Yesterday I needed to determine the pin out of the cable that will connect my device to the camera in order to finish the schematic for my interface board. The cable has a Nikon proprietary connector on one end and a mini USB connector on the other. Using a multimeter, the cable, a USB breakout board and a thin probe it didn't take long to determine the pin out of the cable. I used the information to complete the preliminary version of the schematic diagram (now available here in the gallery).

While I had the cable connected to the breakout board I decided to check if the camera feeds power to the accessory port even when the power switch on the camera is set to the off position. I found out that is the case and that is when I realized I have a minor problem to solve.

The other DIY GPS project for a Nikon camera made me think I would see 5 volts on the accessory connector. When I measured the voltage supplied by my camera I obtained a reading of 5.94 volts. The AVR microcontroller has a maximum supply voltage rating of 5.5V. I am still looking for a datasheet for the voltage regulator used on the GPS receiver module. It might have a maximum supply voltage rating of 5V. I need to knock at least 0.5V off the voltage supplied by the camera to avoid damaging any of the components I'm using in my project. I also need to do so while drawing the least amount of additional current from the camera battery.

I could use a simple resistor to drop the supply voltage but the varying current drawn by the GPS module would result in varying voltage to the AVR microcontroller. A resistor and zener is the next option but that means two more components to add to an already crowded board. The last option is adding a voltage regulator. The regulator must provide an output voltage high enough to satisfy the regulator on the GPS module. I will conduct some tests with a variable power supply to see if a 3.6V output will work.