We were out of town last weekend, and I was out of town for a few days last week. I've spent way too much time trying to sort out the annunciator control and dimming. I was told that 12v bulbs for these annunciators were "unobtainium", and that I needed to use either 5v or 28v, as these were the two voltages available. This meant I needed some sort of DC-DC voltage converter, and a circuit to control the annunciators. The circuit to control the annunciators wasn't too hard to sort out, but the DC-DC converter evaluation board proved more troublesome. I obtained a National Semiconductor DC-DC converter evaluation board, which was advertised as being easily modifiable to work at various input and output voltages. But, this board used tiny surface-mount components, that are way beyond my ability to modify. I would need to hire the modification job out to a professional shop, which would run the costs up even further.

Last weekend I spent some time with my brother, who is a senior guy at a large aircraft mod shop. He said that they often had to change the operating voltage of annunciator bulbs when they did avionics upgrades. He said that they might have some 12v bulbs laying around, as most modern avionics used 5v annunciators.

Learning that 12v bulbs for my annunciators might actually exist caused me to do something I should have done months ago - I contacted the manufacturer. They told me that 12v bulbs were available, at a cost of $5 each. I need four bulbs per annunciator, plus some spares, so figure on $100 total. That isn't a lot in the big picture, as it means I wouldn't need a separate control circuit, nor a DC-DC converter. I would end up with a simpler, lighter, more reliable installation. So, I'm going to order some 12v bulbs, and drop the control circuit and the DC-DC converter. Each bulb will get its power from the box or circuit that drives it. The dimming will be done by switching a zener diode into the common ground for the bulbs. The voltage drop across the zener diode is essentially constant, no matter how much current is going through it. So it can be in a ground line that is common for all four annunciators.

I spent several hours working on the heater box controls. My original concept for routing the Bowden cables proved unworkable, Plan B wasn't any better, but Plan C looked promising. I made up some brackets for Plan C and drilled the holes where they will mount to the firewall. Now I need some warm weather so I can prime them in the driveway. I attached the heater boxes to the firewall, and hopefully I won't have to take them off again.