Telephone Intercom From TTL
This is the only craziest project I ever made, beginning in June 2007 and completed in November 2007. Very complex circuits, and uses a lot of ICs (about 38 ICs). If you think this is a waste, yes! Indeed. However, why do I keep making this project? The reason is, I want to know how the phone working principle and how the analog and digital circuits work.
Circuit Block
Telephone Intercom, which I designed, works as shown in the block diagram below. Intercom system only works with DTMF (Dual Tone Multiple Frequency). Can make two calls simultaneously, but in the process of dialing cannot be done simultaneously. We have to wait for the first call is connected, and then we make a second call.
For example, when "TELP1" wants to call "TELP2", then “CENTRAL CONTROL” will give the offhook condition, then "CENTRAL CONTROL" gave the order to enable "DIAL RECIEVER" just for "TELP1".
DTMF signals from "TELP1" received by "DIAL RECIEVER" and sent to "CENTRAL CONTROL". Then "DIAL RECIEVER" disabled through "DIAL LIMITER", is to prevent connection errors. "CENTRAL CONTROL" will provide dial tone if "TELP2" in a state of "onhook", and will give a busy tone if "TELP2" in a state of "offhook". Dial tone and busy tone is produced by the "SIGNALING DEVICE".
If "TELP2" in onhook condition, "CENTRAL CONTROL" will give orders to "CROSSPOINT SWITCHING" to connect "TELP1", "TELP2", and "RING CONTROL". Then "RING CONTROL" gives ring voltage to "TELP2", to ring the bell on the "TELP2".
When "TELP2" appointed or in the offhook condition and connected with "TELP1", then the "DIAL LIMITER" automatically disabled, and a second call can be made between "TELP3" and "TELP4".
The Circuit
As you can see on the block diagram, this device has several parts, namely; "SLIC (Subscriber Line Interface Circuit)," DIAL RECIEVER "," DIAL LIMITER "," SIGNALING DEVICE "," RING CONTROL "," CROSSPOINT SWITCHING " , and "CENTRAL CONTROL".
From the picture above circuit, each phone has a "CENTRAL CONTROL" itself. "CENTRAL CONTROL" consists of several logic gates, flip-flop, and IC latch 74373. "CROSSPOINT SWITCHING" consisted of analog switch / multiplexer IC MC14066.
"SIGNALING SERVICE / DEVICE" and "RING CONTROL" is astable multivibrator circuit using 556 dual timers IC. "SIGNALING SERVICE" produces 465Hz signal, where the signal is used as a dial tone and busy tone on the intercom network. While "RING CONTROL" produces a voltage pulse with a period of one second "HIGH" and a second "LOW". This is used as a regulator of the bell on the phone who called.
"DIAL LIMITER" consists of inverters and gates "AND", the principle of this circuit is; only be valued logic "1" at the output, if there is only one logic value "1" on input. In addition, the last is "DIAL RECIEVER", its function is to transform an analog DTMF signals from telephone callers to the form of binary code (digital) by IC MT8870.
Using The Telephone Intercom
For example, when you will make a call from "Telephone1" to "Telephone4". You have to press the button “4” on the keyboard "Telephone1", and make sure you hear a dial tone.
If "Telephone4" in a state of "onhook", you will hear the dial tone after you press the button “4” on the keyboard. Meanwhile, if "Telephone4" in condition "offhook" or lifted, it will not hear any tone from "Telephone1".
When the "Telephone4” rings and was appointed (offhook condition), waiting tone on "Telephone1" will stop and now "Telephone1" was connected with "Telephone4" and ready to talk over the phone.
If the conversation was over and one of the phone is closed (the condition "onhook"), then the caller telephone will hear a "broken link" tone as a sign of the conversation was over. For more details, see the table below.
Table 1. Telephone Intercom Dial Command
| Telephone | Dial to Telephone... | Press |
| 1 | 2 | 2 |
| 1 | 3 | 3 |
| 1 | 4 | 4 |
| 2 | 1 | 1 |
| 2 | 3 | 3 |
| 2 | 4 | 4 |
| 3 | 1 | 1 |
| 3 | 2 | 2 |
| 3 | 4 | 4 |
| 4 | 1 | 1 |
| 4 | 2 | 2 |
| 4 | 3 | 3 |
Table 2. Telephone Intercom Dialing Conditions
| Condition | Before push the keypad | After push the keypad | Info |
| Telp make a calling | Hear the dial tone | Hear the waiting tone | Connected |
| Hear the dial tone | Hear nothing | Busy | |
| Hear nothing | - | Dialing in same time |
Summary
- When you hear the tone before you press the keys on the keyboard to make a call, it is called the dial tone. When you hear the tone after pressing the button, it is called waiting tone. In addition, if you hear a tone after the talks finished, it was called the broken link tone.
- Telephone intercom could make two phone calls without interfering with other connections
- Busy condition is when you press the keys on the keyboard to make calls, but you do not hear any tones.
- Telephone intercom cannot make two calls at once at the same time
Conclusion
Although the intercom has been working in accordance with what I have designed, in this complex series I see some problems. One of them is less stable to determine the condition of "onhook" and "offhook". This is probably because I use the relay to determine the condition of "onhook" or "offhook" on the circuit.
In the future and if possible, I want to make this device more simple and compact with a microcontroller to replace the TTL ICs. From this project I learned a lot about how the phone and how digital and analog circuits work. The experience will never be forgotten.
Update
"Telephone Intercom From TTL" came in second place in the Open 7400 competition, organized by dangerousprototypes.com
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