Internal Intercom

This version of the intercom system is designed for users who require a reliable and affordable solution without access to an external telephone line. It is suitable for environments such as preschools, hobby farms, or small workshops, where it is necessary to keep external phone traffic separate and secure from misuse by children and others. The design primarily follows the principles of the Link pulse dial circuit but incorporates modifications and enhancements from the Link A2B+1 for DTMF dialing. The OSL relay and the flip-flop (FF2) that managed its activation and release have been removed, and two additional internal handsets along with their corresponding relays and components have been added. FF1, part of the Ring Trip Circuit (RTC), remains in the design but is positioned at the bottom of the schematic for clarity. The internal wiring for the RTR relay adheres to the Link A2B circuit specifications. When any handset is taken off-hook and a number from 1 to 4 on the keypad is pressed, the DTMF decoder chip (IC2) translates this input to IC3 (1 x 16 decoder). The output from IC3 is then directed to the appropriate base resistors of transistors Q1 to Q4 (R11 through R14). When pin 11 of IC3 goes low, it removes the high signals from pins 12 and 8 of IC1, initiating the impulsing of the selected line relay (LR1 to LR4) through driver transistor Q5 and the relevant buffer transistors (Q1 to Q4). Once the called party answers, the RTC circuit interrupts the ring current, ceasing the ringing tone and allowing the conversation to take place. After the conversation concludes and both handsets are returned to their hooks, the collector of OC1’s phototransistor activates, resetting both FF1 and IC3. Pin 11 of IC3 returns to a high state, stopping the impulsing action of IC1 while providing a dial tone from pin 5 in the reset state, preparing for the next incoming call. An additional feature allows the # output (pin 14 of IC3) to connect to the set (S) input of FF1, which can silence the ringer if a call is received while the user is away. By picking up another extension and pressing the # key, the user can effectively answer the call, preventing the other phone from ringing and the ringtone from interrupting conversations. Overall, this design closely mirrors the features of the Link pulse dialing version but operates on a two-wire circuit between each handset and the central switcher. The tone dialing method is more efficient, and the unused outputs of IC3 can be utilized for various remote control applications, such as activating sprinklers, low-voltage garden lighting, or door entry systems.

The intercom circuit comprises several key components that facilitate its operation. The DTMF decoder (IC2) is pivotal for translating keypad input into a digital signal. This signal is processed by the 1 x 16 decoder (IC3), which controls the line relays (LR1 to LR4) via the driver and buffer transistors (Q1 to Q4). The Ring Trip Circuit (RTC) plays a critical role in managing the ringing state and ensuring that the call is connected once answered. The use of phototransistors, such as OC1, allows for effective feedback to reset the system after a call is completed, ensuring that the system is ready for subsequent calls. The design's simplicity, utilizing only two wires between handsets, enhances reliability and ease of installation, making it suitable for various applications where a robust intercom solution is required. The potential for additional control applications through unused outputs further extends the versatility of this intercom system.This version of the Link is for those who really need a good cheap intercom that will work reliably, but without access to an outside Telco line. This could be in a pre school, a hobby farm or a small workshop or factory, where external phone traffic needs to be kept separate and protected from phone abusers and kids playing aroundG It follow

s mainly the principals of the Link pulse dial circuit, but with the modifications and additions of the Link A2B+1 for DTMF dialing. Basically what IG ™ve done is remove the OSL relay and the flip flop (FF2) that controlled its activation and release, and added an extra two internal handsets and their associated relays and components.

FF1 (as part of the Ring Trip Circuit or RTC) is still there, but is now shown down the bottom of the diagram for simplicityG ™s sake. Internal wiring for the RTR relay is as per the Link A2B circuit. When any phone is picked up off hook and a number from 1 to 4 on the keypad is pressed, the DTMF decoder chip (IC2) decodes this into IC3 (1 x 16 decoder) and the output of IC3 is then fed to the appropriate base resistor of Q1 to Q4 (R11 through R14).

Pin 11 of IC3 goes low, removing the high from pins 12 and 8 of IC1 and impulsing of the selected line relay (LR1 to LR4) begins, via driver transistor Q5 and the appropriate buffer transistor (Q1 to Q4). When the called party answers the call, the RTC circuit G tripsG ™ the ring current, impulsing along with the ring tone is halted, and the conversation can proceed.

When the conversation is completed and both phones are hung up, the collector of OC1G ™s phototransistor goes high and resets both FF1 and IC3. Pin 11 of IC3 goes high again, halting the impulsing action of IC1 but providing dial tone from pin 5 in the reset state, ready for the next call.

One extra nifty little feature involves the # output (pin 14 of IC3). If you connect this to the set (S) input of FF1, you can halt the ringer if someone calls your extension while youG ™re away. You can pick up another extension anywhere else and press the # key and effectively G pick upG ™ the call.

This feature prevents the other phone from continuing to ring and also halts the ring tone from overpowering your conversation. In principal, it follows very closely the features of the Link pulse dialing version, except that thereG ™s only a two wire circuit between each handset and the G black boxG ™ switcher.

In reality, tone dialing is more efficient, and you could easily use some of the other unused outputs of IC3 for remote control purposes (turning on sprinklers or low voltage garden lights, door entry systems etc. ) 🔗 External reference