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In-Car PMR Radio Communications

This is a simple project to enable me to use a PMR-466 radio whilst on track or in a convoy on the road. It uses in-ear headphones and a separate electret microphone installed inside the helmet. These plug into a small box which has a toggle switch to enable transmission and a wired plug that goes into my Motorola PMR radio. I used a toggle switch as it is not safe to keep you hands off the steering wheel for any length of time. Voice operated activation is simply not worth trying in a bike-engined car.

Headphones

This project would work with any headphones but I started looking at it when I was given some Sennheiser CX 500

headphones as a present. These are simply brilliant at passive external noise attenuation because they come with different sized rubber surrounds to seal the ear canal. They have a relatively low impedance of 16 Ohms, which means they need to be capacitor coupled. Input is via 3.5mm stereo jack socket and both left and right channels are in parallel to keep the volume up (this halves lowers the impedance).

Using standard design guidelines, a capacitor of more than 100uF is required for 16 Ohm headphones. One thing to bear in mind though, is that the PMR radio has a mono output, driving both left and right channels in parallel, thus halving the impedance seen. Since this application involves fairly low quality voice audio and not hi-fi quality music, a huge bass response is not required.

Microphone

This can be any 2-pole electret microphone wired to a 3.5mm mono jack socket. It will be passed power up the line.

PMR 466 Wiring

It took me ages to find the information to wire up a PMR radio but I eventually located it on the Internet. My Motorola T5522 radios are wired up in the same way as the Oregon Scientific TP329 radio. The second challenge was to find a 2.5mm stereo plug to go into the radio. Motorola use a non-standard one which is 13mm long (2mm longer than normal) and a standard one bought from somewhere like Maplin will not work. In the end, I found one attached to another old headset, which I used. If you find a source of these plugs, do let me know please.

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 an electret microphone in parallel with a 2K2 resistor to this via a switch. Pushing the switch puts the microphone and resistance onto the middle (MIC) terminal and actives PTT mode.

Circuit Design

This is the basic circuit design and layout I've used:

Packaging

I've installed the whole thing in a 50mm x 50mm x 20mm case (Maplin part no. N53FK), which sits in my shirt or jacket pocket. On the front face is a latching push switch to enable transmission. This makes it easy to operate, with no fiddly switches or buttons. The unit can also be mounted to the harness using velcro.

The inside of the unit.

The outside of the unit, with a switch that is easily operate through clothing.

The way I've designed this, it will also work well on a mountain bike, in a canoe, or whilst skiing .