Bluetooth and VOX Project

Having osteoarthritis in my neck, a RSI condition developed from 25+ years of computer use, I use a Bluetooth wireless headset for most phone calls be they mobile or conventional phone calls. Bluetooth devices transmit in the 2.4GHz part of the spectrum and these particular ones are Bluetooth Class 2 which have 2.5mW output power. Their range is small intended for personal handsfree wireless use so for up to 5m you get good quality; 10m you get scratchy quality and by the time you get 15m away, typically the link has dropped!

Plantronics Voyager docking base for a standard phonePlantronics Voyager 510My Plantronics Voyager 510 headset (seen right) connects to both my mobile phone but also to a standard telephone via a docking station which acts as both a headset charger and Bluetooth interface to the desk phone (see left).

These particular devices are readily available. As I was familiar with these models in my previous work and they have noise cancelling, I decided to replicate that arrangement at home.

Most mobile phones now have Bluetooth built in as standard but a few yearsJabra A210 Bluetooth adapter for connection to Ham Radio equipment ago this wasn’t the case and so Jabra produced an adapter which could be plugged into a mobile phone to allow hands-free operation to a wireless headset (see left).

The Jabra A210 appears not to be available in the UK any more and so I got mine from the US where they often appear on eBay. They’re inexpensive now as they are redundant for their original intended market, however I found US suppliers not overly keen to be bothered posting oversees and had to ask my friend Joe in Connecticut to take delivery and forward on to me.

Application for Amateur Radio useJabra A210 connected to radio transceiver

I came across a write up by Ken K7SFN for using the Jabra A210 to interface with his transceiver. Ken describes the steps on how to do this which sounded extremely easy, particularly if you have any experience with Bluetooth. The device is a duplex wireless transceiver, relaying two-way audio from the headphone socket to the headset ear-piece and from the headset mic into the microphone socket. The connection with the transceiver is via the Jabra’s 2.5mm stereo plug.

I decided to have a go at this project, following Ken’s write up, and found it was extremely easy to achieve after all. The 2.5mm plugs being rather small and problematic to solder in my experience, I bought a 2.5mm-3.5mm adapter and also a 3.5mm-RCA Phono adapter lead. I then chopped off the RCA plugs and fitted a Yaesu 8-pin mic connector to one lead and a ¼ inch headphone jack plug to the other.

I paired the devices (this is Bluetooth terminology for instructing the headset and Bluetooth adapter to connect with each other), and immediately heard the output of the transceiver coming through my headset. I pressed the transceiver’s PTT Lock button to go into transmit and had a very healthy audio coming through the transmitter! I tested the equipment out with some contacts on 20m and worked stations in Cyprus, Iceland and Portugal with all reporting good audio!

The need for VOX and constructing a simple circuit

The approach described above works fine but I thought it would be nice to have VOX so that it was a completely handsfree operation. With VOX, I could go into transmit whenever I spoke and it would revert to reception when I was quiet. The transceiver I intend to use this on is a Yaesu FT-747 and it is low on features!

I found a VOX circuit design on the web by Mike N1HFX. Mike’s circuit was easy to build. It uses one chip, a back-to-back op amp, which detects audio from the mic input and compares that audio with a reference voltage provided from a ‘VOX gain’ potentiometer. When the mic input voltage exceeds the reference voltage it provides power to a transistor working as a switch which simply connects the PTT connector with ground to put the transceiver into transmit mode.

I decided to built it monolithic fashion (ie. didn’t really think about design much!) as it was a simple circuit and chose a snap-top box to house it in. Whilst I wouldn’t advocate this approach for equipment you want to have on show next to your prize transceiver, it suited my purposes. Perforated board was used to hold the components and didn’t bother with a knob for the VOX Gain pot. These snap-top boxes are ideal for this type of project as you can push your plyers through the sides quite easily to make the appropriate hole for controls; the plastic is quite elastic too so the hole has a reasonable seal around the control once fitted.

M0EZP Homebrew VOX based on N1HFX designThe transistor has a capacitor and a resistor in the circuit leading up to it which controls VOX delay, so that transmit mode is kept on for a little time after speaking. Mike advises how to alter this and I did increase the resistance (to reduce the discharge rate of the capacitor) although I found I had to double the 47μF capacitance to around 100μF before the VOX delay was to my liking in my ‘waffle mode’ of speaking! If I was to build the circuit again, I think I would also try something like a 20kΩ trimmer pot in place of the 4.7kΩ resistor before increasing the capacitance.

The controls are very simple: VOX gain potentiometer, on/off switch and input mic socket (I used Yaesu 8-pin). No output socket was used, the output is straight into screened audio cable with a Yaesu 8-pin mic connector on the end. The circuit operates on a supply voltage of 9-14v and I’ve used a 9v battery for simplicity and to isolate this circuit from any RF loops that might otherwise get into the mic audio. I also included an on/off switch and a LED across the supply to remind me to switch off after use! The LED has a 680Ω resistor in series to drop the voltage down to something suitable for the LED.

I got my components from Chris Tredwell of Mode Components (now retired) who always provides a splendid and inexpensive service.

I have included a copy of Mike’s write up just in case Mike’s copy disappears!

Last updated 9th February 2010