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Telephone Circuits

Telephone Circuits:   #'s - B       C - F       G - K       L - Q       R - S      T - Z

Last Updated: October 24, 2017 02:55 PM

Circuits Designed by Dave Johnson, P.E. :

Line Status Indicator for 1 Telephone Line -  This circuit monitors one phone line . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

One-Line Telephone Line Status Indicator -  This circuit monitors one phone line. . . Circuit by David A. Johnson P.E.-June, 2000

Capacitance Proximity Switch
Draws very low power - Ideal for battery-powered applications
6 Models Available - Call 806-368-7747

Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:

Line Cord Resistor replacements -  This page has come about mainly as a result of having open circuit line cord resistors in every single one of my Meck FM converters.  Here, I look at the options for modern substitutes and other schemes for dropping the mains voltage for where a series heater power supply is used.   There is also much misinformation regarding diode and capacitor droppers on the internet, and sadly, some radios are having their valves damaged because of this.  So, I hope to clear this up and show how to calculate the correct components.   __ Contact blehack @ yahoo dot com

Line Status Indicator for 1 Telephone Line -  This circuit monitors one phone line . . . Hobby Circuit designed by Dave Johnson P.E.-June, 2000

Line-Powered Two-Tone Ringer -  Need often arises for having an extra telephone ringer in another room to alert you of an incoming call.  A low-cost, 2-tone ringer IC LS1240, which is commonly used as part...__ Electronics Projects for You

Link Telephone Intercom Every Home Should Have One! -  The Link circuitry is simple and efficient, employing just two IC s, half a dozen transistors, and a handful of garden variety components.  it all runs on 12 volts and is easily assembled.  You can have your own home intercom between the kitchen, the garage, the rumpus room and at your poolside ‘barby’ and all for less than $100!  The Link intercom__ 

Low cost transistorised intercom -  This is a circuit for a low cost intercom that is made by using transistors.   __ Designed by Electronics Projects for You

Low-Cost PCO Billing Meter -  The circuit presented here can be used in PCOs for displaying the actual bill.  The comparative disadvantages of the presented circuit are as follows: 1.  The calculator used along with this circuit...__ Electronics Projects for You

Low-Cost Transistorised Intercom -  Several intercom circuits have appeared in EFY using integrated circuits.  The circuit described here uses three easily available transistors only.  Even a beginner can easily assemble it on a piece of...__ Electronics Projects for You

Low-cost Transistorised Intercom -  The circuit described here uses three easily available transistors only.  Even a beginner can easily assemble it on a piece of veroboard.  The circuit comprises a 3-stage resistor- capacitor coupled amplifier.  When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals.  These ring signals are amplified by transistor T3 to drive the speaker of earpiece.__ 

Measuring Jitter in Digital Systems an 1448-1 -  Application Note__ Aligent

Message Bank & Missed Call Alert -  You'll never miss another Message Bank call again.__ SiliconChip

Message on disabled phone line -  01/20/00 EDN Design Ideas:  Phone companies often disconnect a misbehaving phone line from a complainant's residence for troubleshooting purposes.  With the problem between the residence and the central office, the residence is left with a dead phone line an __ Circuit Design by Kevin Kelly

Message Waiting Flasher -  This circuit starts flashing a neon light when the phone rings and continues flashing it until the phone is picked up to remind you to check your voice-mail.  The flashing neon light drives a voltage doubler (Q2) which keeps it flashing.  Q1 ensures that the __ Designed by Andrew R. Morris

Mobile Cellphone Charger -  Charging of the cellphone battery is a big problem while travelling as power supply source is not generally accessible.  if you keep your cellphone switched on continuously, its battery will go...__ Electronics Projects for You

Mobile Charging With Heat Using A Teg Module -  This circuit can come in handy when you are camping in a remote area where there is no electricity available to charge your mobile phone’s or tablet’s battery. if you can...__ Electronics Projects for You

Mobile Phone Battery Charger -  Mobile phone chargers available in the market are quite expensive.  The circuit presented here comes as a low-cost alternative to charge mobile telephones/battery packs with a rating of 7.2 volts, such...__ Electronics Projects for You

