Simple-OSD notes


Overall I like the Simple-OSD because for now it is simple and provides basic OSD information at a reasonable price. What I don’t like is the lack of instruction and information regarding it’s operation and configuration.

There are a couple of operational issues: the GPS sometimes (too often) displays “waiting for fix” mid flight after working perfectly (no idea what’s going on) and the PPM control input is too sensitive causing the display to flash between lines rapidly at times.

I’ve had one of these and it has worked reasonably well for about a year until one day while working on the plane I plugged the current sensor into the OSD the wrong way, which killed the current sensor and the analog input on the cpu; It then read 6.5 Amps all the time. This also upset the battery mAh readings. For a while I used it with no current or mAh displays until recently I bought a new unit.

The plan was to cut a track and fit a series resistor on the analog input. When the board arrived I found it had been updated and had the resistor already fitted and a few other mods. I had a 30A Alegro current sensor that I planned to use to replace the original now dead Flytron sensor. One of the new mods was to include a series diode on the input to the 5V regulator.

When installed in the plane I spent hours trying to get it the current sensor reading; any reading would do, but nothing. I also noticed that the voltage display was unstable, all over the place. Eventually I took it out of the plane and set up a test rig on the bench with camera and video monitor.

On the bench I connected the scope and looked at some voltages. The 5V rail was a saw-tooth; that’s not right. The 5V regulator was unstable; I’ve seen this before. Remember the series diode before the regulator? there was no capacitor on the input to the regulator so I added a 1uF 25V ceramic. I also added a couple of 22uF 10V tantalums, one to the GPS board and one on the OSD board. This solved the stability problems; the voltage indication is rock solid now.

Now on to the current sensor calibration. How the calibration works is not clear at all. Another couple of hours searching the web and it was not much clearer. There is a suggestion that 0-5V = 0-1024 ADC counts which must be scaled to 0-?? amps. There is an auto-calibration button in the configuration program (also confusing) so I picked the standard 50A sensor and said do it. Nothing appeared to happen and it still didn’t work. I expected some sort of reading but scaled as I was using a 30A sensor rather than 50A.

After another couple of hours of experimenting I discovered a few things that almost make sense: When you click and OK “write cal. values” it re-zeros the current reading from the ADC as 0.1Amps. I’m not sure what the Current-1 cal. value does but there was some suggestion that it should be around 512; half of 1024 for the Alegro sensor which puts out 2.5V at zero amps. My sensor was wired so that +ve current load gave a +ve voltage to the OSD input. The OSD was expecting to see a -ve voltage with increasing current so I had to re-wire the sensor to reverse the current. I also worked out that a simple network of 3 resistors attached to my sensor module could scale the output so that the selected 50A standard sensor displayed 0-30.6 amps as my 30A sensor supplied 2.5-0 volts. Close enough for me.

It would be nice if the standing load of FPV, OSD, RC etc. of about 0.6 amps was displayed. I think the only way to do this is to bypass the current sensor so that it sees 0A flow while calibrating the OSD.

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