[SOLVED] Freezes when RPM signal connected

[bofh]

there is no limitation of pulses / rev

did you scale R4 / C3 according to the wiki article?
https://code.google.com/p/multidisplay/wiki/RPM

[erikiiofph7]

No, I didn't. I missed that part... I used R4=130kOhm and C3=47nF according to https://code.google.com/p/multidisplay/wiki/PartListV2.

According to the datasheet the output voltage of the frequency converter reaches maximum when the pulse frequency reaches f = 1/(R4*C3), which is 164 Hz. I have 2 pulses/rev ==> 164Hz = 4920 rpm.

In real life I don't reach 4920 rpm either, my rpm numbers won't go beyond 3450 rpm, which should correspond to about 3.5 V output from the frequency converter.

Maybe I should test changing the R4 to something with a lower resistance, e.g. the 60 kOhm resistor I just found among the leftovers. With R4=60kOhm 5V output from the frequency converter should correspond to 10600 rpm for my 2 pulses/rev. And if I'm still limited to 3.5 V output it would still cover rpms up to 7460 rpm.

[erikiiofph7]

Now R4 is changed from 130kOhm to 60.4kOhm on MD03 and now the RPM signal works over the full range. However, the rpm signal seems non-linear. I'm reading correct values up to 3000rpm, then the rpm values on the MD goes a little bit high, at 4300 it shows 4500 rpm and at 5300 it shows 5800 rpm. At 5300 rpm the deviation is 9% then.

But I'll look into that later. Maybe I'll just add a line in the code to make the rpm a 2nd degree polynomial instead of a 1st degree. This seems to me to be the easiest way around it...

[erikiiofph7]

By the way, I think it would be good if you point out in the partlist that you need to adjust C3 & R4 to your RPM setup. I know that it is mentioned in the general MD03 buildup page but I missed so I guess there are more people missing it.

On the https://code.google.com/p/multidisplay/wiki/PartListV2 it says in the MD03 RPM section:

| 1x | R4 130k | ...
...
| 1x | C3 47nF | ...

Instead you could have:

| 1x | R4 130k (value depending on your RPM setup) | ...
...
| 1x | C3 47nF (value depending on your RPM setup) | ...

, where "RPM setup" is a link to the RPM measurement wiki page.

[bofh]

done; its in the partlist now!