This document describes the design and testing of a two station aircraft intercom circuit. The circuit uses a summing amplifier to mix audio inputs from the pilot and co-pilot microphones. An audio amplifier then amplifies the mixed signal for the headsets. A comparator circuit ensures the pilot can hear their own voice by comparing the microphone input to a threshold voltage. The author prototypes the circuit on a breadboard, then builds it on stripboard. Extensive testing of each component and the full circuit is described. The completed intercom unit is installed in an aluminum enclosure with controls.

1. Two Station Aircraft Intercom
Aircraft Systems Year 3
Avionics Project
TAN WEN THENG
C00198749
Department of Electronic, Mechanical and Aerospace Engineering
Institute of Technology Carlow, Ireland
18th
February 2015

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TAN WEN THENG
THENG
C00198749
TAN WEN THENG
THENG
24 APRIL2015

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Contents
Declaration i
Contents ii
1. Introduction 1
2. Background Theory 2
3. Circuit Design 7
4. Results and Discussion 11
5. Conclusion 13
References 14

4. 1
Introduction
Two Station Aircraft Intercom
This project was given by Mr David Allan as part of the avionic project in the Bachelor of
Engineering in Aircraft System course at Carlow Institute of Technology, given to Tan Wen Theng.
The aim of this project is to know how pilot make a communication with co-pilot or to contact the air
traffic controller. Through this project I will be producing a complete two station aircraft intercom unit
by the following procedure:-
 Background theory
 Circuit Operation
 Calibration procedure
 Circuit Design
 Board, wiring and panel layout
 Construct the circuit using strip-board and test the complete circuit
 Enclosure design drawing and dimensions
 Construct a suitable aluminium enclosure and install/mount all the components for the
intercom

5. 2
Background Theory
During flight and the engine of the aircraft in operating, the noise will be affected the pilot and
co-pilot to communicate with each other. Thus, an intercom unit is to overcome these problems on the
aircraft. There are different sizes and type of intercoms to fit on different type of aircraft. The project
I’m working on is known as the two station aircraft intercom which is different from other intercom
common intercom that we are using. The difference is when the pilots communicate on the two station
aircraft intercom, the pilot can hear his/her own voice. This is to ensure that the pilots know that the
microphone is functioning properly.
The components used in the project includes resistors, capacitors, diodes, transistors and ICs.
A combination of a summing amplifier, an audio amplifier and a comparator is used to produce this two
place intercom unit.
1. Resistor
Resistor is used in the circuit to increase the resistance in the circuit by opposing the electrons
flow in the circuit. Each resistor have specific resistance value depending on the color and are
measured in Ohms. Resistors are passive component meaning that it does not generate power.
In this circuit, resistor are used to limit current, divide voltage and pull-up I/O lines. Resistors
used in this project are fixed resistors and variable resistors.
2. Capacitor
Capacitor is an electronic component that stores electric charge which more or less like a
battery. Capacitor works by connecting two plate of metals which is separated by non-
conducting substance, or also known as dielectric. When working with capacitors, it’s
important to de-rate the circuits to a lower value than the potentially highest voltage spike in
the circuit. If not, the capacitor might explode. The capacitors we used in this circuit is mainly
the electrolytic type capacitors. They’re used for high-voltage applications because of their
relatively high maximum voltage ratings.

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3. Diode
Diode acts like a switch which allow flow when forward bias and block when reverse bias. It
ensure that current is always flowing in one direction only. Diodes used in this projects are
the signal diodes (1N4148), Zener diode and rectifiers/power diodes which has much higher
maximum current rating.

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4. Transistor
Normally, transistor has two function which they can acts as an amplifier or a switch. In this
project transistor were used as switch. The transistor used in this circuit is known as NPN
transistor. The transistor has three lead, which is Base, Collector, and Emitter. Base are
responsible for activating the transistor and collector is positive lead and emitter is negative
lead.
5. Integrating Circuits (IC)
An integrated circuit (IC), also known as a chip/microchip, is a semiconductor wafer which
consisted of thousands or millions of tiny capacitors, resistors, and transistors. An IC can acts
as an amplifier, oscillator or a counter.

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A combination of a summing amplifier, an audio amplifier and a comparator is used to produce this two
place intercom unit.
Summing Amplifier
A summing amplifier is a circuit built based upon the concept of an inverting operational
amplifier configuration. It operate by combining the voltage from severalinputs into a single output
voltage.
1) Formula to calculate the gain of the circuit.
2) Substitute the values using the formula above
3) Sum of the two amplified signals
The purpose of the summing amplifier in this project is to produce an audio mixer for adding or
mixing together individual waveforms (sounds) from different source channels (pilot, co-pilot, ATC)
before sending them combined to an audio amplifier.

