Intercom with fader

Suitable for tandem bicycles and motor cycles

Connects to iPod and similar MP3 audio players

Circuit diagram:

Intercom with fader for tandems and motor cycles


R1,R2___________22K  1/4W Resistor
R3,R20___________1K  1/4W Resistors
R4______________50K  1/2W Trimmer Cermet or Carbon
R5______________47K  1/4W Resistor
R6,R7,R8_______100K  1/4W Resistors
R9,R10__________68K  1/4W Resistors (See Comments)
R11,R15,R16______1M  1/4W Resistors
R12____________470K  1/4W Resistor (See Comments)
R13,R14________220K  1/4W Resistors
R17,R18________100K  1/4W Resistors
R19____________470R  1/4W Resistor

C1,C2,C5,C7,C8_100nF  63V Polyester or Ceramic Capacitors
C3_____________100nF  63V Polyester or Ceramic Capacitor (See Notes)
C4,C6___________10µF  25V Electrolytic Capacitors
C9_____________100µF  25V Electrolytic Capacitor
C10____________220µF  25V Electrolytic Capacitor
C11____________470µF  25V Electrolytic Capacitor

D1,D2________1N4148   75V 150mA Diodes

Q1____________BC560C  45V 100mA Low noise High gain PNP Transistor
Q2____________BC550C  45V 100mA Low noise High gain NPN Transistor
Q3,Q6_________BC337   45V 800mA NPN Transistors
Q4,Q5_________BC327   45V 800mA PNP Transistors

IC1___________LM358   Low Power Dual Op-amp
IC2____________4016 or 4066 Quad bilateral switch IC

J1,J2___________3mm  Mono Jack sockets
J3,J4,J5________3mm  Stereo Jack sockets

SW1____________SPST  Toggle or Slider Switch

B1_______________6V  Battery (4 x AA or AAA 1.5V Cells in series
                              or any 6V rechargeable battery pack etc.)


A project of this kind was requested by a couple riding a tandem bicycle and looking for a device to install all in one box, allowing them to talk. Furthermore, they liked the option to plug an iPod in as well and having it mute automatically when one of them was speaking.

Circuit operation:

The complete circuit is shown in the diagram and is formed by a microphone amplifier built around IC1A, a simple ac-dc converter (IC1B and related components) driving the dual electronic switch (IC2A and IC2B) and a headphone amplifier formed by Q1 - Q6 etc. For this amplifier only the left channel is shown: obviously, the right channel will be identical (input and output connections are shown into the rectangular box).
The two microphones (small electret types) are connected to J1 and J2 and the two headphone sets (usually 32 Ohms impedance) to J4 and J5. The iPod headphone output is connected to J3 by means of suitable stereo cable and 3mm jack plugs. There is no volume control: the desired level of the music programme is adjusted by means of the iPod control.
R4 acts as a volume control for the microphones and also sets the threshold at which the music will be muted: it should be set once for all and then left alone. As a matter of fact, the music will not disappear completely: it will be attenuated by about 10.5dB if a 68K value is used for R9 and R10 whereas a 100K value will cause an attenuation of about 8dB. In practice, the lesser the value of R9 and R10 the higher the attenuation of the music.
When speaking is stopped, the music will revert to full volume after some time-delay, set by R12 and C6. The value of these components can be varied to suit one's own needs.
The headphone amplifiers, despite the high number of transistors used, are simple enough, efficient and, above all, setup-free. They are able to deliver a full 5V peak-to-peak sinewave into 16 Ohm (i.e. about 200mW into two 32 Ohm headphones wired in parallel) with less than 1% distortion @ 1KHz and 0.7% @ 10KHz.
At the standard 40mW headphone power output capability, distortion figures are 0.6% @ 1KHz and 0.3% @ 10KHz.