MotionMote R3 Guide

[brandock]

I have a MotionMote R3 that I had never soldered up. Just did so today, and it works, but I noticed the assembly guide was supplanted by the latest R4. I'm wondering if the R3 assembly guide could be (or is) posted somewhere. I'd like to refresh my memory on some of the solder jumper options the R3 has. In particular the one that says "Battery". (I am not good at reading schematics, so the assembly guide would be more helpful to me.)

Thanks,
Brandon

[Felix]

Sorry to say that I don't keep the old guides. But if you go to the revisions page there is a summary of the R3 and some photos.
The PIR-VCC jumper just means you can select the power source (vcc) for the PIR. The default is battery, but if you have a strong reason to select the LDO regulated 3.3v source, you can do so.

[brandock]

Thanks! My understanding from the guide is that the 3.3v source is better for the Panasonic PIR, so I did make that change. Open to any updated suggestions there.

I have a follow-up question: can you think of a reason why the reading on A7 with a full battery (4.1V) would be 683 out of 1023 steps, for a calculated voltage of about 2.2V out of 3V3, when the actual voltage at that pin measured with a multimeter is the expected 2.56V. 2.56V/3V3*1023=793.6, so I would expect about 794 steps to be read on A7. Is there anything you can think of that would cause this?

(I did find the guide here: https://github.com/LowPowerLab/RFM69/blob/master/Examples/MotionMote/OLD/MotionMote%20R3%20Guide.pdf)

[Felix]

The multimeter reading can be false if the resistance is too high (like megaohms). That throws off the reading because the DMM has megaohms on its own.

[brandock]

It makes sense that the internal resistance of the DMM (an Extech Ex330, in my case) would affect the reading across a high-resistance voltage divider like the one on the MotionMote.

Here is where I am confused. The two voltage dividers I know of on MotionMotes are 4k7+10k and 1M+2M, with ratios of actual to measured voltage of around 1.47 and 1.5. The ratio I am reading is more like 1.88.

My battery is at 4.1V, and my battery reading at A7 is 683 steps. 683 steps * (3.3/1023 volts/step) * 1.5 = 3.3V.

It is almost as though the voltage divider is being fed the regulated 3V3 VCC instead of the direct battery voltage, which is why I got to wondering what that solder jumper was doing - if somehow it switched the voltage divider source as well as the PIR source.

Any ideas for me as to what may be happening? I tested swapping in a different Motieno MCU and get the same reading.

[brandock]

If anyone is still following along, see if this makes sense.

I think that when I changed to the PIR-VCC jumper, I might have accidentally had a solder bridge to the "Battery solder pad as well. Thus the battery voltage was getting around the regulator and in essence setting the reference voltage of the the entire project to the battery voltage, VBAT.

Thus, when I would read VBAT through the voltage divider, and compare it to the reference voltage (which was running at VBAT), the battery voltage would be in the numerator and the denominator, canceling out and leaving the voltage divider ratio. That explains why my reading was always 683 steps.

683 steps * (VBAT/1023 volts/step) * 1.5 (ratio) = VBAT (+/- tolerance).

When I spent some time cleaning up between the solder pads, the VCC voltage was correct and the battery readings started to make sense. Correct me if I am missing something, but this seems to be the answer to my questions.

[Felix]

That would make sense. Thanks for the follow up and sharing this for others!