Nuts and Volts - January 2016

The basic telephone operation shown in Figure 4 consists of a microphone and speaker connected together in series with a battery. In this "telephone," when switch S1 is closed, direct current (DC) can flow in the microphone and speaker which moves the speaker cone a bit but produces no sound other than an initial "pop." However, when someone speaks into the microphone (MIC 1), alternating current (AC) is produced which moves the speaker cone and "roughly" reproduces the sound spoken into the mic.

As an enhancement, I added an LED with a current-limiting resistor to show when a call was coming in. This is only a slight improvement over the old can and string acoustic phone, but a far cry from modern telephones which use computerized switching, satellite relays, terrestrial microwave links, wireless, cellular technology, and digital encoding.

The schematic for an old line powered phone is shown in Figure 5. This phone receives a 48 volt DC signal from the central switching office to power the phone. To ring the local phone, the central office sends an AC signal of 40 to 150 volts with a frequency of 20 to 40 Hz (cycles per second) superimposed on the 48 volt DC signal. When the receiver is lifted to answer a call, the ringer signal is disconnected and the line voltage between the local phone and the central office drops to eight volts DC (these voltages vary among the different carriers). [NOTE: At my home, the on-hook voltage is 52. 4 VDC and the off-hook is 10. 9 VDC.] To make a call, the off-hook voltage to the central office triggers a dial tone so the caller knows the call connection can be made. Most phones also include additional capacitors to cancel out the inductive reactance of the incoming phone lines.

The multi-line phone is just a normal telephone with multiple lines running to it. A four-line phone has four different lines run by the phone service provider and has the capabilities to switch between lines, put a line on hold, transfer a line, etc. An analog phone uses the amplitude modulation (amplitude of voltage on the line varies as the input voice signal increases) of the 48 volt DC line signal to transfer voice signals versus the digital phones which use digitally encoded signals to modulate the 48 volt DC signal.

The analogy between analog phone and AM radio signals is the radio has a high frequency local oscillator signal which is modulated by the input signal, whereas the phone modulates the DC signal. Thus, a four-line phone is essentially four individual phones in one body. I can imagine using several different frequency bands (frequency division multiplexing) on the two-wire pair to transmit several phone signals simultaneously with filters at the receiver end separating the different "lines," but I do not know of any phone service providers who do this on land lines for voice service to residential users.

FDM is used on long distance trunk lines, satellite communications, terrestrial microwave services, and broadband DSL. This could be a challenge for our readers: Design a multiple frequency/multi-line "phone" system for use in a home or office, but not connected to the phone company’s lines (see Q&A SIDELINES for more details).

Now, let’s answer your question on using an unused line to indicate that you are using your VoIP line. The easy way would be to hook your 5 VDC VoIP phone signal directly into the four-line phone's unused line to see if it worked. However, the easy way is not always the best way since you could damage either or both phones, plus the phone service providers require isolation of devices connected to their networks. First, use my homebrew test device shown in Figure 6 to check the off-hook voltage on the four-line phone.

Hook the test device into the line you are going to test with a short piece of telephone cord with RJ-11 plugs on each end. Most four-line systems use the four-wire phone cable to provide conductors for two lines per cable (red-green on one line and yellow-black on the other), so you will have to determine which line is which to obtain the desired results. Hook a DVM to the red-green or yellow-black pairs, punch in the line you are testing on the four-line phone, take the phone off the hook, and check the off-hook voltage that your phone produces. Also check the off-hook voltage of your VoIP phone in the