Electronic Design

Three-Wire Cables Connect Low-Cost, Low-Power Interphone

The "point-to-multipoint" interphone system shown in Figure 1 and Figure 2 utilizes separate three-wire cables to connect one local station to a maximum of six remote stations. Full-duplex voice communications can be initiated either at the local station (by selecting a remote) or a remote station (via its pushbutton). All remote stations are powered directly from the local station.

A low-dropout linear regulator (IC1) generates 5 V of power on the AUDIO line for distribution to the remote stations. By summing this voltage with the small outgoing audio signal, the system eliminates the need for a separate signal cable. Components C36, R44, R45, and C33 determine the audio performance: the gain (1V/13V in this case) is given by R45/(R44 + R45); the input resistance (2k) is given by R44 + R45; the minimum frequency (80 Hz) is given by 0.159/\[C36(R44 + R45)\]; and the maximum frequency (5.2 kHz) is given by 0.159/\[C33 (R44 || R45)\]. C31 prevents oscillation.

SENSE lines 1-6 (one line to each remote) serve dual functions. When the local station selects a remote by pulling low one of the normally high EN lines, U7 drives the corresponding SENSE line high. This delivers power to the remote station's microphone and associated circuitry.

Pressing the pushbutton of a deselected remote, on the other hand, sends an opposite-direction signal on the SENSE line. The pushbutton closure connects the SENSE line to the AUDIO voltage (4.5 V minimum and always present), creating a logic-high level that tells the local station (via one of its MON lines) which remote station is calling. RPACK2 and RPACK3 serve as protection networks.

When a remote station receives power on its SENSE line, the current drawn from that SENSE line, flowing from the local station via transistor Q7 and the VDD terminal of U7, enables the Q7/Q6 current mirror to act as a mixer stage for audio arriving from the remote station.

Op-amp U1A and the R5/R4/C3 network generate a fixed voltage that polarizes the preamplified microphone X1 in Figure 2. The microphone signal is coupled by C4 to op-amp U1B. U1B is configured as an inverting amplifier with load (R8), but it operates as a transconductance amplifier. Because U1 is a micropower device with negligible quiescent current, its supply current is virtually identical to its output current (the signal current in R8).

Thus, the VDD terminal of U1 (pin 8) serves as the output of a transconductance amplifier, driving audio current via the SENSE line, U7 (in Figure 1), and Q7. The Q7 current is mirrored to the collector of Q6, where R46 converts it to an audio voltage. The values shown for R46 and C34 set the upper frequency of this audio signal at 7.2 kHz. The R46 value (246 ohms) allows no more than two remote stations to be driven simultaneously.

U2 and U3 (in Figure 2) implement the remote station's audio output. U2 regulates the AUDIO line's dc component, producing a fixed output determined by R13 (two resistors can replace this potentiometer). The regulated output should be as high as possible to achieve maximum audio-output power. The voltage between U3's input and output should never be less than the sum of the maximum signal amplitude and U2's minimum dropout voltage.

U3 serves as a power amplifier (PA) that develops 80 mW with a 25-W load. R9 and R12 set the PA's dc output at one-half the U2 output while attenuating the audio to 40% of its incoming value. Because the PA gain is about 20 V/V, the input audio should be about 200 mV p-p to achieve the maximum output power (80 mW and 4 V p-p). C7 minimizes power drawn through the cable by smoothing peaks in the input audio signal.

Note that a low on the SENSE signal lowers the remote-station power consumption to zero by disabling U2 and removing power from the upper section of Figure 2. The 9.1-V zener diodes D1 and D3 provide overvoltage protection for the circuit.

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