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Sunday, 28 May 2017

Hands-free kit for TYT MD380

I have finally got round to it!
To my shame I see it was nearly a year ago that I worked out the microphone and speaker connections on my TYT MD380 handheld - see the blog here:
Mic and speaker connections for TYT MD380

I have a home-made hands-free set up for operating mobile with my analogue handheld (an FT60E) and I would like to use something similar for the digital radio.

The microphone is a small electret condenser capsule, mounted on a microphone boom of galvanised iron wire (coat-hanger wire!) which is attached to a baseball hat. The microphone is quite close to the mouth, but not directly in front to avoid breath-blast. When I made my first version, I stitched the wire to the front of the cap, which was quite time consuming. The current one is glued to the back of the brim with hot-melt glue. Not very pretty, but quite quick to do.

The transmit switch is a toggle switch that is taped to the side of a cigarette-lighter plug that is located very conveniently beside the gear-change stick. This makes it very easy to flip between TX and RX without taking my eyes off the road. And also doesn't involve drilling any holes in car.

I use a Diamond dual-band mobile antenna on the motor car, mounted on the tailgate and connected to the radio via RG58, a BNC and one of those BNC - SMA adaptors.

On the radio, under a plastic cover on the right-hand-side are two sockets, a 2.5mm diameter jack and a 3.5mm diameter jack. These are dual purpose, the 2.5mm one is used for the speaker connection, the 3.5mm is used for the microphone, and the radio is put into transmit by connecting the ground connection of the microphone to the ground connection of the speaker. The two sockets are also used for a USB type serial link for programming the radio - so don't connect to the unused terminals on the connector they are for data transfer. I used a cut-down, ready-made,  2.5mm audio cable, to avoid soldering to a rather fiddly plug.


So here is a diagram showing the connections to the radio. Note how the microphone positive is connected to the ring of the 3.5mm jack, not the tip. Leave the tip unconnected. Also leave the ring on the 2.5mm jack unconnected, these are the data connections. See how the transmit/receive (PTT) switch is wired between the speaker and microphone grounds. It is important for the microphone to be wired the correct way round, because the radio provides power for the FET amplifier in the capsule.
Here is a picture of the wiring before I covered it all with electrical tape. We don't want any short circuits. But it is always best to test it all before covering it with heatshrink or insulting tape.









Here is the complete set-up. A more glamorous baseball cap would do wonders for the appearance of the equipment ... so would a more glamorous radio ham.










A note about the loudspeaker: I use a cheap car radio speaker because it is in a nice plastic pod, which I can put on the centre console of the car. The radio specification calls for a 16 ohm impedance loudspeaker. The car radio speaker is clearly a 4 ohm part. The radio does seem to work ok with an 8 ohm speaker, but I thought 4 ohms was pushing it a bit. I used a transformer to convert the impedance. I am using just the secondary of an audio output transformer. The winding has a centre-tap and I am using it as an autotransformer. The radio is connected across the whole winding, the speaker is connected between one end and the centre-tap. This gives a 4:1 impedance conversion. The primary of the transformer is left unconnected. Hopefully you will be able to find a more suitable speaker and not have to bother with this!

Testing: I first checked the speaker connections, plugging the 2.5mm plug in and listening to the World-wide talk-group on the local repeater. When all was working on the receive side, I made sure the TX/RX switch was "off" and plugged in the 3.5mm jack. Now, on our local repeater GB7AS, we have a special Talk-group on Slot 2 which echos back the transmit audio after a short delay, for people to test their radios. This is called TG9990 Echo Test. If you don't have this, then you will have to get a friend to listen to your audio and check it is working well. you can see if the TX/RX switch is working because the LED on the top of the radio goes red in transmit.  Have fun!
73
Hugh M0WYE


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