For the sake of simplicity and clarity, both a remote set and the central unit electronics is drawn on the same circuit diagram below. Please note that there must be at least two and possibly more remote sets as the one drawn in the diagram below existing in a system, all of them connected in parallel to GND, Rx and Tx lines. In order to understand what happens inside the system when users carry a conversation, it is best to imagine one of the remote sets in "Rx" mode and the other one in "Tx" mode connected to the lines in parallel, and to observe the way that DC and sound signals are being processed and routed by electronic components.

Electronic component values have been calculated so that everything operates reliably despite changes in ambient temperature, battery voltage, the number of remote sets in the system etc. Although the circuit diagram might seam a bit complicated, the fact that it is clearly divided into fully separate subsystems ensures reliable execution of all system functions. Several of these devices have been reproduced and tested in practice and they all worked flawessly from the first start on.

A remote set is represented at the left half of the circuit diagram below, encircled in red. It consists of two subcircuits - a low frequency preamplifier and an LED driver. The preamplifier is made of Q1 and Q2 and the surrounding components. It amplifies low voltage AC sound signal produced by the loudspeaker, which in Tx mode acts as a microphone, and sends it over the Tx wire to the central unit output power amplifier. It then amplifies the signal further and distributes now very strong AC via the Rx line to all other remote sets simoultaneously.

Preamplifier subcircuit is designed so that it draws approximately 5mA DC from the power supply, which drags DC voltage on the Tx line to 3 to 4.5V from around 6V DC it has in stand-by. This voltage drop is monitored by the central unit and used to turn on the output amplifier, which will be explained a moment later. What is interesting to observe is that the preamplifier superimposes (adds) the amplified AC sound signal to the DC voltage on the Tx line enabling the central unit amplifier to receive the user's voice. In effect, DC current flows in one direction while AC signal travels in the opposite direction through the same pair of wires simoultaneously without interfering. Similar method has been practiced in analog telephony public systems (PSTN) for over a century.

If "Talk" pushbutton on a particular remote set is not depressed i.e. if it is left in the default "Rx" position, the loudspeaker of that set is connected via the Rx line to the central unit output power amplifier and does not act as a microphone but as a true loudspeaker. Only while the "Talk" button is depressed i.e. in the "Tx" position, the loudspeaker is connected to a local preamplifier input which enables it to temporarily act as a microphone. Low level sound signal is in that case fed from the loudspeaker through 22nF cappacitor to Q1 base. At the same time, small DC current flows through 560 kΩ resistor from Q1 base via the loudspeaker to the ground polarising Q1 and consequently Q2 and enabling them to perform their preamplifier duties. With the "Talk" button in the default "Rx" position, Q1/Q2 preamplifier draws no current because of the lack of Q1 base polarisation current. Resistors are used for establishing propper DC curents and voltages in the subcircuit while cappacitors limit the frequency band between 250 Hz and 5 kHz providing a bit better sound quality than the traditional analog telephony.

LED driver circuit formed by Q3 and Q4 controls two small LEDs - a green "Rx" and a red "Tx". When one of the users talks so that the central unit is powered up, 3.3V DC voltage exists on the Rx line. Both LEDs use this voltage to glow, but only one at a time. If "Talk" button of a particular remote set is in its "Rx" position, Q3 in it is in that case directly polarised by 100 kΩ base resistor and the green "Rx" LED glows. If so, voltage between Q3 collector and base is a bit negative so that the red "Tx" LED doesn't light up. This is the case in all remote sets except the one which is currently in "Tx" mode. But inside that (active) unit, the "Talk" button is in its "Tx" position, which enables Q4 transistor ito be activated prohibiting the respective Q3 to start conducting. Therefore its "Tx" LED glows while "Rx" LED does not. This little circuit provides a clear indication of whether a particular remote set is receiving or transmitting sounds. LED current is in either case close to 5 mA.

Central unit is drawn on the right side of the diagram above. It is encircled in green and has three subcircuits. The most prominent one is the output power amplifier formed by one half of a dual LM358 operational amplifier and Q9/Q10 complementary transistor pair. This amplifier picks only the AC component of the Tx line signal, amplifies it and sends it via the Rx line to all remote sets listening in "Rx" mode. If necessary, output power amplifier voltage gain can be adjusted by changing the value of 33 kΩ resistor connecting the "Rx" line to the operational amplifier inverting input - higher resistance produces higher gain and vice versa. A reference 3.3V Zener diode connected to operational amplifier noninverting input enables the power amplifier to produce stable DC voltage at the "Rx" line for any given power supply voltage so that battery aging doesn't influence the system operation. All oher 6.8V Zener diodes are put in circuit only to protect semiconductors from power surges and overvoltages induced in long lines by nearby lightning etc. - they just sit there and do nothing in normal use.

Q6, Q7 and Q8 form a chain of DC switches which powers up the output amplifier when any of remote sets is turned into the "Tx" mode. As has been described, remote set preamplifiers draw around 5mA DC current from batteries when operating; this small current produces enough voltage drop over 390 Ω + 100 Ω resistors in the positive power supply line so that Q6 starts conducting. Its collector current energises Q7 which in turn switches Q8 on. Current gains of these three transistors are more than enough to put Q8 into a so called "saturated" conduction state meaning that it behaves as a simple switch having tiny voltage drop between its emitter and collector rather than as a true voltage or current amplifier. This leads to negligible power dissipation on Q8 even when large current necessary for supplying the output amplifier transistors with enough energy for simultaneous driving of several loudspeakers flows through it.

Q5 works similarly to Q6 - it senses DC current in the positive supply line but to turn it on, current higher than 6mA must flow from batteries over the "Tx" line to remote set(s). This happens only in two cases:

The first case is the intended reason for including Q6 into the circuit as it then provides a clean positive logic level signal for turning on the NF ringing oscillator, which is not represented in the circuit diagram on this page. The reason for not drawing it is that it can be formed in numerous ways depending on system designer skills and preferences. This oscillator is expected to inject some kind of low frequency tone signal into the output power amplifier input, but the exact form and shape of this sound is left to future designers to imagine and implement. For example, a simple but reliable square wave audio oscillator can be formed using the leftover second half of the LM358 integrated circuit... A serendipitous consequence of pressing "Talk" buttons on more than one remote set simultaneously is that the ringer circuit will be powered up so intercom users too fast on the trigger will be aware that they erroneously tried to talk at the same time.