolated Telephone Interface:  This circuit allows you to record audio from a Telephone line into a tape recorder or computer soundcard. Most of the parts for this circuit can be scrounged from an old modem, with some work, it is possible to rewire the modem circuitry and use the old modem case.
Laser Telephone Receiver Circuit
Line Status Indicator
Link Telephone Intercom-DTMF Version
Low Cost PCO Billing Meter
Mobile Cellphone Charger
Mobile Phone Battery Charger
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 relay and also to indicate tapping or misuse of telephone lines by sounding a buzzer.
Music On Hold for Telephones
NCDS Interface Schematic for Nokia 3110, 8110, 8110i
Noise Reduction (Filter) Circuit
Nokia 3210 MBUS Interface
Nokia 5190 / 6190 Data Cable
Off Hook Phone Line Supplies 150 MW:  09/01/94 EDN-Design Ideas
Off Line Telephone Tester:  Here is a circuit of an off-line Telephone tester which does not require any Telephone line for testing a Telephone instrument. The circuit is so simple that it can be easily assembled even by a novice having very little knowledge of electronics. FAIR WARNING -- THIS SITE HAS ANNOYING POP UP ADS.
Old Phones As An Intercom
Parallel Telephone With Secrecy and Call Prevention
Parallel Telephones With Secrecy
Parallel Telephones With Secrecy:  This system will enable the incoming ring to be heard at both the ends. The DPDT switch, installed with each of the parallel telephones, connects you to the line in one position of the switch and disconnects you in the other position of the switch. At any one Time, only one telephone is connected to the line. To receive a call at an end where the instrument is not connected to the line, you just have to flip the toggle switch at your end to receive the call, and act as usual to have a conversation. As soon as the position of the toggle switch is changed, the line gets transferred to the other telephone instrument.
Personal Silent Alarm System
Phone "Hold" With Music
Phone Broadcaster
Phone Busy Indicator:  simple busy indicator, does not meet regulations of many countries because this circuit draws excessive continuous current from the ph1-line even when not active and this can cause problems with Telephone system
Phone Hold With Music
Phone in Indicator
Phone in use
Phone In Use indicator
Phone In Use Light
Phone line indicator
Phone Line Monitor
Phone Line to Audio
Phone Off Hook Indicator
Phone Rang Indicator Light
Phone to Audio Interface
Phone-line-voltage monitor meets FCC specs:  When you design equipment that interfaces to a phone line, it is often desirable to be able to monitor the dc voltage on the line. This ability can be useful, for example, to determine whether a line is in use before attempting to go off-hook and possibly interrupting somebody's phone call. FCC regulations place strict limits on the amount of leakage current an on-hook device can draw from a phone line.
Playing a Mac through a Standard Telephone Line:  how to get sound from computer to Telephone line
Precision circuit closely monitors –48V bus:  10/17/2002  EDN - Design Ideas / Ever-greater complexity in communications systems has spurred a need for power-supply management. POTS (plain-old telephone systems) obtain power from –48V supplies backed by arrays of batteries in central offices and distributed throughout copper lines. Although nominally –48V, the voltage on the lines can vary from –40 to –80V, and the voltage is subject to surges and ....
:  Here is a tele-remote circuit which enables switching "on" and "off" of appliances through Telephone lines. The circuit described here can be used to switch up to nine appliances (corresponding to the digits1 through 9 of DTMF the Telephone key-pad). The Telephone can be used to switch on or switch off the appliances also while being used for normal conversation. FAIR WARNING -- THIS SITE HAS ANNOYING POP UP ADS.
:  when the ph1-rings, lights connected to circuit flash
Silent Telephone Alert
Simple Circuit to Connect telephone Equipment to Audio Mixer:
Simple Phone Tap
Simple Phone Tap #1:  will record any conversation on any ph1-on same line, the circuit seems to be a bad design because it does not provide proper isolation and has too low DC impedance (can keep you line off-hook all the Time)
Simple Telephone Recorder:  This recorder can be connected to the telephone lines just about any place, and no external power source is needed. The tape recorders switch terminals are applied to a pair of transistors, connected as Darlingtons, that are used to turn the recorder on and off. When the telephone is off-hook theres usually about50 VDC across the phone thats divided over R1, R2, and R4, so that Q1s base is negative enough to keep the recorder off. Pick up the receiver, and the voltage drops to5 V. That leaves not quite-enough voltage on Q1s base to keep that transistor at cutoff, so the recorder begins. Remember to keep your recorders switch in the on position, and depending on how many people use the telephone, remember to rewind or change tapes occasionally!
Simple Telephone Ring Tone Generator
Smart Ringing Phone Light:  The circuit shown here is used to switch on a lamp when the Telephone rings, if the ambient light is insufficient. FAIR WARNING -- THIS SITE HAS ANNOYING POP UP ADS.
Soft Musical Telephone Ringer
Soft Musical Telephone Ringer
Soft On / Off Switch:  Modern electronic equipment incorporate "push-to-on-push-to-off" switches that do not make the clicking noise as with old equipment. An example of this is the power button on a ATX computer cabinet. Here is a circuit that does the same. It can be used to turn on/off any electronic/electrical equipment that operates on any range of voltages. FAIR WARNING -- THIS SITE HAS ANNOYING POP UP ADS.
Substitute Phone Power Supply Stops Hum During Audio Testing:  06/09/03 Electronic Design - Ideas for Design /When developing quality devices that interface with telephone lines, sensitive audio measurements are sometimes needed. However, both frequency noise and distortion added by the test equipment—as well as low-frequency signals derived from the
Switched Capacitor IC and Reference Form Elegant –48 to +10 Volt Converter:  04/24/03 EDN-Design Ideas /PDF contains multiple circuits - scroll to find this circuit
Talking Phone Dial Monitor
Taxi Phone Automatic Dialler:
Telephone amplifier
Telephone Audio Interface
Telephone Autodialler:
Telephone Call Meter Using Calculator And COB
Telephone Controlled Night Light:  When the telephone rings, or when the handset is lifted, the night light is turned on and remains on while the conversation takes place. When the handset is replaced in the cradle, the light remains on for about11 s. During standby conditions, the -28Vdc bias on the phone line maintains the output of U3 in a high state. When the ac ring signal is applied to the phone line, it is processed by the ring detector U1, producing a negative output pulse at pin2 for each ring. These pulses trigger U2, causing its output to become high and the discharge transistor to turn off.