Correcting a mistake - an update for Rodriguez.

Building an IV plotter from an Arduino Nano - Table of Contents

Rodriguez seems to be my most popular program.

I know of two people who have used Rodriguez, and posted comments where I could find them. Runner up is the Digital D43 software - I know for a fact that one person has used it besides myself.

Both of the folks who have really tried Rodriguez mention getting negative values for the collector current or the diode current.

When I read about the first one, I assumed there was a simple mix up in construction. With two independent people reporting the same thing, I decided to take a look at my hardware and the Rodriguez software.

It turns out that I made a mistake when I drew up the schematic diagram.

I built mine pretty much just “frei Schnauze,” as the Germans say. Just any old way I felt like as I was building it. I, of course, made the software to match the circuit as I had built it.

When I drew up the diagrams, I mixed up two of the analog inputs. Specifically, A2 and A3 for the collector voltage and the collector bias voltage. On mine, A2 is the collector voltage - the voltage directly on the collector of the transistor. In the diagrams, A2 is the collector bias voltage - the smoothed PWM output.

With them swapped that way, the collector current comes out negative. Since Rodriguez uses the same circuit for diodes as for the NPN collector, the diode measurements came out negative as well.

Since there are at least two folks who have built a copy of Rodriguez following my plans, I figure the most polite way to fix it is to correct the software to match the schematic drawings I published. That way all they have to do is download and use the new software rather than modifying their hardware.

The fix is in the Arduino software, not in the Rodriguez Python program.

Download the new Rodriguez.ino, upload to the Arduino, done.

Sorry about that. I’ll try to be more careful about such things in the future.

Building an IV plotter from an Arduino Nano - Table of Contents