Topic: Blitzortung Controller Board - Window Comparator Mod.

[miraculon]

Since I have been unable to track down the source of the interference that occasionally triggers the Blitzortung, I decided on a different approach.

I have noticed with an oscilloscope that when "interference mode" was occurring, it was just barely above the fixed comparator threshold, about 0.5V pk amplitude (1Vp-p). The controller board has a comparator that when the signal amplitude exceeds the threshold, it triggers an ADC read by the micro-controller. By raising the resistance value and making it adjustable, it is possible to raise the trigger threshold above the typical interferences that I have been experiencing.

I replaced the two resistors R4/P and R4/N with a 10K dual potentiometer paralleled with 10K resistors. (I know, I could have used 5k pots).

Finding a dual pot that was linear taper was difficult, since most are "audio" taper. For this application you want a linear response.

The window comparator now adjusts from essentially "zero" to just under 1V pk (2Vpk-pk). I can now adjust out an interference mode (steady yellow LED) until it responds normally to lightning events. I have been running with the adjustable window comparator for about two weeks now.

There have been situations where the interfering noise had extremely high amplitude and there wasn't enough range to stop it. Most of the time, though I have been able to eliminate the interference with a turn of the control. (or to be technically correct, stop triggering the ADC read with the higher comparator threshold.)

I'll attach a schematic from a simulation program. Note that the R3 and R4 are actually R4/P and R4/N in the real schematic of the controller board. I ran 4 conductor shielded wire and removed the existing 2k ohm R4/P and R4/N resistors. The wires from the pots went into the holes where the resistors were mounted. I chose to make fully clockwise the maximum voltage window.

When the control is turned counterclockwise, it will always trigger the "interference mode" and always on yellow LED. The control can be adjusted clockwise until the LED just goes out. This does have the effect of reducing the sensitivity to distant strikes, but in "interference mode", nothing gets through anyways.

Here is a picture showing the installed potentiometers and wiring.



Another shot from the bottom showing the adjustment knob:



Finally the schematic. Again the R3/R4 are incorrect for the real board which have designations R4/P and R4/N.



If anyone is contemplating a Blitzortung build, it might be better to wait for the new "System RED" unit with AGC and remotely adjustable gain.


Greg H

[Weather Spares]

An interesting mod, well done.

I'm a bit disappointed that the new System Red boards are out only a few weeks after I have purchased the same model as yours. As I understand it from the various news posts they've put up there is still likely to be a lot of tweaking and changes to the software and firmware of Red after it comes out, but it looks an impressive project.

There was also a post that said that the pre Red amplifier and controller boards are no longer in stock, so there is a slight delay in waiting for Red to launch.

[miraculon]

Based on the fact that the new System RED is single ended input, I decided to try placing jumpers IA-3 and IB-3 on my PCB 5 V7 amplifier board. I am getting several dB lower on the VU meters* for the normal noise level. When the obnoxious noise occurs, it is now within range of the window comparator adjustment pot (wasn't before).

The moderate level noise issues have settled down and it seems that there has been an improvement in rejecting the noise.

I suspect that the common mode voltage might have been high and single-ended gives the rod antennas a ground reference.

So far, so good. Still not "perfect", but a lot better.

I would recommend trying this jumper setting if you are having noise issues on the system "green" PCB 5, V7 boards.

Greg H

* Linux via sound card and meterbridge program.