July 2003 – the instruments and electrical system.

I had already purchased all the instruments I thought I needed, 3 1/8″ flight instruments and 2 1/4″ engine instruments, so I thought the panel would go together quickly with a simple and basic layout. How wrong I was. The Sonex panel is really small and I soon discovered that I couldn’t fit the basics onto the panel. First thing – I replaced the 3 1/8″ VSI and altimeter with a single 2 1/4″ Stratomaster electronic unit. It works like a charm. But still not enough room. So for some months I researched smaller or combination instruments but got nowhere. Finally I decided to buy a Stratomaster engine information system “EIS”. What a good decision – one instrument with a dozen functions. And the best thing is – only one wire from firewall forward. Everything connects to the “RDAC” unit on the firewall in the engine compartment.

But of course I couldn’t stop there. Now that the panel was electronic, I discovered high brightness LEDs, so I introduced a flashing EIS warning LED, a bright yellow oil pressure warning LED, a master switch LED and a red/green voltage warning LED by way of a voltage sensing circuit mounted behind the master switch.

Then I decided I wanted a good quality voice activated intercom to go with my Microair radio and bought the Australian XCOM unit. Fantastic result. Stop there – oh no. I wanted more. So I bought two electronic noise canceling (ENC) headsets. Then I discovered the problem of carrying a battery box for them – where do you put it? So the culmination of my electronics work was a headset box to take all the connectors as well as supply 9V to the ENC headsets. I installed a 12V-9V-converter circuit inside the box, wired everything properly, added connectors and installed it out of the way behind the seat.

What an epic. But I am really happy with the result. Finished in stipple blue/black it will match the final paint – more on that later.

Panel wiring

A real exercise in wiring layout and basic electronics. I learnt a lot about wire ratings, fuses, over-voltage protection circuits – you name it. Whatever I needed to know, I sought out the information and soaked it up.


I bought the XCOM intercom because it had been designed for simple connection to the Microair. Simple maybe, but the wiring took quite some planning and detailed soldering. Here it is on bench test. Worked first try. Nothing blown up and in for repair. It took a lot of care to prevent that though.

The completed panel

So here it is – the final panel. Only one mechanical instrument left – the ASI. But I figured I needed that. Stipple blue/black for the paint and yellow/black labels from my ink jet printer.

Fitting the panel

Here is the panel installed in the airframe. The glareshield finally supports it, but there’s plenty of work to do before that.

Wiring access

Once the panel is installed, access to the crowded wiring behind is difficult. I made a small removable panel that exposes the wiring and switches for diagnostics and changes.


The remote signal collector for the Stratomaster EIS. Here it is installed on the firewall with the instrument probes connected. 2 EGT, 2 CHT, oil pressure / temperature. Fuel level via the probe in the tank. The alternator regulator is to the right and the air filter can be seen right of picture on the carb heat box.

The battery box

I have installed an Odyssey battery. Lightweight and high cranking power. The plans call for a metal box to mount it on the firewall and here it is complete with securing flap. You can just see the common earth plate with all the earth cables connected to it. And to the right is the starter solenoid. All cables are heavy duty for low voltage loss.

The headset box

I am quite proud of my headset box, although it’s really quite simple. Here it is in pieces. The headsets connect on top. You can see the 12V-9V-converter at lower right and the connectors I used at lower left. All cables are shielded and earthed only at the panel end at the same point. I hope I don’t get any noise problems.

The completed headset box

Here it is all done, mounted on the main fuselage cross member behind the pilot’s shoulders. The headset cables will emerge from between the pilot and passenger. They will not interfere with the seat belts and I am designing the seat cushions to suit.

The next page in Lynn Jarvis’s Sonex project features The cowling.

Lynn Jarvis’s Sonex project

  |  1. Introduction  |  2. Sonex specifications  |  3. Building the Sonex  |

  |  4. The tail and spar  |  5. The wing  |  6. The aft fuselage  |  7. More on the fuselage  |

  |  8. It all comes together  | 9. The canopy  |  10. The engine  |  11. Electrical  |  12. The cowling  |

  |  13. Paint and polish  | 14. Moving  |  15. Finishing  |  16. First flight  |  17. Natfly 2004  |