Radio Transmitter

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Wednesday, April 1, 2009

Intercom with fader

Suitable for tandem bicycles and motor cycles
Connects to iPod and similar MP3 audio players

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.


  • 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.)
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.

  • Sometimes, the use of only one microphone could be desirable. In this case, plug the microphone jack into J1 or J2, but plug a "dummy jack" without cable into the other socket as well.
  • The tone of the microphone channel can be made more "warm" by increasing C3 value to 1µF or more. In this case pay attention, as microphones could pick-up unwanted low frequency motor or traffic noise, causing the activation of the mute circuit even in the absence of speaking.
  • Please note that the control pins of the unused bilateral switches contained into IC2 must be wired to negative ground. Other pins can be left open as shown in the diagram, bottom right.

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