Privacy Telephone Intercom

Description: The circuit presented is an enhancement of the basic Link intercom design. Although pulse dialing may seem outdated, constructing and testing the Link 'P' in this manner is a valuable exercise. This design allows users to utilize vintage rotary dial phones as functional devices or display them operationally. The core of this design includes counting, pulsing, timing, and simple logic circuits, similar to other 'Link' versions. With the prevalence of PIC controllers in modern electronics, this project serves as a reminder of the importance of traditional hard-wired logic design, making it educational as well. Builders of the Link 'P' will gain insights into the logic behind its operation, applicable in various settings such as homes, workshops, and schools. An upgrade to tone dialing is also available if desired. This iteration of the Link represents a significant advancement in telephone projects, featuring genuine ring trip functionality. Future developments aim to enable simultaneous calls, both internal and external, while ensuring privacy for each call. The Link 'P' serves as a transitional stage between the basic Link design and a more advanced version with additional features. The accompanying block diagram illustrates the system's components, including the RTC (Ring Trip Circuit), Reg. U (Register), STG (Service Tone Generator), LLO (Line Lock Out), and Vertical Control. The relay switching matrix resembles a simplified crossbar system. A detailed circuit diagram, spanning multiple pages, will later provide connection and component information for all four phones. The basic Link design allows a phone to receive a dial tone from the STG when picked up 'off hook,' enabling the caller to dial a one-digit number into the Register. The Link 'P' upgrade prevents conflicts when multiple users attempt to dial simultaneously by allowing only one phone to be off-hook for calling purposes.

The Link 'P' intercom circuit is designed to provide an efficient and user-friendly communication system, particularly for environments where multiple telephones are in use. The primary goal of this design is to enhance the functionality of rotary dial phones, allowing them to operate in a modern context while maintaining the charm of their vintage design. The integration of a Ring Trip Circuit (RTC) ensures that incoming calls can be managed effectively, allowing users to pick up the phone and receive a dial tone without interference from other devices.

The Register (Reg. U) acts as a pivotal element in the dialing process, enabling users to input their desired connection through a simple one-digit code. This streamlined approach not only simplifies the user experience but also minimizes the risk of dialing errors, which can occur in more complex systems. The Service Tone Generator (STG) provides auditory feedback to the user, confirming that the system is ready to accept a dialed number.

The introduction of the Line Lock Out (LLO) feature is crucial for maintaining call privacy and managing simultaneous call attempts. By ensuring that only one phone can be off-hook at a time, the LLO prevents confusion and ensures that calls are directed appropriately. Callers attempting to use the system simultaneously will receive a busy tone, indicating that the line is currently in use, thus preserving the integrity of ongoing conversations.

Vertical Control circuitry adds another layer of functionality by determining call priority among users. This logic-based system ensures that the first caller to pick up the phone is granted access to the line, while subsequent callers are held in a queue, effectively managing the flow of communication and preventing overlapping calls.

The relay switching matrix, designed to function similarly to a crossbar system, facilitates the routing of calls between the four connected phones. This matrix allows for efficient switching and ensures that calls are directed accurately based on user input.

Overall, the Link 'P' intercom circuit represents a thoughtful blend of vintage aesthetics and modern functionality, providing users with an effective communication solution that respects the simplicity of traditional designs while incorporating essential features for contemporary use. The comprehensive circuit diagram will provide further details on component specifications and connections, enabling builders to replicate the design accurately and benefit from its enhanced capabilities.The circuit you are about to see is the culmination of some effort at improving the basic Link intercom design. You may think that pulse dialing is `old hat` nowadays, but the exercise of building and testing the Link `P` in this format, and understanding how it all works, is well worth the effort.

Not to mention all those out there who have the old style rotary dial phones as collectables - you can now make them work for you, or display them actually working with confidence. Counting, pulsing, timing and simple logic circuits are at the heart of this design, as with all other `Link` versions.

In a day where PIC controllers are popping up in electronics magazines by the dozen (about half the projects in some magazines now contain a PIC chipset, ) I feel that we could be losing the art of designing in simple `hard wired` logic, and thus this approach is used partly for educational purposes. Many readers who may decide to build up the Link `P` will `see the logic` (pardon the pun) after they have a working model servicing their home, small workshop, factory, preschool or other application.

Besides, there`s an easy upgrade to Tone Dialing if you really want it anyway. Happy switching! AH Downunder. This version of the Link goes beyond anything previously attempted by me in the area of telephone type projects. Having achieved genuine ring trip (not that hard, but a bit tricky to design with `off the shelf` $5 components first time around.

) I`m now heading down the track for `two calls at the same time (either two internal calls, or one internal and one external on the outside line - providing it`s legal in your state/country of origin. ) but several things have to be achieved first. In order to achieve a status between two levels of switching for two calls at the same time, you must first establish privacy on each call.

Otherwise, you will end up with all four phones talking to each other, and that just won`t do. So the Link `P` is an intermediate stage between the basic Link design, and the fully blown one (yet to be realized, but not that far away, ) with all the `bells and whistles` features. In the diagram above, you can see in block format, how this is achieved. The RTC (Ring Trip Circuit, ) Reg. U (Register, which acts like an old style Uniselector, ) and the STG (Service Tone Generator, ) are all part of the basic Link design.

However, I`ve added in two new `blocks` of circuitry, and these are LLO (Line Lock Out, which provides busy tone back to callers who can`t access the Link to make a call, ) and Vertical Control (which governs who can and can`t make a call - simple logic circuitry to determine who`s first in and best dressed!) The relay switching matrix is akin to a simplified `crossbar (XBAR) system. There is a full circuit diagram spread over several pages later on in this article, which gives the details for connections and components for all four phones, and I will refer to these a little later on, but we`ll just stick with the block diagram for now.

When a phone is picked up `off hook` in the basic Link, it receives dial tone from the STG, and then the caller proceeds to dial a one digit number into the Register. This arrangement works well, except for those occasions when someone else picks their phone up `off hook` at the same time, and tries to dial a number too.

The Link `P` upgrade avoids this situation, by enabling only one phone in the `off hook` state for the purposes of making a call, and if you are the first one to do so, you will be connected to the Reg. U and the STG. Any other caller will remain on Line Lock Out (LLO) and will receive Busy Tone, to indicate that the Link is busy with a call.

You can then dial any one of the other three phones, and the