Have you ever wanted to link two local telephone devices without using a telephone line? Now you can!
The following text describes construction, testing and use of an electronic telephone line simulator of my own design. This self-build project can simulate a telephone call / connection between any two local telephone devices. This for a fraction of the outlay of some commercial units, many with cost-increasing features that usually are not required.
Advantages of LineSim
LineSim makes all the following actions possible without occupying a telephone line or incurring any call charges. At some time, you must have wanted to do at least one of these things.
Demonstrate Any Telephone Equipment Without Using A 'Phone Line.
If your work requires you to demonstrate any form of telephony equipment, LineSim can make it a breeze. No need to have one or even two telephone lines available, just connect your equipment to LineSim and away you go!
Fax Machine as Scanner and Printer.
LineSim lets your fax machine function both as a monochrome full-page scanner and as a high-resolution printer.
If a scanner is not available, LineSim can link your fax machine to your modem, allowing you to scan a paper document or image into your computer by sending it to the PC as a fax.
If a good quality printer is not available, LineSim enables your modem to send a document to your own fax machine, resulting in a high-resolution printout.
To initialise a connection, just set the fax or PC to dial any arbitrary number (or no number).
You may have seen overpriced units advertised offering 'fax as scanner / printer' facilities, however these do not provide either dialling tones or a ringing signal. Consequently, you must set your modem for blind dialling and use your fax machine and modem in manual receive mode. (If this is all you require, click here.) Conversely, LineSim simulates a conventional telephone call, producing all the required tones and signals, thereby allowing normal dialling and automatic answering.
Testing Communications Software:
Bulletin Board, Remote Access, Voicemail, Fax on request or Internet server.
If you develop or test any of the above types of software, you will appreciate the ability to simulate exactly the conditions faced by a remote caller. Solution - simply dial in from another local computer linked by LineSim.
Off-line testing of any telephone plus other devices:
FAX / MODEM 
If you suspect a problem with your FAX MACHINE or MODEM, just use LineSim to link it to another compatible device and test it yourself, without having to call, or be called by someone else.
FAX SWITCH
Test or demonstrate your fax switch without using your 'phone line(s).
ANSWERING MACHINE
Perhaps you wish to know how your answering message sounds at the other end of the line.
You may want to test or learn to use the remote control access facilities of your answering machine.
Via LineSim, connect any telephone to your answering machine and access it without dialling in from another line.
Security System Voice Diallers & Dial-In Alarm / Home Automation Interfaces
If you install systems using any of the above, LineSim can let you test these devices and demonstrate systems to potential clients - all without using a telephone line.
Other devices which can be tested or demonstrated by LineSim:
- Call recorder
- Call diverter
- Extension ringer
- Telephone Ring booster
Offline Programming of Telephones
If you need to program several numbers into a telephone and do not have its manual, it may take some trial and error to discover how to program it. Some telephones have to be off-hook in order to be programmed, thus leaving your line effectively engaged. Instead just connect your telephone into LineSim and program it at your leisure, leaving your line free for incoming calls on your other extensions.
Link two local computers to play modem games.
Many computer games allow gaming between two players on different PCs via modem or serial cable.
If a serial connection is not possible, you can link two local computers via LineSim. If you have developed any such software, it will need some real-world testing. Again, LineSim is the answer.
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Requisite Skills & Equipment.
The project is suitable for those with a reasonable level of experience of electronics construction and fault finding. For this reason, there is no blow-by-blow description of the entire construction process. Some knowledge of the working of logic ICs would be beneficial. No expensive test gear is required, although for diagnostic work, a logic probe is highly desirable. I recommend access either to an oscilloscope or frequency counter, however this can be circumvented.
Compatibility
The unit described is designed primarily to be compatible with U.K. telephony equipment, however without circuit modification should be suitable for use with equipment designed for many other countries.
Where applicable, automatic dialling / answering equipment used with the unit, must be able to recognise a 400Hz dial tone and a ring signal of 25Hz.
I cannot provide information as to compatibility with specific national standards.
This unit has many USA-based constructors, none of whom has reported compatibility problems.
Component Availability
The component sources mentioned are U.K. based, however all required components are widely available. The design avoids the need to obtain obscure or 'hard to find' items, or to manufacture intricate parts like coils etc.
Telephone Line Simulator with Dialtone, Busy Tone & Ring Signal
General Circuit Outline
Circuit Diagram
Functional Description
The circuit is centred on two cascaded binary counters clocked by a 555 astable multivibrator. Several of the counters' outputs provide the frequencies, tones and time delays required. A pair of operational amplifiers detect on / off hook conditions at both ends of the simulated line. To detect call answering, a third op-amp monitors the ringing current. R/S latches and logic gates provide event storage and signal control. Discrete circuits provide the voltage multiplication and DC to AC conversion required for the ringing signal. A pair of miniature relays, switch each end of the simulated line between connection and ringing.
Conditions
This project is completely royalty-free as the design is not viable for commercial production. Anyone may build any number of units with no payment to me. Students wishing to build this project, should make it clear to all concerned that the design is not theirs. I shall ignore any question or request whose nature suggests to the contrary.
All I ask is to be notified of each completion of the project.
Please use the PROJECT COMPLETED NOTIFICATION FORM.
Other than for personal viewing, without permission from the author this article or any part thereof may not be reproduced in any form, including the creation of a mirror website.
Disclaimer
The author disclaims all responsibility for any form of injury, damage or loss of any kind associated with the following:
1. The construction, testing, handling or use of the equipment described herein. Prior to commencing work, inexperienced constructors are advised to seek advice / assistance from a knowledgeable person.
2. The following of any directions or advice given herein.
The information on this site is subject to periodic amendment. Users are advised to refresh regularly any cached pages to which they may refer.
Excepting affiliate schemes, the author has no connection of any sort with any company or organisation mentioned anywhere on these pages.
Please contact me if you:
- Complete this project.
- Have any problems or queries connected with this project.
- Discover any errors or omissions on any page.
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