- External Communication Wiring
The circuit uses standard 3-pin microphone connectors and cable to
interconnect the intercom boxes. Pin 1 is connected to the outside shield, and
pins 2 and 3 are the inner pair of shielded conductors. In the intercom
system, pin 1 is ground, pin 2 is the power supply, and pin 3 is the
audio/signal circuit. "Belt packs" are normally fitted with
parallel-connected male and female 3-pin XLR-type connectors so the intercom
line can be extended, "daisy-chain" style, to many intercom boxes. The number
of intercom circuits which can be connected in parallel is limited only by how
beefy the power supply is, and the length (or actually line resistance/loss)
of the cabling.
- Audio and Signaling
The intercom audio signal and the call signal share the same wire. This is
done by AC-coupling all the audio parts of the circuit to isolate them from
whether the signal signal is present or not. The audio signal appears to be
nominally around 250 to 500 mV. The Call signal puts the better part of the
power voltage (a couple dozen volts) on the intercom bus. (Note that when the
call function is activated, you can hear the DC signal voltage turning on and
off as a "thump" in the audio signal.)
- Power Supply
About 28 to 30 volts filtered DC is supplied on pin 3 of the line connector.
D1 protects the intercom circuit from accidental voltage reversal (which would
fry all the ICs and transistors!) R6 and C0 form a simple filter to remove any
garbage that may have been collected in the cable on the way to the intercom
circuit. This supplies the main power ("VCC") to all the circuits.
- V/2 Supply
Because most of the intercom circuit is designed using IC op-amps, and
since we have only a single-ended power supply, we must provide a "virtual
ground" which is halfway between the VCC and ground. The VCC voltage is
divided in half by R17 and R18 and is filtered with C9. This raw VCC/2 feeds
the + input of the mic preamp IC (U1) because it is the most sensitive part of
the circuit. The raw VCC/2 is sampled by U2A which is configured as a simple
unity buffer which provides the main VCC/2 supply to the rest of the circuit.
- Mic Preamp
U1 amplifies the microphone signal to a level high enough to apply to the
intercom bus. There is some fancy frequency shaping done in the feedback
circuit (C4, C8, C11, R9, R10, R11). A microphone mute feature is implemented
by shorting out most of the feedback circuit (with SW1) thereby reducing the
gain of the microphone amplifier essentially to 1 (which would effectively
feed nothing to the intercom bus.) The mic preamp is AC-coupled to the
intercom bus by C3 which isolates the circuit from the DC component of the
- Duplex "Hybrid"
U2B accepts the intercom audio signal from the intercom bus, does some
frequency shaping, and provides a self-nulling "duplex hybrid" functionality.
The intercom bus signal is fed into the non-inverting ("+") input of the
op-amp, and the local microphone signal is fed into the inverting ("-") input.
By adjusting the level of the microphone signal with RV1, you can adjust how
much of your own voice is subtracted from the intercom bus signal. This
effectively adjusts the "Sidetone". This circuit is AC-coupled to the
intercom bus by C15, C16, and C25 which prevent the DC call signal from
screwing up the function of the circuit.
- Output Driver
U3 forms the output circuit which drives the earphone of the intercom
headset. U3A provides several db of voltage gain, and U3B is configured as a
unity buffer to drive low-impedance earphone(s).
Note that this amplifier could also drive a small speaker in a wall-box. But
use an external amplifier unless you are in a quiet environment.
- Call Button
You can send a "call" signal by pressing SW2. This causes base current to
flow through the PNP Darlington transistor Q1 to ground. When this happens, Q1
sends the filtered VCC voltage onto the intercom bus. This is the indication
to all the intercom modules on the bus to activate their signal circuits.
Note that the Call circuit is DC-coupled to the intercom bus, isolated only by
the low-impedance R14. C1 and R2 form an R/C time constant which limits the
Dv/Dt voltage rise on the intercom bus, and C7/R9 form a similar R/C time
constant which limits the voltage decay on the bus. This is done to limit the
magnitude of the "thump" which is heard by all the people on the bus as the DC
"call" signal is applied and removed.
- Signal Lamp
When a high DC voltage is detected on the intercom bus (i.e. when one of
the people on the bus pushes their "call" button), the voltage raises to a
significant fraction of the power supply voltage (depending on how many
circuits are on the bus, and the resistance/loss of the wiring). In any case
this is more than enough voltage to cause base current to flow into Q2. This
causes Q2 to turn on and allow VCC to flow though the signal lamp LP1. R4 and
C2 form a filter to keep the signal lamp from activating on brief noise spikes
on the intercom bus.
R29 can be adjusted to suit the type of indicator desired. Use a
bare-wire jumper if a 24V incandescent lamp is used. Use a resistor to limit
the current if a Light-Emitting Diode (LED) is used. Choose the value to
supply the desired current through your chosen LED.
R30 is required if an electred microphone is used. It supplies the
power to the microphone. Choose the value to supply the required
voltage/current to your microphone.
Line Input can be switched between multiple circuits in a multi-circuit
installation. Use a 3-pole switch unless you have proved that you can
join the grounds from the different circuits without interference.
External signal can be triggered by using an optocoupler or small relay
in place of the signal lamp.
© 2002 Richard Crowley
Updated: 03 May 2003