Fury R1

Design

Build

Drive

Gallery

Contact Me

Misc

Last page update was 24/02/08

Communications

I've experimented with helmet-to-helmet intercomm systems before on both bikes and cars. My experiences in my brother's Striker showed that doesn't matter how good or powerful the system is, the environment is simply too noisy for them to work effectively. The only way such a system will work is if it uses some form of noise cancellation or reduction. There is no point shouting above the noise, when the noise is making your ears bleed!

My system is designed to reduce the background noise levels in the helmet to enable clearer communication. It's also designed to be light-weight! One side effect is that the system could be left on, to simply lower overall noise levels in the helmet, which would be very useful on long journeys. It also interfaces to PMR466 radios an MP3 player and also my mobile phone.

Since my car has been on the road, I've changed my thinking a bit about my design. I've gone with a more modular approach and one that can be better integrated with my in-car video system.

Power

When I built the car, I added a seperate 'accessory' circuit and fuse in the main fuse box. This circuit is not switched by the ignition switch so power is always present. This means that accessory sockets can be used to charge the battery using my Optimate III charger.

I've extended this accessory circuit though and have added some more switched sockets to power the in-car video system and this comms system.

Microphones

I'm not being prescriptive about the microphones used as the signals from them are converted to line-level inputs which are used to drive both my in-car comms system and my in-car video system. The system is going to work better if the microphones are matched in terms of signal levels and frequency characteristics though. The unit assumes microphones with a 2-pin 3.5mm jack plug.

Microphone Amplifier

I've used a stereo microphone amplifer from Maplin (part no. QS41U) which rather nicely uses a 12V power feed. If you only plug in one microphone it automatically sends the output to both the left and right channels, which is good for my in-car video system as it will use the drivers microphone to feed both channels on the video recording.

This part has been re-housed within my comms unit and I've replaced the ¼" jack inputs with 3.5mm ones.

Microphone Processing

Whilst I need full frequency range to reduce ambient noise, I don't want to feed the full frequency range into my in-car video system, as the wind buffeting is quite loud and low frequency noise. I pass it through a 200Hz high-pass filter before it goes on to my recording device.

Mixer

The driver and passenger each have a 4-channel, stereo, line-level mixer taking

Controls

The comms unit has a rotating control switch (Helmet - Off - Radio) and independent volume controls on the unit itself.

There is a latching driver's Push-To-Talk (PTT) switch on the dash which operates the unit in both helmet and radio mode. There is also passenger PTT switch which is momentary.

The advantages of this layout and control system are:

Driver and passenger have independent volume control.
The driver has a locking 'transmit' switch, which can be left on to give a running commentary or dialogue.
Both the driver and passenger can always hear the PMR radio.
Both can talk to each other when the unit is in helmet mode.
With a twist of the control knob the driver can transmit over the PMR radio.

Power Supply

The unit works off of a +12V and -12V power supply and the -12V is derived from the +12V car battery using a dc-dc converter. There is a regulated +9V rail for powering the helmet electret microphone and the power amplifers. There is a regulated +4.5V rail for the PMR radio.

Mixers

There are four mixer circuits, each with four line-level inputs. These provide a combined but stereo signal to be amplified. Each mixer also has two line-level inputs for a music player, PMR radio, phone, etc. The drivers PTT module interfaces a PMR Radio to the mixer/amplifier and supports the PTT function. The comms function needs to be PTT and not VOX operated as the noise levels in a car like this are ridiculous and I don't want wind operated comms.

The pre-amps and filter for each channel are built around the MC3403N quad opamp IC. This IC is also used to create the four mixers.

Power Amplifiers

Two 3W stereo power amplifiers to drive stereo speakers in both helmets. A seperate volume control allows the driver and passenger to adjust the volume independently and presets allow the channel levels to be set-up independently.

PTT Modules

These switch the voice microphone output to the right input (helmet or PMR466 radio) on pressing the talk switch. The drivers talk switch is on the dash and is latching but the passenger talk switch is momentary. A seperate switch on the complete unit selects PMR radio or helmet to helmet operation. The PTT switches remotely operate relays to reduce noise in the circuits and to allow the complex switching required.

Helmet Speakers

Initially I used (50mm diameter, 8mm deep) 250mW mylar cone speakers as they are ultra-thin and fit nicely into the speaker pockets fitted within my Caberg helmet. A quick experiment with my MP3 player showed how terrible this sounded though, with very little bass response and simply not enough volume. My second attempt was using some speakers from a larger pair of Sony headphones, working on the basis that if they sounded right outside them helmet, then they should do the job inside. These worked really well and fitted comfortably into the helmet speaker pockets too. I hacked these apart and glued them into place. They use a stereo 3.5mm jack plug to and the unit assumes this connection method.

Helmet Connections

Originally, the helmet wiring terminated in a mini-DIN (S-VHS) type, 8-way chassis sockets but early tests resulted in these falling out. I'm now using locking 7-pin audio connectors (Maplin part numbers FK28F and FK27E). This connects to the comms unit, via a lead with the equivalent line socket on each end. This set up is used so that the helmet doesn't have wires trailing around, when the system is not required.

There is no standard for wiring up helmets, espeacially using the chosen connectors, so I've created my own. You could have any connector on your helmet but so long as they map to this connector and wiring, then it can be plugged into my car. Hopefully between myself and my two brothers, we can standardise on this configuration.

Ground / Speaker Common (yellow)
Right speaker (orange)
Left speaker (grey)
Voice mic signal (green)
Voice mic ground (brown)
Noise mic signal (red)
Noise mic ground (black)

PMR 466 Headset Wiring

It took me ages to find the information to wire up a PMR radio but I eventually located it on the 466 User

web site. My Motorola PMR466 radios are wired up in the same way as the Oregon Scientific TP329 radio.

The tip (EAR) drives the speaker or headset with the other side connected to the common (GND) terminal. The speaker needs to be high impedence or capacitor-coupled otherwise the PTT will be enabled. Grounding the tip enables PTT. If you use an 8ohm loudspeaker then this can enable PTT.
The middle (MIC) has 2.2V on it to power an electret microphone. Connect this to the common terminal on a SPDT PTT switch. Leave the normally closed side of the switch unconnected and wire an electret microphone in parallel with a 2K2 resistor to normally open side of the switch. Pushing the switch then puts the microphone and resistance onto the middle (MIC) terminal and actives PTT mode.