Mobile Phone Detector using LM358 -  This mobile phone detector can sense the presence of an activated mobile phone from a distance of four to five metres.  So it can come handy in an examination hall or...__ Electronics Projects for You

Mobile Phone Multipower Unit -  Most of the mobile phone complaints are related to the power supply.  This is mainly because of improper charging and use of non-recommended chargers and low-quality batteries.  Depending upon the make...__ Electronics Projects for You

Mobile Phone Sniffer-GSM Telephone Detector -  This handy, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cellphone from a distance of one and-a-half metres.  So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc.  it is also useful for detecting the use of mobile phone for spying __ Designed by Popescu Marian

Mobile Phone TX Demo -  This is a very simple and cheap device that demonstrates mobile phones (US: 'cell phones') generate RF energy (radio waves) strong enough to light an LEDmust register on this site __ Designed by Published in Elecktor July/Aug, 2010

Mobile Shield -  Protect your mobile phone from unauthorised use or theft using this simple circuit.  it can generate a loud chirping sound when somebody attempts to take away the mobile handset.  The added...__ Electronics Projects for You

Mobile Sniffer -  This circuit can detect the use of a GSM mobile in mobile-phone-restricted areas such as examination halls and other ‘do not disturb’ areas.  it can detect the activity of the phone...__ Electronics Projects for You

Modem-access adapter reduces interference -  EDN Design Ideas:  07/22/1999   The modem-access adapter (MAA) in Figure 1 has features that reduce problems when modems and phones share the same line.  The design also overcomes the shortcomings of modem interference protectors.    __ Circuit Design by Thomas Schmidt

Multipurpose for telephone -  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 Box -  U1, an LS3404 melody chip is activated when "hold" S1 is pressed, which causes SCR1 to conduct and hold the telephone line via T1, R1, and LED1.  The voltage across R1 and LED1 is used to activate the melody chip.  Q1 and Q2 form a restart circuit to keep the melody chip going during hold__ 

Null Modem cable 9-pin -  Circuit only, no circuit description included.   __ Designed by Tony van Roon  VA3AVR

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.  __ 

Off-Hook Indicator for Telephone Lines -  A "busy" indicator to prevent annoying interruptions to internet access.__ SiliconChip

Off-Hook Phone Line Supplies 150 m -  09/01/94 EDN Design Ideas:  For systems that require 150 mW or less, the power supply in Fig 1 draws power from ordinary phone jacks without interrupting the voice signal, thereby eliminating the need for batteries and ac adapters.  Built into peripheral equipment such as PCMCiA modem cards, this circuit can spare the battery in a host computer.  The circuit is __ Circuit Design by Ron Clark and Bobnderwood, Maxim Integrated Products, Sunnyvale, CA

Old phones as an intercom -  Talking over the phones is easy.  You put DC current through the phone and it transmits and receives audio.  So two phones and a current source (about 25mA) all in series will give you a talking circuit.  A suitable current source can be as simple as a 9V battery and a series resistor whose value is adjusted (with both phones offhook) till about 25mA flows.  You can then bypass the battery and the resistor with a capacitor to couple the audio straight across and get a loud and clear connection __ Designed by Markus Wandel

Old Telephones as Intercom -  You may wish to use a 12 to 24V battery, like a Gel Cell, since the car power or cheap power wall wart style supply is probably too noisy. You may have to adjust the resistor value (1.2K for 24V). Whatever gets the phones working and does not use more than 30mA total loop current. if you can manage it, get two speakerphone-type phones. Leave them in "speakerphone" mode continuously and pound on the keypad to attract the attention of the other end. __ Designed by Tomi Engdahl

One Cell Flashes White LED -  12/11/03 EDN Design Ideas:  Many portable appliances and other products that must operate from a single cell are restricted to working at very low voltages.  it is thus difficult to drive white LEDs that typically have a forward voltage of 3 to 5V.  The ability __ Circuit Design by Anthony Smith, Scitech, Biddenham, Bedfordshire, UK

One-Line Telephone Line Status Indicator -  This circuit monitors one phone line. . . Circuit by David A. Johnson P.E.-June, 2000


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Telephone Circuits:   #'s - B       C - F       G - K       L - Q       R - S      T - Z


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