openrcforums

Was searching the internet for RPM sensor. But there is not very much to be found.
Stumbled over this old thread here viewtopic.php?f=95&t=4334&hilit=rpm+sensor

LM 358 is still on the way, but I found an PS2561 op coupler in the box here. So used the circuit posted by thomas9x. Connected my oscilloscope to do some tests with a bl motor.

With that 10k resistor in 5V supply the amplitude of the signal to arduino ICP pin drops from 270 Hz (min RPM) to 1250 Hz (max RPM) from 4.5V down to 2.5V. And below 3V the telemetry value brakes down.

Could overcome that by replacing the 10k resistor with a 4.7k resistor and have those values 270 Hz (min RPM) to 1250 Hz (max RPM) from 5V down to 3V with telemetry ok.

My question now, is there no "better" way and get a stable amplitude of the signal. Maybe adding an amplifier .... And could someone here please make a sketch.

br KH (retired mechanical guy, still learning about electronics )

Think, I'll wait for the LM 358 and test that circuit here ....

I just would try without the capacitor or at least with a lower value

Reinhard

LM358 - TA = 0° to +70°C

Don't forget where you live

Maybe better the LM258 or the LM2904

But for testing the LM358 should be fine

João

Don't worry, just for testing

But as always NO RISK - NO FUN!

br

kalle123 wrote:
But as always NO RISK - NO FUN!

Famous last words

João

ReSt wrote:I just would try without the capacitor or at least with a lower value

Reinhard

Thanx for your suggestion Reinhard. You are right.Removing the capacity give a better pic on scope (nearly rectangular and full amplitude, but to much noise), so telemetry display becomes inconsistent.
Tried some smaller capacities and here a 47nF gives best results ....
What do you thing about reducing the 10k resistor? Would 6.8k still be save?

cu KH

With 5 Volt supply voltage and even a 1kohm resistor you will have a current thru the optocoupler transistor of less than 5 mA.
As the absolute maximum current for the transistor is given with 50 mA, that is safe, no problem so far.
I would also try a small capacitor over the input side of the coupler.

Reinhard

Tried to get an improvement this morning by following your advice, Reinhard. But did not help much. You can use that circuit as it is to indicate the bl motor is turning or not. But I would not call it measurement. The schmitt trigger is missing and I don't know how to calculate that one. Missed that on school .....

Meanwhile the LM358 arrived ( @João - ONLY FOR TESTING ) and put it on a breadboard. Clean signal. I'll call that a RPM sensor!

I hope your breadboard is above 0ºC

But it is good that it is working I think I will try it myself after finishing some little projects I have been playing with
Thanks for posting..

João

Took the components from the breadboard and placed those on a smaller board. Not ideal, but will do for a flying test with oXs.

I'm very interessted in a RPM sensor for the oXs but it is quite hard to find information on that. Is the rpm sensor above working? It would be perfect to have a wiring diagram which is working as a summary of all the trials and a short description how to connect and setup the oXs.

Thanks,
Thomas