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Audio Amplifier
An audio amplifier is an electronic device which increasesthe amplitude (strength) of the audio
signals that pass through it. It amplifies low-power signals to a level which is suitable to drive the
speaker in the headsets. Design parameters for audio amplifiers include gain, frequency response,
distortion and noise. In this project, audio amplifier is used to amplify the audio signal from the
microphone. The input signal is amplified about six times the original signal. In this circuit, an IC
358 was used. To amplify a fluctuating signal, a transistor based amplifier is used. With the three
lead coming out from the transistor which is the base,emitter and the collector. When a small signal
is feed between the base and emitter, a larger signal will be generated between the emitter and
collector. Normally, two separate amplifier can be found in audio equipment which is the pre-
amplifier and main amplifier. Pre-amplifier will take the original signal and boosts it to minimum
input signal level that suitable for main amplifier. The main amplifier will then boosts the signal
again into loudspeakers. This application can be found in intercom project. The output signal is then
send to the headphone for pilot and co-pilot or third passengerin the cockpit. Those three headphone
output was label as HA, HB, and HC.
Comparator
Figure 7. Comparator Circuit
Above is an example of a comparator circuit design. Comparator circuit compared two input
voltages and gives an output either 1 or 0 to indicate which is larger. Comparator in this intercom is to
check whether an input has reach its predetermined value or not.

10. 7
Circuit Design
Schematic / Prototyping
Above picture is the circuit design given in class. It consists of two IC, which is MC 358 (top
part) and LM386 (bottom part). This circuit has 36 resistor, 4 transistor, 16 capacitor,5 diode and 2 IC.

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Prototyping
At the starting of the project, I built and design my prototype circuit on a breadboard, and I
tested the circuit in stages. There were 3 stages to be completed and check.

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Strip-board Design
After the breadboard is completely tested , I started to built up my circuit on strip-board.

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Enclosure Design
The enclosure were made from aluminium. As we can see from the enclosure design below, eight holes
were drill for the enclosure. Two for variable resistor, two for main supply and ground point, two for
push-to-talk button, one for the D type-connector, and one for the L.E.D.

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Results and Discussion
Strip-board Results
After the functional test had been demonstrated and approved by the lecturer, I then proceed
using the DIYLC software to arrange the component thus making the process to make my stripboard
easier. After that, I am required to go through 3 stages of tests. To perform this test, we must first put
in an input signal of 2 KHz frequency as the pilot microphone input (PM). The first stage of test is to
test the output of the summing amplifier. The result for the summing amplifier circuit should be
amplified 6 times by the inverter.
The graph shown above is my initial results I get from the oscilloscope. The reason is because of
clipping, due to large amplitude input signal from the signal generator. The problem is solved by
adjusting the amplitude on the oscilloscope.
The second stage is to test the audio amplifier. The output signal must show a gain of 20 times
larger than the input signal.
The third stage is to test the comparator where a DC supply (<5 volts) is feed into the 5th
lead
of the MC 358 IC (AA). The results I was supposed to get is when the current supplied is increased
until a certain threshold voltage, the output signal coming out from the comparator with be 0 Volts.
During the test, I had the signal cut off when my voltages reach 5 volts which is the ideal voltage.
The last and final stage to be completed is to test the two IC together. To test the combined
stages,input signal is feed into PM as usual and the output signal is being observed at the pin 5 of the
second IC. To reconfirm that my volume control is functioning, when the potentiometer (volume) is
adjusted, the output will varies according to the resistance values.
After all the test has been carry out, the final step to complete the intercom project is to install
all the variable resistors, supply and ground connectors, D-type connector and PTT (push-to-talk)
buttons. To find out where the male connector pins for PM (Pilot Mic) and CM (Co-pilot Mic) is

15. 12
connected to the female connectors of the microphone in the Headsetconnectordevice, a continuity test
is being conducted.
During the test, the captain’s headphones and mic is working perfectly but my co-pilot’s mic
doesn’t seem to work. After a while of troubleshooting, I found out the mic is faulty and I replaced the
co-pilot’s mic with another new headphones sets. The intercom set works perfectly after the co-pilot’s
mic had been replaced.

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Conclusion
This intercom project has help me how to read a circuit diagram, understanding different type of
amplifier such asthe summing amplifier, audio amplifier and the comparator,improvement of soldering
skills, and also troubleshooting the circuit by carrying out the test step by step. During the testing
process,I learned how to use the testing equipment such as signal generator,oscilloscope, powersupply,
and multi-meter. I also familiar to use the tools to build up my enclose cases by bare hand with safety
issues in the workshop. This all activity that I completed in this project are experience which could not
earn from book.

17. 14
References
Sarasotaavionics, (2015), Figure 1. PM 1200 2-Place Intercom Unit [ONLINE]. Available
at:http://sarasotaavionics.com/avionics/pm1200 [Accessed 15 April 15].
Electronics Tutorial. 2015. The Summing Amplifier. [ONLINE] Available at: http://www.electronics-
tutorials.ws/opamp/opamp_4.html. [Accessed 18 April 15].
Hyperphysics. 2015. Amplifiers. [ONLINE] Available at: http://hyperphysics.phy-
astr.gsu.edu/hbase/audio/amp.html. [Accessed 18 April 15].