Major modifications to my comms system. 10th of December 2010.

Firstly MK I

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I removed the box mounted at the rear for several reasons.

1. The canopy frame hit the head phone cables when I opened and closed it.

2. It was a pain to plug my head phones in and out with my seat belt on.

3. The long cables back to the radio were not ideal.

So all in all not a great idea.

My new box will mount under the panel with some Velcro so I can easily remove it for service MKI is just connectors no mic switching relays or noise cancelling. I built this one to get me in the air and prove that everything works. The jack socket on the back is for my recorder and the pot is to vary the amount of signal sent to the recorder. The 15 pin plug is for the radio and the 9 pin is for all the connections to the panel.

MK II

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MKII was designed from the Microair circuit in the manual with relays to switch off the unused microphone while transmitting. Then I came up with the idea of adding a noise cancelling circuit to reduce the transmitted cabin noise.This circuit was to be plugged into the side of the main box because I could not fit it all in the one box.

THEN I realized I could achieve all the microphone switching for the radio AND intercom with one DPDT relay which meant I could fit the whole thing in one box. In fact I have built the whole thing on one PC board.

MK III

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So far I have had it working on the bench but not in the air.

MK IV under construction.

Yes we have MK IV ,unfortunately MK III did not perform as well as I had hoped , the noise canceller needs a bit more work.

No noise canceller for the time being. MK IV has 3 relays 1 to switch the microphones and the other two to switch the radio PTT's for radio transmission and intercom. On MK III I used isolation diodes to allow the switching of the radio and the microphone change over relay but I discovered the radio PTT's are better taken to ground not + 0.6 v as they were with a diode in the line. The last 2 shots the completed article, The DB9 plug connects to all the buttons etc ,the DB15 connects to the radio and the power plug gets AV power from the AV bus.

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MKV February 2013

I think I have made more of these things than Tony Spicer made panels. The more I think about this stuff the more improvements come to mind. I have decided to dispense with the second box for the co-pilot and install the sockets in the end of the main box. Also I am not happy with the Velcro mounting system so have installed some nut plates in the cross channel under the panel and screw the comms box to them.

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If I turn the box up side down and put the screws up into the box and then screw the lid on it turns out very neat as you cannot see any screws.

Another addition to MkV is a hand held microphone which plugs into the 5 pin din socket I was using for the second box , mainly to use when on the ground but could also be used in an emergency if the headset mike failed or if I find myself without a headset for what ever reason. It,s a modified Phillips 828 mobile radio microphone. I took the Phillips dynamic insert out and replaced it with an electret similar to the headsets.

The sockets for the co-pilots headset.

Must tidy all the wiring up before I go flying.

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The schematic of my comms box. The intercom switch alows both mics to be operated at the same time. In PTT mode each pilot has to push his Intercom PTT before speaking. In practice it works much better than any VOX system I have ever used.(Once you get use to it) The switches are on the headphone leads at chest height.

June 2019 Modification to MKV , I will call it MKVa as it is not a major change.

The headset mounted switch was not ideal for the pilot because he often has his hands busy and to go fishing for the dangling switch took time and often by the time you found it the thought was gone.

No problem for the passenger because he can be holding his all the time and just press it when he wants to say something.

What I have done is install a DPDT momentry toggle switch on the joystick where the radio PTT was. Now if I want to transmit on the air band I move the switch left and if I want to talk to the passenger I move the switch to the right. Problem solved , your hand is always / mostly holding the joystick.

I don,t need the socket for pluging the intercom switch into so I took it out and put a flashing red LED in it,s place the LED comes on if either button is pressed or the switch on the box is in the intercom position.

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The new toggle switch on top of the joystick and the old push button. I had to make a new aluminium mount for it because the push putton hole was too big for the minature toggle switch. It,s just machined out of a small piece of 3/16" 6061 left over from the build. Next photo is the 3 pin plug I had to install to replace the 2 pin , Pink is intercom red is radio PTT and black is ground. My new circuit diagram. I used pin 6 , I was saving this for the stall warning system which is has been half finished for too many years , one day!